Proton Transfer Reaction Rates
Proton-transfer reactions are fundamental in chemistry and have important applications in enzyme catalysis and energy transduction and storage. We teach acid-base reactions to beginning chemistry students as if these reactions are simple and straightforward proton-exchange processes, but they are not. Proton-transfer reactions can involve quantum tunneling of the proton, solvent control of the reaction coordinate, and strong anharmonic couplings of the proton to many degrees of freedom of the complex. As a result, these reactions pose a significant challenge to even the most sophisticated quantum rate theories. Researchers have developed a number of theoretical methods and applied these to the same model system, but these models show little agreement about the chemical reaction dynamics. Although experiments have probed proton-transfer reactions in excited electronic states, such systems remain theoretically inaccessible. We are developing a model system for studying proton-transfer reaction dynamics in the ground electronic state using 2D IR spectroscopy and want to use that system to test theoretical predictions about the factors that control the reaction dynamics. This project is in its preliminary stages. More information to come as we continue to make progress on this interesting and important problem!
Asymmetric, Strongly H-Bonded complexes
Strong hydrogen bonds in asymmetric complexes such as formic acid with bases like pyridine or pyrazine are interesting because of their relationship to proton-transfer reactions which underlie many chemical phenomena including acid-base chemistry, biological energy transduction, and enzyme catalysis. Infrared absorption spectroscopy has long been used to characterize hydrogen bonds because of the distinct red shift and line broadening of the proton-stretching vibration of hydrogen-bonded complexes. These effects result in line shapes that span 1000 cm-1 or more, but, in spite of extensive experimental and theoretical study, the origins of these unusually broad absorption bands are not fully understood. The number of potential contributions to the proton-stretching line shape complicates the vibrational spectroscopy of hydrogen-bonded complexes. First, the distribution of hydrogen-bonding environments may inhomogeneously broaden the band. Second, the population lifetimes of hydrogen-bonded systems, typically sub-picosecond, can broaden the absorption band. Third, the proton-stretching vibration can couple to low-frequency, intermolecular motions of the hydrogen-bonded complex giving rise to a Franck-Condon-like progression of transitions in the low-frequency mode built on the proton-stretching excitation. Finally, the proton-stretching vibration may be Fermi-resonance-coupled to nearby combination and overtone transitions allowing them to borrow intensity from the proton stretch. All of these effects will contribute to the vibrational line shape of proton-stretching vibrations of strongly hydrogen bonded complexes to some extent, but their relative influence on the line shape cannot be determined from the absorption spectrum alone. We have used time-resolved vibrational spectroscopy to probe the contributions to the rich and complex lineshape.