Solvation and Hydrogen-Bonding Effects on Proton Wires

In this paper, the multiconfigurational molecular dynamics with quantum transitions (MC-MDQT) method is used to simulate the nonequilibrium real-time quantum dynamics of proton transport along water chains in the presence of solvating water molecules. The model system consists of a protonated chain of three water molecules and two additional solvating water molecules hydrogen-bonded to each end of the chain. Nonequilibrium initial configurations are generated with an extra proton stabilized on one end of the water chain, and proton transport along the chain is induced by variations in the hydrogen-bonding distances between the solvating water molecules and the ends of the chain. These simulations indicate that solvation and hydrogen bonding significantly impact the proton-transport process and that quantum effects such as hydrogen tunneling and nonadiabatic transitions play an important role. Moreover, this model system exhibits a wide range of mechanisms, including both concerted and sequential double proton transfer, both strongly and weakly coupled double proton transfer, and both adiabatic and nonadiabatic pathways. The MC-MDQT approach provides a clear physical framework for interpreting and analyzing these different types of mechanisms.