Kinetic isotope effects for nonadiabatic proton transfer reactions in a polar environment. 1. Interpretation of tunneling kinetic isotopic effects

A theoretical study of primary kinetic isotope effects (KIEs) is presented for proton transfer (PT) reactions in a polar environment in the nonadiabatic, i.e., tunneling, regime. This treatment differs from traditional descriptions for PT most notably in the identification of a solvent coordinate as the reaction coordinate. The theory explicitly addresses KIE features that are extremely sensitive to the proton donor-proton acceptor mode dynamics. Besides KIE behaviors that are not consistent with nontunneling PT, individual KIE aspects in some cases, such as magnitude, temperature dependence, variation with reaction asymmetry, and Swain-Schaad behavior can yield results consistent with nontunneling PT. However, a combination of KIE aspects-with particular emphasis on KIE variation with reaction asymmetry or temperature-can clearly identify tunneling in PT systems. In addition, PT via excited proton vibrational states is shown to significantly contribute to the reaction rate and KIEs, especially for extremely asymmetric reactions, where it can dominate.