|
SUMMARIES OF FY 1996
RESEARCH IN THE CHEMICAL
SCIENCES |
Chemical Physics:
Offsite Projects
University of Akron
Akron, OH 44325
Department of Chemistry
Generalizations Concerning Vibrational and Rotational Energy Redistribution within
Polyatomic Molecules
This project employs high-resolution infrared single- and double-resonance spectroscopy to
explore the possibility of establishing broadly based generalizations about the rate and mechanism
of intramolecular vibrational energy redistribution (IVR). The presence of rapid IVR determines
the collisional and reactive properties of vibrationally excited molecules in combustion systems or
wherever they occur. The role of molecular flexibility in accelerating IVR, the dependence on the
nature of the initially prepared vibration, and a possible correlation between rate and mechanism
are explored in this work. Specific molecular systems include propyne, methanol, and
methylamine, which will be studied in the 3000 to 7000 cm-1 energy range. The
experiments resolve clumps of discrete molecular features, called molecular eigenstates, for which
the good quantum numbers are completely assigned. The needed information about the rate and
mechanism of IVR is contained in the frequencies and intensities of these discrete features. The
experimental work is supported by random matrix calculations, which are capable of quantifying
the contributions of anharmonic and Coriolis (3 types) coupling mechanisms.
Arizona State University
Tempe, AZ 85287-1604
Department of Chemistry
Electronic Structure and Reactivities of Transition Metal Clusters
| Investigator(s) |
Balasubramanian, K.
|
$95,000 |
| Phone | 602-965-3054 |
| E-mail |
KBalu@ASU.Edu |
The objective of this research is to seek answers to fundamental and intriguing questions pertinent
to the electronic structure and reactivities of transition metal clusters. The geometries, binding
energies, energy separations of excited states, ionization potentials, and other properties of
clusters, including their reactivities, are theoretically computed as functions of cluster size.
Theoretical studies on the dimers and trimers are focused on the energy separations
(Te) of several excited states and their spectroscopic constants
(re, 
e, µe). Computations on the potential
energy surfaces are undertaken to shed light on the reactivity of these species. Spectroscopic
constants of several low-lying states of WN, and other transition metal nitrides, as well as carbides such
as RhC, TaC, etc., are computed including spin-orbit effects. The observed spectra are too
complex to explain without theoretical studies. Comparisons with observed spectra are
made.Transition metal carbides such as YCn, TaCn,
LaCn, LaCn+,
TaCn+, HfCn, RhCn,
Y2Cn for the values of n between 3 and 13 are studied. The
geometries, binding energies, energy separations of low-lying electronic states, etc., are being
computed. The geometries of these carbide clusters are being fully optimized. Geometries and
energy separations of transition metal clusters such as Rh4, Rh5,
W3, W4, Zr5, Hf4, etc.,are being
computed using high-level relativistic ab initio methods. The potential energy surfaces of
transition metal dimers with carbon monoxide (CO) will be computed to gain insight into the
nature of surface+CO interactions. Specific reactions being studied are Ir3+CO
and Rh3+CO, which are being investigated through the computation of the
potential energy surfaces. These studies use complete active space MCSCF (CASSCF) followed
by multireference configuration interaction (MRCI) computations that include several million
configurations. Spin-orbit effects are included using the relativistic configuration interaction (RCI)
method.
Arizona State University
Tempe, AZ 85287
Department of Chemistry
Generation Detection and Characterization of Gas-Phase Transition-Metal Aggregates and
Compounds
Characterization of the gas-phase products generated in the reaction of early transition metals (Sc,
Ti and Y) and late transition metals (Ir and Pt) with alkanes, H2S,
NH3 and N2O has been the main focus of recent investigations.
The early transition metal containing systems are expected to be more readily modeled because of
their limited number of valence electrons and serve as excellent tests for the reliability of
semi-empirical and ab initio bonding predictions. The late transition metals are exceedingly
important in metal-mediated transformations of alkenes and alkanes, but because of the near
degeneracy of the 6s2 5dn, 6s1
5dn+1, and the 6s0 5dn+2 configurations and the
similar radial extent of the 6s and 5d orbitals the description of bonding is very complex. A laser
ablation supersonic molecular beam spectrometer, using LIF detection, has been constructed. The
optical Stark effect in PtX (X=O, N, S and C) and IrX (X=C and N) molecules were measured
and the permanent electric dipole moments determined. A simple, single configuration, molecular
orbital correlation diagram was developed to explain the observed trends. New polyatomic
molecules, YCC and ScNH have been detected. This is the first spectroscopic detection of a
gas-phase metal dicarbide.
University of Arizona
Tucson, AZ 85721
Department of Chemistry
Chemical Activation of Molecules by Metals: Experimental Studies of Electron Distributions
and Bonding
The continued purpose of this research program is to obtain detailed experimental information on
the different fundamental ways metals bond and activate organic molecules. Our approach is to
directly probe the electronic interactions between metals and molecules through a wide variety of
ionization spectroscopies and other techniques, and to investigate the relationships with bonding
modes, structures, and chemical behavior. The following specific research accomplishments have
taken place during this year of the project. (a) We have published our study of the
ligand-mediated metal-metal interactions in bimetallic fulvalene complexes. The metal-metal
interactions, which are entirely through the pi orbitals of the ligand, are different for cobalt and
rhodium because of different orbital delocalizations, and this controls the electrochemical behavior
of the complexes. (b) We have acquired high resolution photoelectron spectra of CpNiNO. The
quality of these spectra provide a definitive answer to a dispute concerning the electronic
interactions in this system, which has attracted widespread attention. (c) We have studied the
influence of acetylide substituents and metal electron richness upon the interactions of acetylides
with metals. In addition to the common occurrence of metal-acetylides in catalytic systems, these
complexes are also of interest for nonlinear optical properties. Others have stressed the ability of
acetylides to withdraw pi electron density from the metal center, but these experiments show that
the acetylide ligand behaves more as a pi electron donor. (d) We have investigated the electronic
communication between two metals through a bridging acetylide ligand. Again, the interaction is
primarily through the pi bonding framework. (e) We have studied the electronic perturbations
caused by cyclopentadienyl perhalogenation in metal complexes. Electronic and steric
perturbations caused by chemical substitutions on cyclopentadienyl ligands are an important
means of tuning chemical and catalytic reactivity. We find that the electron withdrawal that
accompanies highly electronegative halogen substitutions is tempered by the pi donor nature of
the halogens toward the cyclopentadienyl pi electrons. During this period, there have also been
numerous developments of the equipment for these investigations, and a number of projects are
underway which give a promising outlook for the coming period of the program.
University of California, Los Angeles
Los Angeles, CA 90095-1569
Department of Chemistry and Biochemistry
High-Resolution Raman Spectroscopy of Complexes and Clusters in Molecular Beams
The project objectives are twofold. The first is to develop methods of nonlinear Raman
spectroscopy for application in studies of sparse samples. The second is to apply such methods to
structural and dynamical studies of species (molecules, complexes, and clusters) in ultracold
supersonic molecular-beam samples. In the past year we have made significant progress in
extending the application of mass-selective ionization-detected stimulated Raman spectroscopies
(IDSRS) to the study of intermolecular vibrational transitions in molecular clusters. This progress
has resulted from several developments. First, we have increased the spectral resolution of our
apparatus by an order of magnitude to about 0.02 wavenumbers. Second, with this higher
resolution we have succeeded in observing the manifestations of the pendular states that arise via
the interaction of high-intensity optical fields with species having permanent polarizability
anisotropies. Such observations have been a considerable aid in the assignment of observed
intermolecular Raman bands to intermolecular vibrations in the clusters. Third, higher resolution
has enabled us to observe the weak, narrow intermolecular Raman bands that gain their intensity
from the intermolecular-interaction-induced modulation of a cluster's polarizability. Previously,
we had been effectively blind to these important vibrational modes. Finally, the increased
sensitivity available from the higher-resolution apparatus has enabled the observation of
intermolecular combination bands and overtones. Characterization of such transitions is critical to
the elucidation of intermolecular potential-energy surfaces. In the year ahead we plan to extend
our Raman methods to the study of species that are relevant to combustion processes (i.e.,
radicals and ions).
University of California, Santa Barbara
Santa Barbara, CA 93106
Department of Chemistry
Interactions of Highly Vibrationally Excited Molecules with Clean Metal Surfaces
In the last 12 months we have commissioned a new ultrahigh vacuum surface science machine for
use in the title experiments. The surface science machine is fully operational and all components
are functioning properly. In order to obtain important information on one of the most heavily
studied prototypical surface reactions, we have undertaken the study of the associative desorption
of atomic Hydrogen from Cu(111). We have determined that the reaction of two Hydrogen atoms
adsorbed on a Cu(111) surface produces molecular Hydrogen which leaves the surface with a
"helicoptering" motion; that is, a motion where the rotation of the desorbing
molecules is preferentially aligned in a plane parallel to the surface. Furthermore, we find that this
alignment increases strongly with increasing rotational angular momentum. These results imply a
clear steric effect in the time-reversed dissociative adsorption process for high j-states which
disappears at low j. Specifically, adsorption of high j-states of molecular Hydrogen occurs
preferentially for collisions with the internuclear axis parallel to the surface. Adsorption of low
j-states exhibits little or no steric effect, perhaps due to molecular realignment by the adsorption
potential energy surface. These conclusions result from observations of single rovibrational states
of H2, D2 and HD desorbing from a Cu(111) permeation source
under ultrahigh vacuum conditions. To achieve this, the output of a doubled dye laser is tripled in
Xenon to generate tunable radiation in the range 104.5 - 110 nm. This radiation is used to excite
specific one-photon transitions in the three isotopic variants of the Hydrogen (B-X) system, while
the fundamental (313.5 - 330 nm) subsequently ionizes the B-state. This 1+1' REMPI scheme is
sensitive to the rotational alignment of the desorbing Hydrogen molecules relative to the
polarization of the radiation. A photo-elastic modulator is used to control the polarization of the
radiation so that it is either parallel or perpendicular to the surface normal. Our results are in
qualitative agreement with recent calculations. They will provide an important experimental
benchmark for further theoretical comparison.
Catholic University of America
Washington, DC 20064
Department of Chemistry
Studies of Combustion Kinetics and Mechanisms
| Investigator(s) |
Slagle, I.
|
$111,000 |
| Phone | 202-319-5383 |
The goal of this research is to obtain new quantitative knowledge of the kinetics and mechanisms
of the elementary reactions of polyatomic free radicals, which are important in hydrocarbon
combustion processes. Polyatomic free radicals are generated in a heatable (up to 1100 K) flow
reactor by the photodecomposition of suitable molecules using a pulsed uv-laser. The ensuing
reactions are monitored in time-resolved experiments using photoionization mass spectrometry. In
order to obtain basic information regarding the reactivity of these free radicals, reaction rate
constants are measured as a function of temperature and pressure (0.5 to 25 torr) and, when
possible, the primary reaction products are determined and their branching fractions measured.
These experimental studies are coupled with theoretical ones to obtain an improved understanding
of the factors governing reactivity and to provide a rational basis for extrapolating the observed
kinetic behavior of free-radical processes from laboratory conditions to the harsher environment
of actual combustion processes. Recent studies have focused particularly on the reactions of the
vinyl radical, the simplest unsaturated hydrocarbon free radical and one that has been implicated
as a soot precursor. Reactions studied include the unimolecular decomposition of
C2 H3 and its reactions with O2,
H2 and acetylene.
University of Chicago
Chicago, IL 60637
James Franck Institute
Chemical Reaction Dynamics of Combustion Intermediates and Products
The studies develop our predictive ability for bimolecular and unimolecular reactions of
combustion intermediates and products. The first experiments investigate the photolytic
generation and reactivity of bimolecular reaction intermediates important in combustion such as
the HOCO intermediate of the OH + CO 
H + CO2 reaction and the HONO intermediate of the OH +
NO H +
NO2 reaction. The second set of experiments investigates how state-specific
parent vibrational excitation of molecules such as CH3SH and
CH3NH2 can, upon photodissociation, alter the branching ratio
between competing product channels. The change in branching between the product channels is
determined with photofragment velocity and angular distribution analysis in a crossed laser
molecular beam apparatus. The final experiments probe how the relative orientation of the
reactants in bimolecular reactions like H + O2 OH + O, a key rate limiting, chain branching step in
combustion processes, can influence the reaction cross section. Orienting the diatomic reagent in
analogous systems, H + XY X + YH, with respect to the velocity vector of the H atom reagent allows us
to investigate how the effective barrier changes with relative orientation and how nonadiabatic
recrossing influences the reaction cross section.
University of Chicago
Chicago, IL 60637
James Franck Institute
Quantum Dynamics of Fast Chemical Reactions
In this research we want to determine accurate values of thermal rate constants and state-to-state
cross sections for elementary bimolecular reactions in the gas phase including photodissociation.
In these studies we develop and use accurate and efficient quantum methods. The thermal rate
constants have been calculated both from the quantum thermal averaged flux-flux correlation
function (evaluated by diagonalizing the Hamiltonian in a three-body DVR) and from the
cumulative reaction probability, N(E). Recently we have developed methods that use only real
square integrable representations on a finite (small) range in order to represent the continuum
scattering processes. We have applied these to the photodissociation of van der Waals molecules
and the UV photodissociation of a model for methyl mercaptan. Applications to other 3-D
reactions are in progress. We have also treated the dynamics of electronically nonadiabatic
collisions using the two diabatic surfaces, together with an appropriate interaction term.
Nonadiabatic effects are common in photodissociation and in some chemical exchange reactions.
An additional focus has been on the development of a "quantum transition state
theory" based on operator expressions for N(E). Time evolution of the "transition
state wavepackets" once yields their contributions to the cumulative reaction probability at
all energies. Applications to the H2 + OH reaction in full 6-D yields excellent
results. Finally we have developed a new efficient method for correcting or interpolating potential
energy surfaces using spectral data or the ab initio values at relatively few points.
University of Colorado
Boulder, CO 80309-0215
Department of Chemistry and Biochemistry
Laser Photoelectron Spectroscopy of Ions
The combustion of aromatic compounds is an important process. The manner in which an
aromatic ring is cracked by dioxygen is an intriguing one for chemists. There is a recent proposal
that a dioxiranyl might be the key intermediate in the ring opening of benzene. We have begun to
study the chemistry and spectroscopy of two possible intermediates in benzene oxidation, the
C6H5 radical, and the phenyl peroxy radical,
C6H5OO. We have measured the IR absorption spectrum of the
C6H5radical in an Ar matrix and we have photodetached the
HO2- and
(CH3)3CO2- ions. In order to
understand the spectroscopy and thermochemistry of the phenyl peroxy radical, we plan to
measure the gas phase acidity of C6H5OOH and to photodetach
the C6H5OO- ion. Phenylhydroperoxide
(C6H5OOH) is a very dangerous molecule to prepare
consequently we have begun our initial studies with tert-butylhydroperoxide,
(CH3)3COOH. The
(CH3)3CO2- ion, m/z 89, was
prepared by a discharge with tert-butylhydroperoxide. We observe photodetachment 
2A''
(CH3)3CO2
2A'
(CH3)3CO2- and find EA
[(CH3)3CO2]=1.1 ± 0.2 eV which
compare with EA[HO2] = 1.078 ± 0.010 eV. The 
E(2A',2A'')
splitting is 1.2 ± 0.2 eV in the organic peroxide which contrasts with 0.867 ±
0.001 eV for HO2. In the future, a measurement of acidG298[(CH3)3COO-H] will afford us acidH298[(CH3)3COO-H] which can be combined with
EA[(CH3)3CO2] to yield
H298[(CH3)3COO-H]. Since fH298[(CH3)3COOH] is known, our value of
H298[(CH3)3COO-H] will provide us with the
absolute heat of formation of the tert-butyl peroxyl radical, fH298[(CH3)3COO]. This value will immediately yield
D0[(CH3)3CO-O] and
D0[(CH3)3C-OO]. The phenyl radical
(C6H5) is an important species in organic chemistry and
combustion processes. The energy of this radical has recently been reported [fH0(C6H5) =
84.3 ± 0.6 kcal mol-1] and is derived from the bond energy of benzene
[D0(C6H5-H) = 112.0 ± 0.6 kcal
mol-1]; the high reactivity of C6H5 has made its
spectroscopic characterization very difficult. We have measured the infrared absorption spectrum
of the phenyl radical in an argon matrix at 12 K and propose assignments for both the frequencies
and intensities for all of the 24 IR active modes. We observe the IR absorption spectra of the
phenyl radical in an Ar matrix at 12 K when we photolyze either the acyl peroxide,
C6H5CO2-OCOC6H5 or the anhydride,
C6H5CO-O-COC6H5. Benzoyl
peroxide is by far the most effective precursor of phenyl. Our irradiated matrix has an EPR
spectrum that is identical with the known EPR spectrum of the phenyl radical;
C6H5. The EPR spectrum of phenyl is consistent with a
C2v "
radical" whose ground electronic state is 2A1. To find the IR fundamentals of
C6H5, we monitored the time course of an extensive number of
infrared spectra as we photolyzed
C6H5CO2-OCOC6H5. We identify 24 prominent bands that are formed at the same rate as the benzoyl peroxide
bands disappear. Then we check our 24 bands by monitoring the rate of destruction of the
C6H5 features as we warm the matrix up to 30 K and observe the
appearance of the IR spectrum of biphenyl, (2). Only those bands that a) are produced at the same
rate as benzoyl peroxide is destroyed and b) disappear at the same rate as
C6H5-C6H5 is produced are
identified with C6H5. A comparison of the integrated intensity of
the CO2 bands and those of phenyl permits us to estimate the absolute infrared
intensities of C6H5. Our matrix frequencies are expected to be
within ± 1% of the gas phase values (
/cm-1) while our intensities (A/km mol-1) are
within ± 10% those of the isolated C6H5.
University of Colorado
Boulder, CO 80309
Department of Chemistry and Biochemistry
Time-Resolved FTIR Emission Studies of Laser Photofragmentation and Chain Reactions
Spectrally resolved infrared emission is used to study combustion processes of radical-radical
reactions and radical-molecule collision events. A time-resolved Fourier transform infrared
emission apparatus has been developed for these measurements. New studies involve the detailed
investigation of a novel five-membered ring transition state in the reactions of O atoms with
molecules such as ethyl iodide and propyl iodide, which forms HOI. The reaction proceeds by
attack of the O atom at the iodine followed by a potential surface crossing that allows the O atom
to abstract an H atom from an adjacent carbon. Detailed spectroscopic information has been
obtained to determine the structure of the HOI product molecule for the first time. A jet-cooled
source of molecules has been developed to study infrared emission from photofragments and
reactions. In preliminary work, the dynamics of NH2 from the photodissociation
of NH3 has been explored under the highly compressed spectral condition of
jet-cooling.
Columbia University
New York, NY 10027
Department of Chemistry
Energy Partitioning in Elementary Gas-Phase Reactions
The reactions of O(3P) with a series of alkynes has been studied by laser induced
fluorescence detection of the nascent H atoms and CO. The most startling discovery is that the
CO products are invariably vibrationally and rotationally cold in spite of the fact that the reactions
are very exothermic. The general explanation is as follows. The O atom attaches itself to an
unsaturated carbon atom forming a vibrationally excited C-O bond. However, a hydrogen atom or
alkyl radical is bound to that same carbon atom; the CO molecule cannot be liberated until the H
atom or alkyl radical migrates to an adjacent carbon atom. The energy within the CO bond is used
to overcome the potential barrier to the 1,2 migration. The substituted ketene which is then
formed decomposes too rapidly for vibrational energy to flow back to the CO. On the other hand,
the linearity of the ketene CCO group produces a rotationally cold CO molecule. The larger
alkynes yield a variety of radicals whose identity and ionization potentials are being determined by
vacuum ultraviolet photoionization. Experiments similar to the above are planned for the reactions
of O(3P) with alkenes.
Columbia University
New York, NY 10027
Department of Chemistry
Laser-Enhanced Chemical Reaction Studies
This project employs extremely high resolution infrared diode lasers to study fundamental
combustion and collision dynamics and photochemical reaction processes. High energy atoms,
molecules, and chemically reactive radicals, produced by excimer laser photolysis or dye laser
excitation, are used as reagents to investigate collisional excitation, collisional quenching, and
chemical production of individual rotational and vibrational states of molecules. Translational
energy recoil of the target molecules is determined by measuring the time dependent Doppler
profile of the molecular infrared transitions. This experimental method has been used to probe the
quenching step in the famous Lindemann unimolecular reaction model in which vibrationally hot
molecules with chemically significant amounts of energy are cooled by collisions with a cold bath
molecule. Loss of energy by these highly excited molecules has been found to proceed via two
distinct mechanisms. The first vibrational energy transfer mechanism is characterized by very large
bath rotational and translational energy uptake with no vibrational excitation, the clear signature
of an impulsive short range force collision. In the second mechanism, vibrationally excited states
of the bath molecule are excited in a collision with a highly excited donor. Quite amazingly, when
the vibrations of the bath molecule produced in the collision are high frequency (energy spacing
large compared to kT) almost no energy is imparted to the rotational or translational degrees of
freedom of the molecule. This is the clear signature of a long range force mechanism in which soft
collisions dominate the energy transfer. Indeed, temperature dependent studies confirm this
conclusion, since the excitation probabilities increase as the temperature decreases. Perhaps even
more surprising is the fact that vibrationally excited states of the bath that are infrared inactive,
having no dipole transition matrix element to the ground state, are also excited by a long range
mechanism. This suggests excitation of the bath can be mediated by quadrupole interactions as
well as dipole interactions. The detailed chemical and collision dynamics of a number of processes
are being studied using optical parametric oscillators to prepare single initial quantum states of the
reactants. A general super radiant laser process has been discovered using this apparatus. The
data obtained in all of these experiments is of fundamental as well as practical interest in testing
theoretical computations based on approximate potential energy surfaces and in providing an
improved understanding of combustion and atmospheric chemical processes.
Columbia University
New York, NY 10027
Department of Chemistry
Single-Collision Studies of Energy Transfer and Chemical Reaction
This research project addresses the dynamics of chemical reactions that are important in
combustion processes, or that serve as prototypes of important combustion reactions. Our current
interest is in reactions of free radicals, such as H atoms, with vibrationally excited hydrocarbons,
such as CH4. We are trying to learn how reactions of vibrationally excited
molecules differ, both qualitatively and quantitatively, from reactions of ground vibrational state
molecules. This is an important issue in understanding combustion processes, since vibrationally
excited species are relatively abundant at combustion temperatures, and reactions with them can
make a large contribution to the overall rate of reaction. Our primary interest is in radical +
polyatom combustion reactions. However, in carrying out benchmark studies of atom + diatom
reactions we have found that current understanding of even these simple reactions is not very
good when the reactant diatom is vibrationally excited, so we are putting effort into studies of
atom + diatom reactions as well. We do both experimental and computational studies. The
experiments--state-to-state dynamics experiments under single-collision conditions--use a
sequence of precisely timed nanosecond laser pulses to create the free radical reactant, to
vibrationally excite the reactant molecule, and to detect the products and determines their energy
distributions. The computational simulations are quasi-classical trajectory calculations. The
state-to-state reactive cross sections that come from our experiments are used to provide a
rigorous test of the accuracy of the computational simulations of the reactions. When validated by
comparison with experimental results, the computational simulations provide insight about the
dynamics of combustion reactions, and are used to develop models of these reactions that have
predictive power.
Cornell University
Ithaca, NY 14853
Department of Applied and Engineering Physics
Spectroscopy of Combustion Radicals
A knowledge of reaction pathways leading to the formation of chlorinated aromatic compounds in
the combustion of halogenated organic compounds and the development of monitoring methods
for hazardous emissions will assist the DOE in the management and disposal of hazardous
chemical wastes. Selective laser ionization techniques are used in this project for the measurement
of concentration profiles of radical intermediates in the combustion of chlorinated hydrocarbon
flames. A novel flame-sampling tunable VUV photoionization mass spectrometer is in use for
these studies. Both CH4/O2 and
H2/O2 flames seeded with chlorocarbons including
C2HCl3, C2H3Cl,
CH3Cl, CHCl3, CH2Cl2, and
CH3Cl are under study. A number of radicals, previously unmonitored in a flame
environment, are detected in the work to date. These include Cl, CCl, CHCl,
CH2Cl, C2Cl, C2HCl,
C2HCl2, C2H2Cl, and
C2Cl2. Profiles of the relative concentrations of flame species are
compared with the predictions of kinetic models for chlorocarbon combustion to confirm and
extend postulated reaction mechanisms. Near-threshold photoionization spectroscopy with the
tunable VUV laser system is used for the measurement of ionization potentials for chlorocarbon
radicals.
Cornell University
Ithaca, NY 14853
Department of Chemistry
Studies of Combustion Reactions at the State-Resolved Differential Cross Section Level
| Investigator(s) |
Houston, P.L.
|
$100,603 |
| Phone | 607-255-4303 |
| E-mail |
PLH2@CORNELL.EDU |
The technique of product imaging is being used to investigate several processes important to a
fundamental understanding of combustion. The imaging technique produces a
"snapshot" of the three-dimensional velocity distribution of a state-selected reaction
product. Research in three main areas is planned. First, differential cross sections are being
measured for several reactions, perhaps the most important of which is the H +
O2 reaction. Second, the imaging technique is being used to detect only zero
kinetic energy fragments from a photodissociation near threshold. Since these fragments are
produced primarily when the photolysis light is resonant with an internal level of the activated
complex, it should then be possible by scanning the photolysis light to obtain a
"spectrum" of the transition state. Third, the imaging technique is being used to learn
the distribution of translational energy when a highly vibrationally excited molecule collides under
well-defined single-collision conditions with a partner. This distribution of translational energy is
directly related to the distribution of vibrational energy removed by collisional deactivation, a
quantity of importance to a theoretical understanding of two important combustion processes,
unimolecular dissociation and radical recombination.
Cornell University
Ithaca, NY 14853
Laboratory of Atomic and Solid State Physics
Photochemical Dynamics of Surface-Oriented Molecules
This project focuses on the elucidating of the detailed mechanisms of nanosecond and
femtosecond laser induced surface reactions and the determination of the time scales involved in
the breaking and formation of chemical bonds on the surface. The depth of our understanding of
the experimental results was obtained via modeling and simulations of nanosecond and
femtosecond laser induced dynamical quantum processes involving molecules adsorbed on solid
surfaces, which provide detailed information on energy related processes such as catalysis,
combustion, and materials synthesis and processing. Our approach is to investigate: 1.
unimolecular reaction (photodesorption) and 2. bimolecular reaction (photoreaction). For the
study of photodesorption, the adsorption systems were O2 adsorbed on Pt(111),
O2/Pt(111), and NO/Pt(111), while O2 coadsorbed with CO on
Pt(111), O2/CO/Pt(111), leads to a bimolecular reaction to produce
CO2. The translational distributions were obtained as a function of the laser
fluence, wavelength, and the time delay between pairs of femtosecond laser pulses in two-pulse
correlation measurements. A number of remarkable hallmarks of femtosecond laser induced
surface chemistry were revealed when compared to that induced by nanosecond laser pulses. In
addition, the desorption rate reaches a maximum value about 500 fs after the laser pulse.
Emory University
Atlanta, GA 30322
Department of Chemistry
Theoretical Studies of Combustion Dynamics
The objectives of this research project are the development and application of rigorous theoretical
methods to describe dynamical processes of importance in gas-phase combustion. The focus of
the research is on bimolecular and unimolecular reactions of importance in combustion. The
dissociation of HCO and DCO via Feshbach resonances has been studied using a high quality ab
initio potential. The recombination rate of H+CO to form stable HCO (in the presence of an Ar
buffer gas) has also been calculated using a new theory of Professor W. H. Miller (still within the
strong collision approximation). Scattering calculations of state-to-state energy transfer in
Ar+HCO collisions have also been done, using a model Ar-HCO interaction potential. These
calculations have been extended to energies above the dissociation threshold, and have revealed
the importance of resonances in the dissociation mechanism. Reduced dimensionality quantum
calculations have been done on the reaction OH+CO H+CO2. This reaction is dominated by HOCO
resonances, and thus represents an important prototype radical-radical reaction system.
Satisfactory agreement with the experimental thermal rate constant was obtained by lowering the
exit channel barrier slightly.
Emory University
Atlanta, GA 30322
Department of Chemistry
Kinetics of Elementary Processes Relevant to Incipient Soot Formation
Kinetics and mechanisms of elementary processes important to incipient soot formation have been
investigated experimentally and theoretically. In the experimental studies, the cavity ring-down
method, which has been developed in this project for kinetic measurements of
NH2 and C6H5 radical reactions, was extended
for C6H5O detection and the
C6H5O + NO reaction. In addition, we have employed a
laser-photolysis/mass spectrometric method to measure for the first time the absolute rate
constant for the recombination of C6H5 radicals, k =
2.0E13.exp(-190/T) cm3/mole.s. Theoretically, we have carried out ab initio
molecular orbital calculations using a modified Gaussian-2 and other advanced levels of theory to
study the kinetics and mechanisms of C6H5O + NO =
C6H5ONO, O + C6H5O =
O2 + C6H5, CH3 +
C5H5 = fulvene + 2H and the fulvene-to-benzene isomerization
reaction. The results of these studies are relevant to the formation of the first aromatic ring in
combustion and also cast doubt on the existing data for D(C5H5
- H) and the activation energy for the fulvene isomerization reaction.
Florida State University
Tallahassee, FL 32306
Supercomputer Computations Research Institute
Ab Initio Electronic Structure Studies of Pyrolytic Reactions of Polycyclic Aromatic
Hydrocarbons and their Derivatives.
| Investigator(s) |
Cioslowski, J.; Moncrieff, D.
|
$125,000 |
| Phone | 904-644-4885 |
| E-mail |
jerzy@scri.fsu.edu |
Substituted phenyl radicals produced from (poly)chlorobenzenes by abstraction of hydrogen or
chlorine are the subject of our continuing theoretical research on the pyrolysis of simple polycyclic
aromatic hydrocarbons (PAHs) and their derivatives. These radicals recombine to form
polychlorinated biphenyls (PCBs) that, because of their carcinogenecity and persistence in soil, are
one of the most dangerous industrial pollutants known. Electronic structures, energies and
geometries of all the possible isomers of the (poly)chlorophenyl radicals are being calculated at
the BLYP/6-311G** level of theory. Systematization of the computed data, which
involves the development of simple additive rules for the prediction of the energetically preferable
sites of the C-H and C-Cl bond dissociations in chloro-derivatives of benzene, is currently under
way. In a separate but related research effort, studies of the thermal rearrangements of PAHs are
about to be initiated. Such rearrangements, which occur in the course of combustion processes at
higher (ca. 1500°C) temperatures, are believed to provide an important pathway to the
formation of carcinogenic PAHs from biologically less active precursors. Thermal rearrangements
of several model compounds will be studied in detail and their mechanisms will be carefully
examined.
University of Georgia
Athens, GA 30602
Department of Chemistry
Spectroscopy at Metal Cluster Surfaces
Microscopic metal clusters composed of a variety of pure component systems and metal mixtures
are produced and studied in a molecular beam environment. These same methods are used to
produce metal complexes, which have small molecules "physisorbed" on the surface
of the metal cluster particles. Electronic spectroscopy is applied to these clusters to investigate the
fundamentals of metal-metal and metal-adsorbate bonding. These studies produce vibrational
frequencies, bond distances, and bond energies for metal clusters and their complexes. Recently
studied systems include Cu-Li, Ag-Li, Ag-Ar, Ag-Kr, Ag-Xe, Cu-Ar, Cu-Kr, Cu-Xe and
In-N2. Studies of larger clusters focus on the metal-carbon systems known as
"met-cars" clusters. In these systems, the M8C12
stoichiometry is formed preferentially, and a cage-like structure has been proposed to explain this
preference. We are measuring dissociation products and ionization potentials for different
M8C12 metal analogues (Ti, Fe, V, Nb). Additional studies have
identified preferential formation of M14C13 clusters believed to
have face-centered cubic crystallite structures. Recent progress in this area includes the first
measurements of ionization potentials for M/C clusters (e.g.,
Ti8C12, IP=4.9 eV) and the preparation of new metal carbide
clusters (silver, antimony, bismuth). The measurements of the fundamental interactions exhibited
by clusters are used to evaluate their potential as models for bulk surface chemistry and catalysis.
University of Georgia
Athens, GA 30602
Center for Computational Quantum Chemistry
Theoretical Studies of Elementary Hydrocarbon Species and Their Reactions
High level quantum mechanical methods are now a significant source of specific predictions
concerning molecular systems that may be very important, but inaccessible to experiment. An
important example is the study of molecular species and chemical reactions of fundamental
importance in combustion processes. This research involves both the development of theoretical
methods and their application to hydrocarbon chemistry. Quantum mechanical electronic structure
methods intermediate between extensive configuration interaction and high level perturbation
theory are being developed. Chemical reactions being studied in molecular detail include the
dissociation of HNCO and ketene and the different aspects of the
C2H5 + O2 reaction. Other work in progress
includes studies of the elementary reaction of quartet methylidyne (CH) with methane, the highly
strained tetra-tert-butylethylene molecule, the ring opening of cyclopropylidene, a comparison of
the cyclic and chain forms of the HO2 dimer, and the secondary deuterium kinetic
isotope effects on the isomerization of the trimethylene diradical to cyclopropane.
Harvard University
Cambridge, MA 02138
Division of Applied Sciences
Fundamental Spectroscopic Studies of Carbenes and Hydrocarbon Radicals
This research provides definitive identification of reactive hydrocarbon radicals and carbenes and
fundamental spectroscopic data needed for diagnostic probing in combustion systems. The
reactive hydrocarbons are produced in a pulsed supersonic molecular beam and observed with a
newly constructed Fourier transform microwave (FTM) spectrometer. Radical chain reactions are
the dominant mechanism for the decomposition of acetylene and polyacetylenes at high
temperatures, but little is known about the thermodynamic and kinetic properties of
CnH radicals other than C2H. Our recent detection of the
C7H, C8H, C9H, and C11H
radicals establishes that highly unsaturated reactive chains containing up to 11 carbon atoms are
stable and allows refinements in the estimates of thermodynamic properties assumed in chemical
models. The cumulene carbenes H2C5 and
H2C6 (isomer of triacetylene, HC6H) in their
singlet electronic ground states, which we also detected by the same technique, probably play a
prominent role in the synthesis of unsaturated hydrocarbons. The number densities of the
CnH radicals, cumulene carbenes, and cyanopolyynes HC11N and
HC13N in our molecular jet are sufficiently high to detect the electronic spectra
by standard laser spectroscopic techniques. This combined program of microwave and laser
spectroscopy of reactive hydrocarbons in supersonic jets, should help guide four-wave mixing and
other laser diagnostic techniques now being developed in other laboratories to understand the
detailed processes that occur in combustion.
University of Illinois at Chicago
Chicago, IL 60680
Department of Chemical Engineering
Kinetics of Combustion-Related Processes at High Temperature
The purpose of this project is to determine rates and mechanisms for fuel hydrocarbon pyrolysis
and other reactions at high temperatures. The measurements are made in a shock tube (providing
arbitrary, precise, and externally set temperatures) with two very high resolution laser diagnostic
techniques: laser schlieren measurement of density gradients (net endothermic rate) and the new
method of excimer laser flash absorption, which provides absorption profiles in the UV with 0.05
microsecond resolution. Previous work included a study of the dissociation of vinylacetylene,
which led to the proposal of a new mechanism for acetylene polymerization. Also, studies of
large-molecule dissociation at extreme temperatures, such as the retro-Diels Alder dissociation of
cyclohexene, tetrahydropyridine, and norbornene, have provided the first observations of
unimolecular falloff in such dissociations. The norbornene study also offered the first
measurements of incubation times in a large-molecule dissociation. Additional work involves
measurements of vibrational relaxation in large molecules; dissociation, isomerization, and
aromatic formation in allene/propyne; and dissociation rates in several halocarbons. A theoretical
analysis of large anharmonic effects (restricted internal rotations) on the rate of dissociation of
unsaturated hydrides has been developed and applied to HCN,
C2H2, and now to C3H4. Recent
work includes a study of the pyrolysis of pyrazine and pyrimidine, as well as an investigation of
decomposition, isomerization, and incubation in the oxirane/acetaldehyde system.
University of Illinois at Urbana-Champaign
Urbana, IL 61801
Department of Mechanical and Industrial Engineering
Investigation of Saturated Degenerate Four-Wave Mixing Spectroscopy for Quantitative
Concentration Measurements
A combined experimental and theoretical approach is used for the development and evaluation of
strategies for quantitative degenerate four-wave mixing (DFWM) concentration measurements in
flames. The DFWM process is investigated theoretically by solving the time-dependent density
matrix questions by direct numerical integration. During the past year our studies on the effect of
Doppler broadening on concentration measurements was concluded. A corrected Abrams-Lind
reflectivity expression was developed that will be tested experimentally in our laboratory in the
coming year. The use of short-pulse lasers for quantitative DFWM concentration measurements
was investigated theoretically; the use of picosecond lasers for DFWM in atmospheric pressure
flames has been suggested as a means of overcoming complications due to collisions. Our studies
indicate that even when the laser pulse length is much shorter than the characteristic collision
time, the DFWM signal becomes independent of collisions only for resonance lines that are
predominantly Doppler-broadened. We have begun a theoretical investigation of the effects of
level degeneracies (Zeeman states) on DFWM signal generation in the saturation regime.
Collisional effects and different laser polarization schemes will be investigated using our
degenerate-level DFWM code. We have also ported our DFWM code to a massively parallel
computer and will investigate the effects of multi-frequency-mode laser radiation in the coming
year. Experimentally, we have constructed a low-pressure flame facility and have begun to
perform OH DFWM measurements. The results of OH DFWM measurements over a wide range
of flame pressures and stoichiometries will be compared with our theoretical calculations of
DFWM signal levels, saturation intensities, and detection limits.
Johns Hopkins University
Baltimore, MD 21218
Department of Chemistry
Investigations of Reactions and Spectroscopy of Radical Species Relevant to Combustion
Reactions and Diagnostics
Chemical reactions that do not conserve total electron spin, spin-forbidden reactions, play an
essential role in combustion chemistry. Several spin-forbidden reactions are currently under
investigation. The mechanism of the electronic quenching reaction CH(a4
-) + CO CH(X2
) + CO has been analyzed. This reaction proceeds
through an intermediate complex model. The mechanism of the reaction N(4S) +
CH3(2A') H2 + HCN is currently under consideration. Ultimately the
electronic structure data necessary for the theoretical prediction of the rate constants of these
reactions will be determined. Recently it has been shown that the Berry phase effect, induced by
conical intersections, can have important implications for molecular reaction dynamics. The
reaction OH + H2 H2 + H2O is a key combustion reaction. The
potential impact of the Berry phase effect on this adiabatic ground state reaction is being
considered.
Massachusetts Institute of Technology
Cambridge, MA 02139
Department of Chemical Engineering
Aromatics Oxidation and Soot Formation in Flames
The oxidation of aromatics and the formation of soot in flames are being studied with emphasis on
experimental identification of important molecular species, including fullerenes, characterization
of soot structure, and measurement of concentration profiles of molecular species and soot
through the reaction and post flame zones of low-pressure one-dimensional flames. The species
identifications, soot structures characteristics, and net reaction rates calculated from the
concentration profiles are used to test and to refine hypothesized reaction mechanisms. Proposed
mechanisms of benzene oxidation are being tested, and refined as appropriate, using measured
concentration profiles of radical and stable species present during benzene oxidation in flames.
The research on soot formation is concerned with the particle inception or nucleation stage and
the study of soot structure at all stages of growth in order to obtain mechanistic information from
evidence of growth steps recorded in the structure of particles. The ultimate objective is to
understand how nascent soot particles are formed from high molecular weight compounds,
including the roles of planar and curved PAH and the relationship between soot and fullerenes.
The objective of the research on fullerenes is to identify the range of fullerenes formed in flames,
the nature of the precursor species, and the mechanisms and kinetics of the formation reactions.
Massachusetts Institute of Technology
Cambridge, MA 02139
Department of Chemistry
Spectroscopic and Dynamical Studies of Highly Energized Small Polyatomic Molecules
| Investigator(s) |
Field, R.W.; Silbey, R.J.
|
$150,000 |
| Phone | 617-253-1489 |
| E-mail |
rwfield@mit.edu |
Studies of intramolecular vibrational redistribution (IVR) and unimolecular isomerization have
focused on the acetylene molecule (C2H2) and have utilized the
spectroscopic techniques of dispersed fluorescence (DF), stimulated emission pumping (SEP), and
infrared-ultraviolet (IR-UV) double resonance DF and SEP. Sensitive and selective
absorption-based spectroscopic techniques, suited for both detection and characterization, have
also been under development. These techniques are based on the combination of magnetic
rotation spectroscopy (MRS), which provides selectivity to the lowest rotational levels of free
radicals, and frequency modulation (FM) spectroscopy. In a collaboration with T. Sears and G.
Hall at Brookhaven National Laboratory, the velocity and internal state distributions of
photolytically generated free radicals (CN) were sensitively monitored by transient FM
spectroscopy. In a collaboration with E. Eyler at the University of Delaware, the sideband phase
and amplitude stability, on which the sensitivity of FM spectroscopy depends, was shown to be
preserved when an FM'ed cw dye laser was pulse amplified and frequency tripled. The study of
IVR in acetylene is based on a superpolyad model in which the initial stages of IVR are described
by a few spectroscopically determined resonance parameters. The superpolyad model describes
the frequency and intensity patterns in the spectrum and the rates and pathways for energy flow in
a computationally simple form, which is explicitly scalable in Evibration. The
model also guides selection of initial states, accessible via IR-UV-SEP, that are optimally coupled
to dynamical features such as the acetylene-vinylidene isomerization coordinate. Despite severe
overlap of polyad features in the DF spectrum of acetylene, it has proven possible to
"unzip" the spectrum into separate polyads. This provides a quick survey of the
unimolecular dynamics at energies where breakdown of the predicted polyad patterns would
signal the onset of isomerization.
University of Massachusetts at Amherst
Amherst, MA 01003
Department of Chemical Engineering
Probing Flame Chemistry with MBMS, Theory, and Modeling
Elementary reactions in combustion are studied using molecular-beam mass spectrometry
(MBMS) of free-radical and stable species in flames, ab initio calculations of thermochemistry and
transition states, new kinetics from reaction theories, and tests of mechanisms using whole-flame
modeling. The present focus is on oxidation and molecular-weight growth chemistry in forming
aromatics. Complete MBMS data sets have been mapped for the first time in high-temperature
ethene flat flames, 
=0.75 and =1.90. We obtain new measurements of the
kinetics for H and OH + C2H4,
C2H3 decomposition, and
C2H3+O2. With the BAC-MP4 method, we have
calculated thermochemistry and rate constants for these reactions and those of
CH3CHO, CH3CO, CH2CHO,
C3H5 isomers, and C3HxO
formation and destruction.
University of Massachusetts at Amherst
Amherst, MA 01003
Department of Chemistry
Theory of the Dissociation Dynamics of Small Molecules on Metal Surfaces: Finite
Temperature Studies
Realistic theoretical models are used to examine the dynamics of some molecule-surface reactions
important in catalysis. Time-dependent techniques are used to treat the necessary degrees of
freedom quantum mechanically. The dissociative adsorption of molecular hydrogen and its
isotopes on metals has been studied in detail. The dissociation of methane is currently under
study. New methods are being devised to evolve the wave function and to include the effects of
lattice vibrations, in an attempt to resolve some long-standing disputes. Dissociation probabilities
are being computed as a function of beam energy, molecular state, and surface temperature.
Studies have been made of Eley-Rideal processes in which a gas phase H or D atom reacts with
an adsorbed H, D, or Cl atom. Experimentally observed activation energies have been explained in
terms of enhanced reactivity due to adsorbate vibrational excitation. Fully quantum
three-dimensional calculations have been implemented, allowing for the determination of reaction
cross sections and product translational and ro-vibrational state distributions. Detailed studies of
the reaction dynamics have been made for several potential energy surfaces and isotopic
combinations, and excellent agreement has been found with experiment. The dynamics have been
elucidated in terms of a competition between reaction and trapping. More detailed studies are
examining the effects of impact angle and surface corrugation.
University of Michigan
Ann Arbor, MI 48109
Department of Atmospheric, Oceanic, and Space Sciences
Energy-Transfer Properties and Mechanisms
This project studies the mechanisms and properties of energy transfer involving moderate-sized
molecules. A fuller understanding of highly excited molecules is obtained by a combination of
experiments and modeling. The aim is to develop a theoretical model for prediction of energy
transfer properties. In the experiments, the excited molecules are monitored with various
techniques, including time- and wavelength-resolved IR emission and resonance-enhanced
multiphoton ionization. The disposal of energy in translational, rotational, and vibrational degrees
of freedom is monitored as highly excited molecules are deactivated. In the modeling effort,
collisional/reaction master equation formulations are developed and used to investigate how
molecular energy transfer affects chemical reaction systems in combustion and in other systems
that experience temperature and pressure extremes.
University of Minnesota
Minneapolis, MN 55455
Department of Chemistry
Variational Transition State Theory
This project involves the development and application of variational transition state theory
(VTST) and semiclassical transmission coefficients for gas-phase reactions. The work involves
new theoretical formulations and the development of practical techniques for applying the theory
to various classes of transition states and for interfacing reaction-path dynamics calculations with
electronic structure theory and applications to specific reactions. We have developed new general
strategies for obtaining potential energy surface information for reaction-path dynamics
calculations, and we are developing practical methods that eliminate the need for fitting electronic
structure data to analytic forms. The new methods are called direct dynamics. We have developed
one approach to direct dynamics that we call interpolated variational transition state theory
(IVTST). In this approach, we base the dynamics calculations on electronic structure data,
including energies, gradients, and hessians, at the reactants, products, saddle point, and zero, one,
or two additional points near the saddle point. Our newest direct dynamics scheme is called VTST
with interpolated corrections (VTST-IC) or dual-level direct dynamics or triple-slash (///)
dynamics. This approach involves using straight direct dynamics with NDDO-SRP
parameterization or a low-level ab inito calculation as a first step; in the second step this is
augmented by high-level data at selected geometries along the reaction path. Applications are
being made to combustion processes, including the reaction of OH with propane. In addition the
new techniques are being incorporated in our portable POLYRATE program.
National Institute of Standards and Technology, Gaithersburg
Gaithersburg, MD 20899
Chemical Science and Technology Laboratory
Optically Driven Surface Reactions: State-Resolved Probes of Surface Dynamics
Lasers and state-resolved diagnostics are used to initiate and follow chemical processes on solid
surfaces. Optical excitations allow the study both of thermal and non-equilibrium chemistries that
might arise naturally during catalytic reaction and materials processing. Laser wavelengths ranging
from the IR through the UV are available to initiate chemical transformations by creating thermal,
adsorbate-localized, or substrate-mediated excitations. Quantum state resolved diagnostics of the
reaction products allow for a better understanding of the detailed reaction mechanism(s) that
follow and the dependence of reaction pathway(s) on the excitation mechanism. Previous work in
this laboratory clearly demonstrated the first evidence for the importance of hot-carrier-driven
chemistry on a metal surface [NO/Pt(III)], surface state-driven chemistry on a semiconductor
[NO/Si(III) 7x7], and substrate and adsorbate quenching effects in adsorbate photolysis
[Mo(CO)6/Si(III)]. Current work is directed at understanding the dynamics of
photostimulated oxidation of carbon surfaces and testing theoretical models of molecular
desorption induced by femtosecond laser pulses.
National Institute of Standards and Technology, Gaithersburg
Gaithersburg, MD 20899
Chemical Science and Technology Laboratory
Kinetics Database for Combustion Modeling
The computer simulation of combustion processes provides a means of making optimum use of
increasing capabilities in computational power and fundamental understanding to supplement
physical testing. It can therefore have important influences on design and optimization of
combustion devices for energy efficiency and pollution minimization. The key ingredient for
taking advantage of this technology is the availability of a reliable database of chemical kinetic and
thermodynamic information bearing on all the species, stable and unstable, that are formed in the
course of the reaction. This project seeks to fulfill this need through the development of a
database of evaluated and estimated chemical kinetic and thermodynamic information. The
procedure has been to start with the simplest of hydrocarbons, methane, and then add increasingly
complex and more realistic fuel molecules containing the functional groups that are the key to
their reactivity and the basis for estimation. Thus smaller saturated alkenes, alkynes, and aromatics
have now been examined. An important aspect of this work is the examination of techniques for
estimation. Much progress has now been made in developing an understanding of the limitations
of the general procedures for extrapolation of data to the high temperatures required for
combustion applications.
National Institute of Standards and Technology, Gaithersburg
Gaithersburg, MD 20899
Physics Laboratory
Spectroscopic Investigation of the Vibrational Quasi-Continuum Arising from Internal
Rotation of a Methyl Group
This project studies the vibrational quasi-continuum in acetaldehyde, methanol, and related
molecules because internal rotation is important in promoting intramolecular vibrational
redistribution (IVR). It aims to understand: (1) torsional motion below and above the barrier, (2)
traditional vibrational states, and (3) interactions involving levels with excitation of both kinds of
motion. Although a few unresolved problems remain, work on the purely torsional problem is
gradually winding down, while work on IVR-enhancing interactions is being intensified. A
vibration-torsion-rotation formalism for treating these interactions quantum-mechanically has been
developed, in which internal rotation is assumed to be slow compared to the other vibrations, so
that Born-Oppenheimer considerations now lead to a separation of rotation and internal rotation
from the other vibrations, rather than a separation of rotation from internal rotation and the other
vibrations. A verdict on the usefulness of this new formalism, which gives rise to some unusual
group theoretical features apparently related to Berry's phase, should emerge during applications
this coming year. A new computer program is being applied to torsionally mediated vibrational
interactions in acetaldehyde (skeletal bending region) and methanol (C-H stretching region) to
determine if a two-state Fermi-resonance model can explain (qualitatively or quantitatively) the
"large and inverted" torsional splittings which are observed.
University of New Orleans
New Orleans, LA 70148
Department of Chemistry
Identification and Temporal Behavior of Radical Intermediates Formed during the
Combustion and Pyrolysis of Gaseous Fuel
| Investigator(s) |
Kern, R.D., Jr.
|
$90,000 |
| Phone | 504-286-6847 |
| E-mail |
RDKCM@UNO.edu |
The pyrolyses of nitrogen containing aromatic ring compounds are of interest in combustion
science particularly with regard to coal and its products. A complementary shock tube study of
the decomposition of the simplest of these ring compounds, pyrazine, using laser schlieren
densitometry (John Kiefer,UIC) and our time-of-flight mass spectrometric analysis has revealed
nonlinear growth of a key product, cyanoacetylene. The temperature dependent maxima and
subsequent decay rates of cyanoacetylene and its sensitivity to the initial concentration of
H2 played an important role in formulating a new chain mechanism for pyrazine
decomposition. The laser schlieren experiments provided excellent rate data for the initiation
reaction, C-H fission, as well as the early rate of chain acceleration over the temperature range
1460-2300K. A value of 96.6 kcal/mol was established for the initiation step. The major products
are HCN and C2H2; the minor products are
C4H2 and cyanoacetylene. The existence of a chain reaction with
CN radical as the major chain carrier was confirmed. A value of 96 kcal/mol for the heat of
formation for cyanoacetylene was selected to match the late time, high temperature profiles as
observed by time-of-flight mass spectrometry.
New York University
New York, NY 10003
Department of Chemistry
Accurate Polyatomic Quantum Dynamics Studies of Combustion Reactions
New computational methods are being developed for the time-dependent approach to polyatomic
combustion reactions. The new development is aimed at tackling more complex reaction systems
that require a large number of basis functions in quantum dynamical approach. The proposed
method (normalized angular quadrature scheme) enables one to obtain stable and accurate results
without the need for large-matrix storage and large-matrix multiplications. As a result, the
benchmark H2 + OH and its isotopic reaction HD + OH can now be calculated
efficiently on a medium-sized workstation. Application of this new method to other combustion
reactions is in progress.
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599
Department of Chemistry
Unimolecular Reaction Dynamics of Well Characterized Ionic Reactions
| Investigator(s) |
Baer, T.
|
$103,225 |
| Phone | 919-962-1580 |
| E-mail |
BAER@UNC.EDU |
The dissociation dynamics of energy selected ions are investigated by photoelectron photoion
coincidence (PEPICO). Molecules are prepared in a molecular beam so that their internal as well
as translational temperature is near 0 K. The primary experimental information includes ionization
and fragment appearance energies, and ion time-of-flight (TOF) distributions. The latter permit
the measurement of dissociation rates and product energy distributions. One of the broad goals of
this research project is the development of simple methods for modeling unimolecular reactions
with ab initio molecular orbital calculations and the RRKM statistical theory. Progress in recent
years has shown that the dissociation rate constants of simple reactions with well established
dissociation energies can be successfully calculated. However, many unimolecular reactions
involved in combustion processes are complex in that the molecules isomerize to lower energy
isomers prior to dissociation. We have investigated a number of similarly complex ionic reactions
and have developed the experimental and mathematical tools to extract valuable information
about such processes. In the case of the pentene ion and tri-methyl-borate ion dissociation, we
have succeeded in modeling these reactions with three wells and have managed to extract all six
rate constants (4 isomerization and 2 dissociation) from the data. The results have been analyzed
by a combination of ab initio molecular orbital, and RRKM statistical theory calculations. These
experiments point the way toward analyzing complex reactions in combustion systems, where in
general much less information is available. New experiments are planned at the Chemical
Dynamics Beam Line of the Advance Light Source. The greater photon flux should make possible
PEPICO experiments on dimer ions which dissociate to interesting free radicals. These studies
should provide heats of formation of free radicals important in combustion processes.
Alternatively, they yield ground state energies of neutral and ionic dimers.
North Dakota State University
Fargo, ND 58105
Department of Chemistry
Infrared Laser Studies of the Combustion Chemistry of Nitrogen
The project under investigation is part of a broad effort to understand at a molecular level the
detailed kinetics of combustion processes. The studies in our laboratory concentrate on
radical-radical and radical-molecule chemical reactions which either form or destroy
NOx. Many of these reactions are important in NOx control
strategies such as thermal de-NOx and NO-reburning. The emphasis is primarily
on the quantitative determination of product branching ratios for reactions that have several
possible channels. We used UV laser photolysis to initiate these reactions, and infrared diode laser
spectroscopy to probe reaction products. Some of the reactions recently studied and/or currently
under study include NH + NO, NH + NO2, NH2 +
NO2, and CH2 + NO.
University of Oregon
Eugene, OR 97403
Department of Chemistry
Dynamical Analysis of Highly Excited Molecular Spectra
A framework for analysis of highly excited vibrational states of polyatomic molecules is
investigated using techniques of nonlinear dynamics, especially bifurcation theory. The goal is
classification of the motions of molecules excited to the regime of chaotic dynamics, and
determination of patterns resulting from these motions in experimental spectra. Several research
areas are being investigated with application to species and methods of interest in combustion
processes. The first is a method based on diabatic correlation diagrams applied to polyatomic
spectra with chaotic dynamics, specifically, triatomics such as H2O, and larger
molecules such as acetylene. For the triatomics, the critical points of an effective Hamiltonian
used for fitting spectra are analyzed, giving the large-scale bifurcation structure of the molecular
phase space. An assignment procedure using the bifurcation analysis, in tandem with the
correlation diagram procedure, is being investigated. A more phenomenological extension to
acetylene is being investigated, where the bifurcation structure is harder to obtain, and at present,
unknown. The final research area, with application to experimental spectra of
CS2, is bifurcation and semiclassical quantum analysis of two degree-of-freedom
systems with such strong coupling that earlier methods of analysis of chaotic systems are
inapplicable.
Pennsylvania State University, University Park
University Park, PA 16802
Department of Chemistry
Metal Cluster Alloys and Oxides Elucidating Structural and Electronic Effects in Governing
the Reactivity and Catalytic Role of Matter in Finite Dimensions
| Investigator(s) |
Castleman, A.W., Jr.
|
$104,000 |
| Phone | 814-865-7242 |
| E-mail |
awc@psuvm.psu.edu |
Catalytic processes pervade our society, being involved in the efficient utilization of the energy
supply of the nation, providing new ways of producing fuels, effecting the control of pollutant
emissions, and improving the efficiency of many industrial processes. The enormous benefits
which would arise from having the ability to design catalysts with a high degree of selectivity, and
ones that could yield the facile formation of a desired reaction product, have been long realized.
The present program is addressed to investigations that are intended to provide new insights into
the physical basis for catalysis. There is growing awareness that systems comprised of oxides --
both in terms of their own innate catalytic activity, as well as their use as supports for other metal
catalysts -- and in carbides which often display considerable selectivity as catalysts, represent
promising materials for further development. An understanding of their general role as catalysts is
still in a stage of infancy, but it is recognized that their oxidation state, stoichiometry and charge
are intimately associated with their effectiveness as catalysts. Cluster research provides one
valuable approach and is the basis for the research being conducted herein. The research program
encompasses four general lines of inquiry: (i) investigation of the reactivity of metal-carbon and
metal-oxide clusters to determine the influence of oxidation- and charge-state on their reactivity,
(ii) determination of the kinetics of absorption and the reaction kinetics of selected molecules that
are catalyzed by these metal compound clusters, (iii) thermochemical studies of adsorption onto
metal-carbide and metal-oxide cluster systems, and (iv) studies of catalytic reactions which have
counterparts to ion-molecule reactions. Investigations are directed to certain well-defined classes
of oxides and metal-carbon compounds in terms of the reactions of species formed in combustion
processes whose abatement will contribute to the enhanced quality of the environment, and to a
study of the build-up of potential higher hydrocarbon fuels through oxidative conversion reactions
of small organic molecules. During the last year we gained a great deal of new insight into certain
reactive processes involving carbides and oxides of various stoichiometries. Considerable
attention has been focused on the reactivity of Met-Cars (M8C12,
where M = early transition metals) which are unique cluster materials discovered several years
ago in our laboratory. Particularly noteworthy is work reported dealing with gas-phase reactions
of Met-Cars with acetone and methyl iodide and also related studies of these complexes which
provide new insight into the structure of Met-Cars and into adsorption reactions. It is found that
the formed complexes bear analogies to the nature of bonding involved in the attachment of
-bonded molecules to metal surfaces. In
another interesting development in the Met-Car field, we discovered that selected Met-Car cations
can be produced through the oxidation of neutral metal-carbon clusters. In other studies of the
catalytic properties of niobium oxide clusters, we completed some recent investigations of the
oxidation of alkanes and reactions of ketones (acetone) with these species which are common
catalysts in related surface reactions. The findings have shown specific cluster size dependencies
for various oxygen atom transfer reactions, providing new insights into these important classes of
reactions.
University of Pennsylvania
Philadelphia, PA 19104
Department of Chemistry
Collisional Energy Transfer of Highly Vibrationally Excited Molecules
Using a recently developed Time-Resolved Fourier Transform Emission Spectroscopy technique
in the IR region we have studied collisional deactivation of highly vibrationally excited molecules.
Collisional energy transfer is crucial for activating and/or deactivating thermal and photo-induced
unimolecular and bimolecular reactions of molecules that are important to energy production and
environment concerns. Collisional energy transfer of NO2 (excited to as high as
22,000 cm-1), SO2 (32,000 cm-1),
CS2 (32,000 cm-1), and pyrazine (40,000 cm-1)
to a variety of collisional partners has been characterized. Among the many interesting
observations are the ones that point to a surprising revelation that the mechanism responsible for
energy transfer of highly excited molecules, in contrast to those excited at lower energies, is
dominated by long range, Coulombic interactions. These observations are: 1) The
transition-dipole coupling model quantitatively describes the V-V transfer probability. 2) Even
though the V-T transfer probabilities for molecules excited at lower energies appear to be
dominated by kinematic effects associated with hard sphere collisions, for molecules excited at
much higher energies it seems that collisional partners with higher polarizability are more
effective. 3) For excited molecules such as NO2 and CS2 with
strong vibronic coupling at higher energies, the amount of energy transferred appears to increase
dramatically with molecular excitation passing a threshold. The threshold energy for a specific
excited molecule is always at the same value for all collision partners and correlates well with the
beginning of vibronic coupling within the excited molecule, indicating again the effect of transition
dipole coupling. These observations have pointed to a most effective mechanism for energy
transfer out of highly excited molecules: the large amount of energy transferred, wither through
V-V or V-T/R channels, to an ambient collider is through long range, Coulombic interactions!
University of Pennsylvania
Philadelphia, PA 19104
Department of Chemistry
Intermolecular Interactions of Hydroxyl Radicals on Reactive Potential Energy Surfaces
This program is focused on the characterization of the interaction potentials between the hydroxyl
radical in its ground X 2
and excited A 2+ electronic states with collision partners of relevance to
combustion. Recently, binary complexes of OH and H2/D2 have
been stabilized in the attractive well region of the entrance channel to the OH +
H2
H2O + H reaction and probed via electronic spectroscopy. These studies are
providing a detailed picture of the OH (A 2+, X 2) + H2/D2
potential energy surfaces and the reaction dynamics taking place on these surfaces. Current work
is aimed at understanding the OH (A 2+) + H2/D2 and OH (A
2+) + N2 systems and the curve crossings
and/or reactive channels that lead to collision-induced quenching of OH (A
2+).
University of Pittsburgh
Pittsburgh, PA 15260
Department of Chemistry
Optical Imaging in Microstructures
Our research is focused on developing morphology-dependent stimulated Raman scattering
(MDSRS) into an analytic optical imaging tool that can be used to monitor spatial variations of
chemical composition and molecular structure in axisymmetric microstructures. MDSRS relies on
using the cavity modes associated with microstructures to enhance optical signal generation. Since
different cavity modes occupy different regions in space, location-specific spectra can be
generated, i.e. the effect is analogous to that seen in magnetic resonance imaging. In the past year
we have demonstrated that MDSRS imaging is feasible. We did this by imaging water structure in
the diffuse part of a charged water droplet's electric double layer.
Princeton University
Princeton, NJ 08544
Department of Chemistry
Analysis of Forward and Inverse Problems in Chemical Dynamics and Spectroscopy
This research is concerned with fundamental issues in molecular science and practical solutions in
chemical kinetics engineering problems. The first part of the research aims to develop and apply
new algorithms to optimally draw on ab initio quantum chemistry calculations and available
laboratory data in order to gain a quantitative understanding of the effect of molecular interactions
on spectroscopic, dynamical, and kinetic events. Specifically, mathematical and numerical tools
based on functional sensitivity analysis, inverse problem theory, and reproducing kernel theory
have been developed for identifying the relationship between potential energy surfaces and
observables (forward analysis), and for constructing physically acceptable potential energy
surfaces from either laboratory data or ab initio calculations. The second part of the research aims
to develop effective dimension reduction methods for constructing simplified, yet accurate,
combustion models. Here a recently formulated algebraic-based method, along with nonlinear
perturbation theory, has been employed to develop rigorous and efficient nonlinear lumping
schemes for combustion model simplification.
Princeton University
Princeton, NJ 08544
Department of Mechanical and Aerospace Engineering
Comprehensive Mechanisms for Combustion Chemistry: An Experimental and Numerical
Study with Emphasis on Applied Sensitivity Analysis
This program addresses improving understanding of combustion chemistry through experimental
flow reactor studies in the temperature range 550-1200 K, the pressure range 0.3-20 atmospheres,
and with characteristic reaction times from 0.02-3.0 seconds. Through the use of techniques based
on elemental gradient-feature sensitivity and path analyses, computations are performed to obtain
elementary rate information and to develop and study comprehensive chemical kinetic
mechanisms. Elementary kinetic data are obtained from perturbation studies of the
CO/H2/ oxidant reaction system by small amounts of hydrocarbons and/or
hydrocarbon oxygenates. Of special interest here are the reactions of HO2 with
CH3 and other species. Reaction systems of interest include those for pyrolysis
and oxidation of simple oxygenates (especially formaldehyde,acetaldehyde, and dimethyl ether),
simple olefins (especially ethene and propene), and ethane. The research emphasizes the extension
of the present knowledge, based on reaction mechanisms of these small molecules, to pressures
and temperatures where the reaction of radicals with oxygen and the reactions involving
RO2 and HO2 are important.
Princeton University
Princeton, NJ 08544
Department of Mechanical and Aerospace Engineering
Aromatic-Radical Oxidation Kinetics
Elucidation of the details of the oxidation of the aromatic ring of monocyclic aromatic species,
key elements in automotive and industrial fuels, has been the main focus of this research effort.
Special attention has been focused on the oxidation of C5 species, in particular
cyclopentadiene (CPD), that early work in this laboratory has shown to play an important role in
the sequence of steps that bridge the opening of the aromatic ring and the eventual production of
carbon dioxide and water. Other species, phenol and anisole, shed light on the chemical
mechanism of aromatics oxidation and have also been examined. A brief summary of recent
results is now given. After extensive characterization of the Princeton Flow Reactor in order to
determine the dependence of previously reported anomalous CO2 on
temperature, equivalence ratio and concentration of reactants, a set of operating conditions was
determined that maximizes homogeneous gas phase chemistry and minimizes catalytic surface
effects that have plagued previous experiments. At these operating conditions data were obtained
at lean, stoichiometric and rich conditions that allow comparison with detailed chemical kinetic
model predictions. Preliminary results of the modeling show good agreement with respect to the
product profiles, through not the rates which are somewhat slow. Work on improving the model
is in progress. A complementary study of anisole is being conducted as a means of more
thoroughly examining the oxidation chemistry of the cyclopentapentadienyl radical in the context
of a different radical environment. The decomposition of anisole to the cyclopentadienyl radical is
accompanied by the production of the methyl radical which is accomplished via cleavage of the
methylphenoxy bond followed by a ring contraction to yield CO and cyclopentadienyl.This
chemistry has been examined in the Princeton Flow Reactor at a variety of temperatures and
stoichiometries that provide reliable data for comparison with models of the pyrolysis and
oxidation of anisole. The development of these detailed chemical kinetic models is in progress.
Cyclopentadienyl reactions are also being probed though oxidation studies of phenol. Loss of
hydrogen from phenol to produce phenoxy is followed by expulsion of CO and ring contraction to
form cyclopentadienyl radical. The chemistry of the cyclopentadienyl radical is being inferred from
the stable species profiles obtained at a variety of stoichiometries during the oxidation of phenol in
the flow reactor.
Purdue University
West Lafayette, IN 47907-1393
Department of Chemistry
Multiresonant Spectroscopy and the High-Resolution Threshold Photoionization of
Combustion Free Radicals
This project is aimed at characterizing the thermochemistry, spectroscopy and intramolecular
relaxation dynamics of an important set of combustion intermediates, which includes radical
alkoxides and hydrides of the first-row non-metals. We will prepare these radical systems by the
photolysis of established precursors in the expansion region of pulsed free-jets in our differentially
pumped laser-ionization source quadrupole mass spectrometer and similarly configured zero
electron-kinetic-energy (ZEKE) threshold photoionization spectrometer. Using multiresonant
methods which have been well established in our laboratory, we will select individual rotational
states and overcome Franck-Condon barriers associated with neutral-to-cation geometry changes
to promote transitions to individual autoionizing series and state-resolved ionization thresholds.
Systematic characterization of rotational structure and associated lineshapes will provide
experimental data on autoionization dynamics as input for theoretical modeling. Extrapolation of
series combined with direct threshold photoelectron detection will yield precise ionization
potentials that will constitute an important contribution to the thermochemical base of information
of these critical combustion intermediates.
Purdue University
West Lafayette, IN 47907-1393
Department of Chemistry
Probing Activated Chemical Reactions in Diacetylene and Vinylacetylene
The spectroscopy and chemistry of the metastable state(s) of diacetylene
(C4H2) and vinylacetylene
(C4H4) are the focus of this research project. Cavity ring-down
spectroscopy is being used to study the very weak absorption spectrum of the spin-forbidden
3u1g+ transition in
C4H2. The upper state of this transition is thought to be
responsible for much of the photochemistry of C4H2in the
ultraviolet. To study the chemistry of these metastable states, a two laser pump-probe scheme is
employed in which metastable diacetylene (C4H2*) is formed
following ultraviolet excitation by intersystem crossing from the singlet manifold. Primary reactive
encounters with various small hydrocarbons are probed by exciting
C4H2 while the gas mixture is in a short reaction tube attached to
a pulsed valve. Reaction occurs during the traversal of the reaction mixture down the tube and is
quenched as the mixture expands into vacuum. Products are identified using either single-photon
vacuum ultraviolet ionization or resonant multiphoton ionization time-of-flight mass
spectroscopy. Recent results indicate that the reaction of C4H2*
with 1,3-butadiene produces two aromatic products: benzene and an as yet unidentified product
of mass C8H6. These reactions may be important as alternative
routes to the first aromatics in soot formation.
Rice University
Houston, TX 77251
Department of Chemistry
Infrared Absorption Spectroscopy and Chemical Kinetics of Free Radicals
| Investigator(s) |
Curl, R.F.; Glass, G.P.
|
$95,000 |
| Phone | 713-527-4816 |
| E-mail |
rfcurl@rice.edu |
This research is directed at the detection, monitoring, and study of the chemical kinetic behavior
by infrared absorption spectroscopy of small free radical species thought to be important
intermediates in combustion. Recently, investigation of the chemical kinetics of the nitrogen
containing radical, HCCN, has been initiated. This species has a triplet ground state and a very
interesting balance in electronic structure between a diradical structure with one unpaired electron
on the terminal C and the other on the N and the triplet carbene structure with both unpaired
electrons on the terminal carbon. Thus it is likely to exhibit chemistry with attack at both the
terminal C atom and at the N atom. HCCN has been found to react rapidly with NO and
O2 and the rate constants at room temperature for both reactions have been
measured as 3.8(3)x10-11 cm3s-1 and
2.3(2)x10-12 cm3s-1 respectively. Reactivity with
ethylene, acetylene, methane, carbon dioxide, and hydrogen at room temperature was found to be
slower than the smallest rate we can measure which is approximately 10-13
cm3s-1. A search for reaction products is being carried out. Work
previously described on the rate of reaction between singlet methylene and acetylene has been
completed and recently published.
University of Rochester
Rochester, NY 14627
Department of Chemistry
Low-Energy Ion-Molecule Reactions and Chemiionization Kinetics
Crossed ion-beam--neutral beam reactive scattering experiments are being performed with the
goal of using product quantum state distributions, energy disposal measurements, and state-
resolved angular distributions to extract dynamical information on collision mechanics and
features of the potential surface mediating the reaction. Attention has been focused on hydrogen
atom transfer reactions of O- with D2 and CH4,
where either complete or partial vibrational state resolution of the products has been
accomplished. In the O- + D2 systems, product vibrational states
of OD- up to v' = 4 are resolved over the full collision energy range from 0.25 eV
to 1.2 eV. The extent of product vibrational excitation increases with increasing collision energy,
and vibrational surprisal plots are consistent with a negative vibrational temperature that
decreases in magnitude with decreasing collision energy. In conjunction with angular distributions
that become more symmetric with decreasing energy, the data are consistent with incipient
statistical behavior at the lowest energies. The O- + CH4 reaction
forming OH- shows that the product is sharply backward scattered with significant
vibrational excitation at 0.34 eV, with the products maintaining their vibrational excitation but
becoming forward scattered with increasing collision energy. A series of experiments on
O- with vibrationally excited molecules prepared by laser excitation is planned.
University of Southern California
Los Angeles, CA 90089-0482
Department of Chemistry
Reactions of Atoms and Radicals Using Pulsed Molecular Beams
Unimolecular and bimolecular reaction dynamics of atoms and small radicals are being studied
using pulsed molecular beams. Exothermic and endothermic reactions of atomic carbon in its
ground electronic state are examined using laser ablation of graphite. Endothermic hydrogen
transfer reactions of C(3P) induced by translational energy with
H2, HCl, HBr, H2S, CH3OH,
CH3OD, C2H2 and CH4 yield
CH radicals. Center-of-mass collision energies with all species except H2 peak at
~ 2 eV but can exceed 6 eV. The rotational distributions with all reactants are rather similar and
can be fit to temperatures in the range 1500-2200 K. In the reaction with methanol there are two
reaction sites, methyl and hydroxyl. In the reactions studied so far, the spin-orbit and the
Lambda-doublet ratios are statistical. Mechanisms are discussed using the detailed calculations
available for the C + H2 system, which indicate that an insertion route is open for
these reactions with no or only a small barrier. The reaction with chloroform is being studied as a
prototype of a system in which both exothermic and endothermic channels (yielding CCl and CH
products, respectively) are open at the employed collision energy. Only a very small CH signal is
detected, but large signals from CCl are readily observed. In preparation for the studies of free
radicals decomposition, an ion imaging detector has been introduced into a crossed molecular
beams TOF machine. With this technique the product state distributions of one product correlated
with a specific quantum state of a second product can be determined. Preliminary images of NO
from the photodecomposition of nitrogen dioxide and H and CO from the decomposition of
isocyanic acid were obtained.
University of Southern California
Los Angeles, CA 90089
Department of Chemistry
The Stabilization Theory of Dynamics
We have developed a new method of carrying out computations on complex systems as
HO2 and NO2. The former we have published on and the latter
we are working on. They are complex because once the first fifty or so vibrational states are
exceeded all eigenfunctions (both bound and resonance) become very topologically complex and
seemingly unassignable because of the same reasons that cause classical chaos in these energy and
configuration regions. For HO2 the chaos is caused by pure mode coupling. For
NO2 the same reason exists but in addition, chaos and complexity caused by the
surface crossing and the conical intersection occurs. The complexity translates into the need to
use for eigenfunction representation, very dense and large basis sets or grids. This in turns causes
the Hamiltonian matrix to be so large that it cannot be fit into core memory as required by
standard methods of diagonalization. We have developed a new iterative recursive polynomial
method to compute the action of the Green function on a vector which avoids storing the
Hamiltonian matrix in the core. This is what is needed to compute bound, resonant and scattering
information. The method is extremely powerful and can be used on present day work stations.
Armed with this formalism we developed a two part computational program and we worked on
the important to combustion, HO2 molecular species. We have computed its
bound vibrational states, many of its resonant states (with lifetimes), its' density of states and its'
total photoadsorption cross section and its' recombination rate. These calculation used our Green
function computational scheme in a mode that generates a greatly reduced basis which then uses
conventional diagonalization to finish the problem. This not only yielded bound and resonant
states but gave a spectral representation that enable us to compute the other mentioned
properties. Additionally, we have made a breakthrough in the interpretation of complex spectra.
By applying a scaling technique we are able to construct the bifurcation diagram directly from
quantum spectra and to compare the results with classical calculations. The method allows the
identification of important motions. More diverse systems of greater complexity and
dimensionality will be investigated in the future.
University of Southern California
Los Angeles, CA 90089
Department of Chemistry
Reactions of Small Molecular Systems
The focus of this work is complex reaction mechanisms of prototypical molecules and radicals
that display phenomena important in combustion chemistry. In the case of NO3, a
detailed understanding is sought for the two unimolecular reaction pathways: NO +
O2 and O + NO2, both separately and in competition with one
another. This is a case of nearly identical energy thresholds and qualitatively different transition
states (i.e., tight versus loose). For the NO + O2 channel, which has a tight
transition state, tunneling will also be examined. The main experimental tools are
double-resonance and time-resolved pump-probe methods, detecting NO3 and
NO via LIF. C2H2 experiments will examine the threshold
region, specifically, the roles of intersystem crossings and reactions via zeroth-order triplet versus
singlet surfaces. For the case of C2HD, the H/D ratio provides a good diagnostic.
In this case H atoms are detected via 1+1 photoionization or by using the high-n Rydberg
time-of-flight (HRTOF) method. With the latter, detailed product state distributions are obtained
from the c.m. translational energy distributions, and high sensitivity and good resolution have
been demonstrated. Photolytically prepared radicals such as C2H are also
examined.
Stanford University
Stanford, CA 94305
Department of Mechanical Engineering
Spectroscopy and Kinetics of Combustion Gases at High Temperatures
This program involves two complementary activities: (1) development and application of cw laser
absorption methods for sensitive detection of species and measurement of fundamental
spectroscopic parameters at high pressures and temperatures; and (2) shock tube studies of
chemical reactions relevant to combustion. Species of interest in the spectroscopic portion of the
research include OH, HO2, CO2 and CH3.
Recent shock tube studies of reaction kinetics have been focused on the reactions HNCO + OH
CO2 +
NH2, HNCO + OH products, H + O2 HO2, and CO + OH CO2 + H.
State University of New York at Stony Brook
Stony Brook, NY 11794
Department of Chemistry
Ionization Probes of Molecular Structure and Chemistry
Ionization processes in intense, wavelength-tunable laser fields are being used in new
spectroscopic techniques to investigate the spectroscopy and photochemistry of ions and
molecules. Resonant multiphoton ionization, laser threshold ionization spectroscopy, and
photoinduced Rydberg ionization(PIRI)spectroscopy provide sensitive tools for the detection of
transient species and for examining the excited state structure and dynamics of molecules. These
methods also provide means for the detection and identification of minute quantities of molecular
species in difficult environments such as the mixtures produced in combustion reactions. Newly
developed primary tools in these studies are mass analyzed threshold ionization spectroscopy
(MATI), which provides high resolution ion vibrational spectra of selected molecules, and PIRI,
which gives access to the excited states of ions. In MATI the various ionization thresholds that
mark the energy states of an ion in transitions from a neutral state are measured by using the fact
that very highly excited neutral states near each threshold can be ionized by an electric field. In
PIRI the final ionization is accomplished by the absorption of a further photon. The ions produced
in either method are sent through a mass spectrometer so the optical spectrum of each mass is
obtained with high sensitivity. These techniques are being used to refine our knowledge of the
molecular orbital structure of benzene and substituted benzenes, molecules which play an
important role in many combustion processes.
University of Utah
Salt Lake City, UT 84112
Department of Chemistry
Thermochemistry and Reactivity of Transition-Metal Clusters
The objective of this project is to obtain information regarding the thermodynamic properties of
transition metal clusters, their binding energies to various ligands, and their reactions by using a
metal cluster guided ion beam mass spectrometer and a cluster ion photodissociation
spectrometer. Recent progress includes studies of the reactions of vanadium, chromium, and iron
cluster ions with D2 and O2. In the D2 reactions,
metal cluster monodeuteride ions are observed as the major product for all systems. The kinetic
energy dependence of these reaction cross sections is analyzed to yield cluster ion-deuterium atom
bond energies. Larger cluster ions also react with D2 to form dideuteride cluster
ions in what is believed to be an activated dissociative chemisorption process. In the
O2 reactions, cluster dioxide product ions are formed efficiently in exothermic
processes for all three metal systems examined. Less abundant are cluster monoxide product ions
which are formed in endothermic reactions. Analysis of the kinetic energy dependence of these
reactions is particularly complicated, but thermodynamic information about the binding energies of
one and two oxygen atoms to the clusters can be obtained. The trends in this thermodynamic
information provide clues regarding the geometric and electronic structures of the bare clusters.
University of Utah
Salt Lake City, UT 84112
Department of Chemistry
Spectroscopy of Polyatomic Transition Metal Clusters and Unsaturated Transition
Metal-Ligand Complexes
In this project we use electronic spectroscopy to investigate the chemical bonding and electronic
structure of unsaturated transition metal-ligand complexes and polyatomic transition metal
clusters. Our aim is to develop an experimental knowledge base which will elucidate the chemical
bonding and electronic structure of these species, enabling a correlation between electronic
structure and reactivity to be drawn. A second aim is to investigate several molecules in sufficient
detail to provide stringent tests of theoretical calculations on these computationally challenging
molecules. Throughout this grant we have used pulsed laser ablation in a supersonic expansion of
helium along with resonant two-photon ionization spectroscopy to investigate species formed
when metal is ablated in the presence of methane. Our spectra of RuC have led to a definitive
identification of its ground state along with 9 excited electronic states. Our spectra of MoC have
enabled its ground state symmetry and bond length to be determined, and studies of FeC have
permitted many excited states to be characterized. Most interesting, however, is our observation
of complex organochromium molecules which are formed in the chromium-methane laser-induced
plasma. In some, such as CrCH3 and CrCCH, chromium replaces hydrogen in an
otherwise stable molecule. Others have a more complicated structure, as in
CrCH2 and CrC3H2. Work is in progress to
completely analyze the spectra of these organochromium species.
University of Virginia
Charlottesville, VA 22901
Department of Chemistry
Photon and Electron Stimulated Surface Dynamics of Single Molecules
The aim of this proposal is to initiate fundamentally new studies of the surface dynamics of
individual molecules. Recent technological advances in the field of low temperature scanning
tunneling microscopy (STM) have made it possible to image and even manipulate single
molecules adsorbed on metal surfaces. Thermal drift problems inherent to the construction of
STMs from piezoelectric materials have impeded the development of low and variable
temperature STMs which might be able to follow the kinetics and possibly the dynamics of
thermally activated processes of single adsorbates in real time. Rather than studying thermally
activated processes, we propose to examine the dynamics of photon and electron induced
chemistry of single molecules using low temperature STM. The ability to study individual events
occurring for molecules absorbed at specific surface sites or next to coadsorbed molecules with
specific relative stereochemical geometries provides us with radically new dynamical information
which is unattainable from conventional studies involving ensembles of molecules. The specific
goals of our project are to examine, at the level of individual molecules, (1) the photodissociation
dynamics of adsorbates whose photofragments remain trapped on the surface, (2) the dynamics of
localized adsorbate chemistry induced by electrons emitted from the STM tip, (3) bimolecular
photoreactions between coadsorbed molecules with prearranged relative stereochemistry. The
results of our work should improve our fundamental understanding of surface chemistry and the
technological prospects for spatially localized processing of materials by photons from focused
laser beams and electrons emitted from STM tips.
University of Washington
Seattle, WA 98195
Department of Chemistry
Atomic Probes of Surface Structure and Dynamics
In the last year, we have extended our reversible work-based formulation of transition state theory
(TST) for calculating transition rates in multidimensional systems. One important challenge has
been to extend TST to systems with quantum degrees of freedom. The most commonly used
quantum TST has been the centroid density formulation originally suggested by Gillan. This had
been tested and found to work well for symmetric transitions. We have, however, found that the
centroid density approximation does not work well for asymmetric systems at low temperature
when tunneling becomes an important transition mechanism. We have formulated a new quantum
transition state theory where the transition state is defined as a NP-1 dimensional cone in the
space of all closed Feynman paths represented by P beads, rather than a surface in classical
N-dimensional space of the system coordinates. The free energy barrier is evaluated by reversible
work in this `action-space' and we refer to the method as Reversible Action-space Work Quantum
Transition State Theory (RAW-QTST). It has been found to work well on test problems,
including asymmetric transitions at low temperature. We have also applied the method to a very
large system, H2 desorption from Cu(110) surface, where the two H atoms and
eight Cu atoms where treated fully quantum mechanically and about 200 Cu atoms were included
classically. In a harmonic limit, applicable at very low temperature, the theory reduces to so called
instanton theory. At high temperature, above the crossover temperature for tunneling, the theory
reduces to the centroid density approximation and in the classical high temperature limit the
classical variational TST is recovered. This work is carried out in collaboration with Dr. Greg
Schenter and Dr. Bruce Garrett at the EMSL of Pacific Northwest National Laboratories.
University of Wisconsin at Madison
Madison, WI 53706
Department of Chemistry
Vibrational State Control of Photodissociation
The fundamental and practical importance of highly vibrationally excited molecules in combustion
processes, atmospheric chemistry, plasmas, and a host of other environments motivates their
detailed experimental investigation. This research uses a combination of laser excitation to prepare
highly vibrationally excited molecules with single-quantum-state resolution, and spectroscopic
detection to monitor the excited molecule or its decomposition product, in studies of the
unimolecular reaction, photodissociation, and bimolecular reaction dynamics of vibrationally
energized molecules. A collection of state preparation and detection techniques gives these
measurements broad scope. The excitation approaches are vibrational overtone excitation,
stimulated emission pumping, and stimulated Raman excitation, and the detection methods are
UV and VUV laser-induced fluorescence and laser-induced grating spectroscopy. By selectively
preparing vibrational states and subsequently dissociating or reacting them, these experiments
explore normally inaccessible regions of both the ground and electronically excited potential
energy surfaces. These approaches have even achieved laser control of the course of a chemical
reaction. The experiments provide new insights into the structure and dynamics of vibrationally
excited molecules, which play an important role in fundamentally and practically important
processes.
University of Wisconsin at Madison
Madison, WI 53706
Department of Chemistry
Spectroscopy Studies of Methyl Group Internal Rotation and of Organic Free Radical
Vibration
One long-term goal of this work is to develop new techniques for measuring vibrational spectra of
polyatomic neutral-free radicals. Toward that end, an optical parametric oscillator (OPO) has
been constructed based on the crystal KTP and pumped by the second harmonic of an
injection-seeded Nd:YAG laser to provide tunable near-IR. Testing of this device is underway. IR
absorption will be detected by depletion of laser-induced fluorescence from =0 radicals formed by photolysis of suitable
precursors upstream in a pulsed nozzle expansion. In addition, work will be initiated to address
the spectroscopic problem of internal rotation of methyl and silyl groups attached to aromatic
rings. Statistical modeling of chemical reactions of molecules that include methyl rotors requires
good intuition about whether internal rotation is nearly free or hindered. Internal rotors present
relatively tractable examples of noncovalent interactions that determine the energetics of different
molecular conformations as well. Such rotors also accelerate intramolecular vibrational
redistribution and alter photochemical pathways. From a combination of spectroscopic
experiments and ab initio calculations, an understanding is developing of the pattern of rotor
barriers vs. electronic state and also the effects of chemical substitution at different locations on
the benzene ring. The result is a growing intuition about the mechanism by which noncovalent
interactions within a molecule determine the potential energy function for internal rotation.
Last updated by Harry J. Dewey, (hd@lanl.gov) on December 23,
1996.