A Computer Simulation Model for Proton Transport in Liquid Imidazole

A multistate empirical valence bond (MS-EVB) model is developed to simulate proton transport in liquid imidazole. This approach allows proton transfer to simultaneously occur on both reaction sites (donor and acceptor) of the imidazole molecule. The underlying imidazole and imidazolium models are described by the generalized Amber force field (GAFF). The imidazole MS-EVB model was parametrized to reproduce the ab initio proton shuttling potential energy surface (PES) of a protonated imidazole dimer in the gas phase. In bulk phase at 393 K, the MS-EVB simulation yields a proton diffusion coefficient of 0.20 angstrom(2)/ps and a Grotthuss hopping rate of 1/36 ps(-1). Both results are in good agreement with experimental results. Despite the prevalence of a classical-like imidazolium structure with highly localized protonic charge, charge delocalization is not a negligible process in the simulations. Rather, it is shown to enhance the rate of proton diffusion by approximately 40% through Grotthuss shuttling. Analysis of the EVB states reveals that the imidazolium ion's first solvation shell by imidazole molecules is highly ordered through the formation of hydrogen bonds, while the second solvation shell is highly disordered. Together with the importance of charge delocalization, this result suggests that reorientation of imidazole rings in the second solvation shell is the rate-limiting step for proton transfer.