A proton between two waters: insight from full-dimensional quantum-dynamics simulations of the [H2O-H-OH2]+ cluster.

The dynamics of a proton between two water molecules is studied by full-dimensional (15 dimensional) quantum dynamics using the multiconfigurational time-dependent Hartree (MCTDH) method. The collision of H(3)O(+) and H(2)O fragments is followed by an ultrafast and nearly irreversible energy transfer from the degrees of freedom that define the hydrogen bond (oxygen-oxygen distance and central proton position) to the rest of the degrees of freedom. The vibrations of the oxygen-oxygen distance are damped within the first 300 fs while the vibrations of the shared proton along the hydrogen bond are damped within the first 150 to 200 fs. Collisions in which the fragments arrive with a high momentum to the interaction distance lead to more recrossing of the transferring proton than collisions with a lower momentum. Slow coordinates, e.g. pyramidalization of the water monomers, have less time to adapt to the incoming or outgoing proton in the case of a high momentum, which leads to an enhanced recrossing effect with respect to slower collisions. In order to understand the energy flow dynamics between the vibration of the shared proton and other degrees of freedom a 5-state model is constructed and exactly solved. The energies and couplings of the states of the model are obtained from the analysis of the infrared spectroscopy of the H(5)O(2)(+) cation, namely from splittings and shifts of the most important spectral lines. The model qualitatively reproduces the key aspects of the full dynamics related to the vibrations of the shared proton, indicating that the proposed coupling scheme is correct.