Double proton transfer reactions at the transition from a concerted to a stepwise mechanism: a comparative ab initio study

Abstract

The double proton transfer reactions between some substituted pyrazoles and guanidine were studied by ab initio calculations up to the CCSD(T) level. In contrast to standard textbook reactions, two of the systems show broad plateaus of almost constant energy along the minimum energy path (MEP) instead of well localized transition states. The impact of zero point vibrational energy (ZPE) corrections on the energy profiles are discussed in detail. Analyses based on the reaction path Hamiltonian (RPH) were used to gain further insight into the properties of the reaction paths. For one system RPH dynamics simulations were used to determine the life time on top of the plateau and the conversion coefficient entering into the overall rate constant. One dimensional (1D) tunneling effects were studied along the MEPs and a linear reaction path.