Ultrafast Librational Relaxation of H₂O in Liquid Water

Abstract

The ultrafast librational (hindered rotational) relaxation of a rotationally excited H2O molecule in pure liquid water is investigated by means of classical nonequilibrium molecular dynamics simulations and a power and work analysis. This analysis allows the mechanism of the energy transfer from the excited H2O to its water neighbors, which occurs on a sub-100 fs time scale, to be followed in molecular detail, i.e., to determine which water molecules receive the energy and in which degrees of freedom. It is found that the dominant energy flow is to the four hydrogen-bonded water partners in the first hydration shell, dominated by those partners' rotational motion, in a fairly symmetric fashion over the hydration shell. The minority component of the energy transfer, to these neighboring waters' translational motion, exhibits an asymmetry in energy reception between hydrogen-bond-donating and -accepting water molecules. The variation of the energy flow characteristics with rotational axis, initial rotational energy excitation magnitude, method of excitation, and temperature is discussed. Finally, the relation of the nonequilibrium results to equilibrium time correlations is investigated.

Solvation Dynamics in Liquid Water. 1. Ultrafast Energy Fluxes

“…Here we can draw a parallel with the results for pure rotational excitation in Ref. 53: there it was found that rotational transfer is extermely rapid, so that energy does not sit on any water molecule for long; in addition, rotational excitations of up to 15 kcal/mol did not produce any noticeable changes in mechanism (in terms of percentages of energy flowing into the different modes).…”

“…This is an issue that will arise several times, although we attempt not to be repetitive. The energy flow from the produced rotationally excited central water molecule was characterized in detail as well 53 . It is less local in character than that directly from the bend to the first hydration shell: approximately 80 % of its rotational kinetic energy being transferred to water molecules in the first shell, with an even sharing by molecules accepting or donating hydrogen bonds.…”

Fluctuations of local electric field and dipole moments in water between metal walls

“…This indicates that the dipole motions are governed by the hydrogen bond dynamics in liquid water. In the previous simulations on liquid water 15,16,[36][37][38][39][40][41] , distinctly separated slow and fast molecular motions have been observed due to the hydrogen bond network.…”

Solvation Dynamics in Water: 2. Energy Fluxes on Excited- and Ground-State Surfaces

“…Several previous reports have shown that this energy flow approach results in a clear-cut understanding of molecular mechanisms involved in rotational/librational and vibrational relaxation of neat liquid water. [14][15][16] (The rotations in liquid water are of course hindered rotations, i.e. librations; we will use both appellations).…”