The dynamics of proton transfer in a water chain

We perform quantum molecular dynamics simulations to study proton transfer along small water aggregates, such as a chain of hydrogen-bonded water molecules (proton wire) which is an important mechanism for charge species permeation. The electronic structure of the system is calculated concurrently with the nuclear motion using Born-Oppenheimer molecular dynamics within the framework of density functional theory. The simulations are performed on protonated linear chains of six water molecules, a linear water chain containing the water molecules, and an ammonia molecule. We discover that proton transfer along the chain is an extremely fast process, occurring in subpicosecond time scales. The translocation mechanism of the proton is neither a concerted mechanism in which the donor-acceptor pattern would occur over the entire chain in a single step, nor a result of a single proton hopping along the chain. The process takes place through a series of semicollective motion during which rapid fluctuations of the hydrogen-bond lengths along with reorganizations of water molecules are observed. The proton is translocated after a series of successive protonation-dissociation steps along the chain where hydrogen ions hop from oxygen to oxygen. We also discover that H3O+ and H5O2+ are the dominant species found during the course of the process. These simulations allow the study of dynamical properties of the systems at finite temperatures. (C) 1999 American Institute of Physics. [S0021-9606(99)51026-X].