THEORETICAL-STUDY OF H+ TRANSLOCATION ALONG A MODEL PROTON WIRE

Authors
Citation
R. Pomes et B. Roux, THEORETICAL-STUDY OF H+ TRANSLOCATION ALONG A MODEL PROTON WIRE, Journal of physical chemistry, 100(7), 1996, pp. 2519-2527
Citations number
44
Categorie Soggetti
Chemistry Physical
ISSN journal
00223654
Volume
100
Issue
7
Year of publication
1996
Pages
2519 - 2527
Database
ISI
SICI code
0022-3654(1996)100:7<2519:TOHTAA>2.0.ZU;2-5
Abstract
The mechanism of proton translocation along linear hydrogen-bonded wat er chains is investigated. Classical and discretized Feynman path inte gral molecular dynamics simulations are performed on protonated linear chains of 4, 5, and 9 water molecules. The dissociable and polarizabl e water model PM6 of Stillinger and co-workers is used to represent th e potential energy surface of the systems. The simulations show that q uantum and thermal effects are both important because the height of th e barriers opposing proton transfer are strongly coupled to the config uration of the chain, which is, in turn, affected by the presence of a n excess proton. For characterization of the quantum effects, the ener gy levels of the hydrogen nucleus located at the center of a protonate d tetrameric water chain are calculated by solving the Schroedinger eq uation for an ensemble of configurations which were generated with pat h integral simulations. Analysis shows that the first excitation energ ies are significantly larger than the thermal energy k(B)T and that qu antum effects are dominated by the zero-point energy of the proton. Th e quantum correlations between the different proton nuclei are found t o be negligibly small, suggesting that an effective one-particle descr iption could be valid. Potential of mean force surfaces for proton mot ion in relation to the donor-acceptor separation are calculated with c lassical and path integral simulations for tetrameric and pentameric w ater chains. The mechanism for long-range proton transfer is illustrat ed with a simulation of a hydrogen-bonded chain of nine water molecule s, During the simulation, cooperative fluctuations which modulate the asymmetry of the chain enable the spontaneous translocation of protons over half of the length of the chain.