( CH2OH). The kinetic equilibrium
determinations were obtained from the forward and reverse rates of the reactions:Jeffrey W. Hudgens and R. D. Johnson III
Physical and Chemical Properties Division
Chemical Science and Technology Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899 USA
E-mail: Hudgens@nist.gov
E-mail: RDJ3@nist.gov
The CH2OH radical plays important roles in combustion of
hydrocarbon fuels, atmospheric pollution chemistry, and interstellar chemistry [1]. Nearly
120 papers have described its properties and reactions. Even so, the heat of formation of
CH2OH is uncertain. Figure 1 summaries the experimental determinations of ( CH2OH). The kinetic equilibrium
determinations were obtained from the forward and reverse rates of the reactions:
X (X= Cl, Br, I) + CH3OH
HX + CH2OH
The ion appearance determinations were obtained from the threshold
appearance energy of CH2OH+ from CH3OH. After 30 years of
study, determinations of ( CH2OH)
obtained from these methods seemed irreconcilable.
Figure 1. Determinations of (
CH2OH) from kinetic equilibria and ion data.
Experimental determinations of (
CH2OH) use heat capacities and entropies of the CH2OH radical and
cation. These thermodynamic values are estimated from spectroscopic data with standard
formulae. Several groups have noted that these values are uncertain because the
spectroscopic data does not clearly establish the internal rotation energies. Various
groups have treated the internal rotation as a harmonic oscillator, a hindered rotor, and
as a free rotor. For derivations of ( CH2OH)
from kinetic data, these diverse treatments of internal rotation will vary 
( CH2OH) by 15 J-(mol-deg)-1
and ( CH2OH) by 5.6 kJ-mol-1.
We have conducted experiments and performed ab initio
calculations to obtain accurate partition functions for hydroxymethyl radicals and
cations. Our results show that previously-accepted spectroscopic data contain substantial
conceptual errors and do not include vibrational states residing below 400 cm-1.
We measured these missing vibrational levels by experimentally observing the extremely
weak "hot" bands exhibited in the 
2A(3p)
<--- 
2A resonance enhanced
multiphoton ionization (REMPI) spectra of CH2OH, CH2OH, CD2OH,
and CD2OD produced by the reaction of fluorine with methanol. Using optimized
MP2/6-311G(2df,2p) calculations, we constructed potential energy surfaces for the radical
and cation. Eigenvalues obtained from these potential energy surfaces reproduce the REMPI
data and show that the missing levels arise from strongly coupled v8 torsion
(internal rotation) and v9 CH2-wag modes.
Using the new experimental and ab initio results, we calculated
heat capacities and entropies for the radical and cation using the improved partition
functions. The new thermochemical values differ significantly from previous ones.
Recalculations of older kinetic equilibrium data using the new entropy, 
( CH2OH) = 244.170 0.018 J-(mole-deg)-1,
and new heat capacities lower ( CH2OH)
by 2-4 kJ/mole. Re-evaluated mass spectrometry data and kinetic equilibrium data lie in
agreement and suggest that ( CH2OH) =
-17.8 ± 1.3 kJ-mol-1. Our recommended value is also supported by
extensive ab initio CBS-QCI/APNO calculations that predict ( CH2OH) = -18.4 ± 1.3 kJ-mol-1.-1
Previous studies have regarded all vibrational modes as distinct
oscillators and rotors. In fact, the v8 torsion (internal rotation) and v9
CH2-wag modes are strongly coupled and governed by nonharmonic potential energy
surfaces. In the radical the CH2-wag coordinate has a 156 cm-1
barrier at the planar configuration and two 1643 cm-1 barriers to internal
rotation. Accurate predictions of the v8 and v9 energy levels
require simultaneous solutions for motion along both coordinates.
Several previous studies have regarded the hydroxymethyl radical as a totally nonsymmetrical species, i.e., as a member of the C1 point group. In fact, the wavefunctions of hydroxymethyl species belong to the Cs point group in the rigid molecule picture and to the G4 permutation-inversion group (which is isomorphic to the C2v point group) in the non-rigid molecule picture. Therefore, the proper symmetry number of hydroxymethyl radicals and cations is sigma = 2. This symmetry misassignment has also increased errors in some previous entropy calculations.
References
1) R. D. Johnson III and J. W. Hudgens, J. Phys. Chem. 100, 19874 (1996).
Presentation mode: We prefer to present this paper in a poster session.
Optional Information:
Author's Personal URL on the World Wide Web:
Johnson: none available
Hudgens: http://www.nist.gov/kinetics/hudgens
URL's for the Author's Institutions on the World Wide Web:
Johnson: http://www.nist.gov
Hudgens: http://www.nist.gov/kinetics