Transition State Theory in the Treatment of Hydrogen Transfer Reactions

Variational Transition State Theory in the Treatment of Hydrogen Transfer Reactions 

Transition state theory (TST) [1—4] is a widely used method for calculating rate constants for chemical reactions. TST has a long history, which dates back 70 years, including both theoretical development and applications to a variety of reactions in the gas phase, in liquids, at interfaces, and in biological systems. Its popularity and wide use can be attributed to the fact that it provides a theoretical framework for understanding fundamental factors controlling chemical reaction rates and an efficient computational tool for accurate predictions of rate constants. 

The need to include quantum mechanical effects in reaction rate constants was realized early in the development of rate theories. Wigner [8] considered the lowest order terms in an -expansion of the phase-space probability distribution function around the saddle point, resulting in a separable approximation, in which bound modes are quantized and a correction is included for quantum motion along the reaction coordinate - the so-called Wigner tunneling correction. This separable approximation was adopted in the standard ad hoc procedure for quan- 

Hydrogen-Transfer Reactions. Edited by J. T. Hynes, J. P. Klinman, H. H. Limbach, and R. L. Schowen Copyright 2007 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-30777-7 

H-transfer reactions are of great interest because they play important roles in a variety of systems, from gas-phase combustion and atmospheric reactions of small molecules to complex catalytic and biomolecular processes. Many enzyme reactions involve proton or hydride transfer in the chemical step [26], and we know from experience with simpler reactions that multidimensional treatments of the tunneling process are essential for quantitative accuracy and sometimes even for qualitative understanding. 

---

Intramolecular hydrogen transfer is another important class of chemical reactions that has been widely studied using transition state theory. Unimolecular gas-phase reactions are most often treated using RRKM theory [60], which combines a microcanonical transition state theory treatment of the unimolecular reaction step with models for energy redistribution within the molecule. In this presentation we will focus on the unimolecular reaction step and assume that energy redistribution is rapid, which is equivalent to the high-pressure limit of RRKM theory. 

---