Mechanisms of double proton transfer. Theory and applications

An analytical two-dimensional (2D) potential-energy surface based on two equal hydrogen bonds coupled by a correlation term, recently introduced [J. Chem. Phys. 127 (2007) 174513] to describe the dynamics of double proton transfer, is reviewed and generalized. It is then applied to the evaluation of proton transfer dynamics in a number of realistic systems, namely several molecules and dimers that exhibit various degrees of correlation between the motions of the two protons. The three parameters required to generate this 2D potential are derived from electronic structure and force field calculations, such that they include implicitly the effect of coupled skeletal modes. It follows that explicit introduction of such coupled modes is not required to obtain the basic relations that define the stationary points of the 2D surface, and thereby the reaction mechanism. Based on these relations, a detailed analysis is reported of a variety of systems exhibiting double proton transfer, including, apart from previously investigated porphine and porphycene, representing weak correlation, and the formic and benzoic acid dimers, representing strong correlation, two newly investigated systems which shed light on the hitherto not represented intermediate correlation category, namely naphthazarin, and the 4-bromopyrazole dimer.