Transport and spectroscopy of the hydrated proton: A molecular dynamics study

In order to study the microscopic nature of the hydrated proton and its tra nsport mechanism, we have introduced a multi-state empirical valence bond m odel, fitted to ab-initio results [J. Phys. Chem. B 102, 4261 (1998) and re ferences therein]. The model makes it possible to take into account an arbi trary number N of valence states for the system proton+water and the electr onic ground-state is obtained by diagonalization of a NxN interaction matri x. The resulting force field was applied to the study, at low computational cost, of the structure and dynamics of an excess proton in liquid water. T he quantum character of the proton is included by means of an effective par ametrization of the model using a preliminary path-integral calculation. In the light of the simulations, the mechanism of proton transfer is interpre ted as the translocation of a privileged H5O2+ structure along the hydrogen bond network, with at any time a special O-H+-O bond, rather than a series of H3O++H2O --> H2O+H3O+ reactions. The translocation of the special bond can be described as a diffusion process with a jump time of 1 ps on average and distributed according to a Poisson law. A time dependent correlation f unction analysis of the special pair relaxation yields two times scales, 0. 3 and 3.5 ps. The first time is attributed to the interconversion between a delocalized (H5O2+-like) and a localized (H9O4+-like) form of the hydrated proton within a given special pair. The second one is the relaxation time of the special pair, including return trajectories. The computed diffusion constant (8x10(-5) cm(2)/s) as well as the isotopic substitution effect (1. 15), are in good agreement with experiment. The broad infrared absorption s pectrum which characterizes the excess proton in liquid water is also compu ted and interpreted. The main contribution to the broad bands between 1000 and 1800 cm(-1) is a combination of the bends and asymmetric O-H+ stretch o f the H5O2+ complex. The continuum of absorption between 2000 and 3000 cm(- 1) is attributed to the interconversion between symmetric and asymmetric st ructures within a given special bond. (C) 1999 American Institute of Physic s. [S0021-9606(99)30432-3].