Separation of quantum and classical behavior in proton transfer reactions: Implications from studies of secondary kinetic isotope effects

In this article the separability of the nuclear degrees of freedom into mixed quantum and classical components is examined by looking at secondary kinetic isotope effects in a model proton transfer reaction. To explore this issue, the nature of secondary kinetic isotope effects is investigated by means of molecular mechanics and ab initio simulations of a model system in which intramolecular proton transfer occurs in a region whose chemical topology is similar to that of malonaldehyde. Isotope effects are calculated using importance sampling Monte Carlo techniques designed to improve die statistical efficiency of ab initio simulations within the framework of quantum centroid transition state theory. The ab initio results for kinetic isotope effects are contrasted with those obtained using two molecular mechanics potential energy functions. It is demonstrated that the calculated isotope effects are extremely sensitive to subtle features of the potential energy surface, which suggests that information from ab initio structure and energy calculations about configurations along the reaction path must be utilized in the construction of classical potentials to obtain accurate secondary kinetic isotope effect predictions. It is also demonstrated that quantum nuclear degrees of freedom of all secondary atoms that move significantly as a chemical reaction proceeds should be treated explicitly, as even secondary heavy-atom tunneling effects could be significant, (C) 2002 Wiley Periodicals, Inc.