Accurate Affinities

A molecule's tendency to grab hold of a proton--its proton affinity--comes into play in areas such as biology (enzymes, ion transport through membranes), chemistry (flames, plasmas, interstellar environments), and the environment (atmospheric reactions, pollutant formation, hydrocarbon fuel combustion), to mention just a few. In addition to addressing such practical concerns, experimentally determined proton affinities provide feedback to theorists who need more accurate data to test their calculations. Whether or not a given chemical reaction proceeds is determined to a large extent by the change in energy that results as the reactants transform into the products. The dynamics of complex systems such as flames can involve hundreds of chemical reactions, each of which proceeds at its own rate. In many cases, it is not practical to measure each of these rates, so workers who model complex systems rely on a knowledge of the species' thermochemical properties, such as heat of formation and proton affinity, which permit them to either calculate the unknown rates or to at least determine whether or not a reaction is feasible.

PFI-PEPICO takes advantage of the multibunch time structure of the ALS storage ring (512 ns of synchrotron radiation followed by a 144-ns dark gap). The ALS photons excite C₃H₇Cl molecules to energies in the vicinity of the ion dissociation threshold. While promptly produced electrons and ions are extracted by a small electric field, some neutral molecules in high-n Rydberg states remain. These need just a small energy boost to become ionized. That energy is provided by a pulsed electrical field (pulsed-field ionization, or PFI) during the 144-ns dark gap. The resultant photoelectrons provide the start signal for time-of-flight measurements of the corresponding photoions (photoelectron-photoion coincidence, or PEPICO). By measuring the relative abundance of C₃H₇Cl⁺ vs. C₃H₇⁺ over a range of photon energies, the researchers were able to determine very precisely at what energy the abundance of C₃H₇Cl⁺ goes to zero (i.e., where C₃H₇Cl⁺ dissociates completely into C₃H₇⁺ and Cl).

Left: Time-of-flight distributions for C3H7Cl+ and C3H7+ at selected photon energies. Right: Relative abundance of C3H7Cl+ vs. C3H7+ in the vicinity of the ion dissociation threshold, or "appearance energy" (AE) of C3H7+ at 0 K.

This dissociation threshold energy represents the energy change that occurs when the parent molecule (C₃H₇Cl) splits into the products (C₃H₇⁺ and Cl) or vice versa (in which case this energy is called the heat of formation). Because the heats of formation of C₃H₇Cl and Cl are well known, the heat of formation of C₃H₇⁺ can now be determined with an accuracy limited by the error in the heat of formation of C₃H₇Cl. Then, because C₃H₇⁺ is made up of C₃H₆ (propene) and H⁺ (a proton), the heats of formation of C₃H₇⁺ (from this work) and H⁺ (well established) yield the change in energy involved in attaching a proton to propene (i.e., its proton affinity). As mentioned above, propene provides one of the absolute reference points for the scale of relative proton affinities.

Similar measurements and calculations were performed for ethylene (C₂H₄), another "anchor" molecule for the proton affinity scale. These more accurate heats of formation led to proton affinities of 742.3 kJ/mol for propene and 682.0 kJ/mol for ethylene, in good agreement with the latest theoretical calculations and about 8 kJ/mol lower than the previously accepted standard values. An example of the effect of these measurements for selected molecules having proton affinity values between 700 and 800 kJ/mole is shown above.

Research conducted by T. Baer (University of North Carolina at Chapel Hill), Y. Song and C.Y. Ng (Ames Laboratory), J. Liu and W. Chen (Berkeley Lab).

Research funding: Office of Basic Energy Sciences (BES), U.S. Department of Energy (DOE). Operation of the ALS is supported by BES.

Publications about this experiment: T. Baer, Y. Song, C.Y. Ng, J. Liu, W. Chen, "The Heat of Formation of 2-C₃H₇⁺ and Proton Affinity of C₃H₆ Determined by Pulsed Field Ionization-Photoelectron Photoion Coincidence Spectroscopy," J. Phys. Chem. A 104(9), 1959 (2000). T. Baer, Y. Song, J. Liu, W. Chen, C.Y. Ng, "Pulsed field ionization-photoelectron photoion coincidence spectroscopy with synchrotron radiation: The heat of formation of the C₂H₅⁺ ion," Faraday Discuss. 115, 137 (2000).