Plateau Reactions:  Double Proton-Transfer Processes with Structureless Transition States

The double proton-transfer reactions of the model base pair systems pyrazole−pyrazole and pyrazole−guanidine have been studied by computational methods up to the CCSD(T)/aug-cc-pVTZ level. In contrast to the synchronous proton transfer of the pyrazole dimer, the reaction profile of the second system shows a plateaulike transition region instead of a well-defined transition state characterized by one structure on the energy hypersurface. According to the computational results of this study, such plateau reactions with structureless transition regions behave differently in comparison to standard systems with gauss-shaped barriers. Energy partitioning schemes were used to analyze the unusual reaction profile, and one-dimensional tunneling effects were accounted for by a numerical approach. An empirical scheme has been derived to predict plateau reactions for double proton transfer systems.