Analysis and dynamics of unusual double proton transfer reactions based on the reaction path Hamiltonian

Abstract

The double proton transfer reactions of the model base pair systems pyrazole–guanidine and pyrazole–cyanoguanidine were investigated at the MP2/[aug]-cc-pVDZ and CCSD(T)/[aug]-cc-pVDZ levels of theory. The dynamics of these two systems were studied within the reaction path Hamiltonian (RPH) formalism including the most important curvature coupling elements and all mode-mode couplings. In contrast to the proton transfer of the pyrazole–cyanoguanidine complex, the pyrazole–guanidine system is an example for a plateau reaction with a structureless transition region instead of a well localized transition state. We discuss the effects of the different forms of the minimum energy paths (MEP), of the curvature and of the couplings on the probability of trajectory reflections and thus on the conversion coefficient, entering into the overall rate constant. A comparison with variational transition state theory (VTST) is provided. Furthermore, the effects of different isotopes at various temperatures are presented.