A realization of ‘‘V structure’’ in liquid water

Abstract

Results obtained from a molecular dynamics computer simulation of ST2 model water are used to generate a representation of liquid water which corresponds closely to the so‐called ’’V structure’’ described by Eisenberg and Kauzmann. The representation is obtained by calculating a sequence of mean molecular positions, each mean being obtained by averaging the true dynamical center of mass and angular coordinates over a time comparable to the periods of hindered translational and rotational motion. For the averaging time used (τ_A = 0.2 psec), the results show significant narrowing in the distribution of nearest neighbor distances and of hydrogen bond angles. The distribution of molecular interaction energies manifests an associated shift to more negative values, and is somewhat narrower. Correspondingly, for a reasonable energetic criteria for an intact bond, the occurrence frequency of hydrogen bonds is dramatically increased in the V structure. The degree of connectivity of the hydrogen bond network shows parallel behavior. However, it is further shown that the energetic description of the V structure parallels that of the original, unmodified, structure, if corresponding intact hydrogen bond energy criteria are chosen reasonably. To obtain corresponding energetic structures, the criterion in the V structure must be shifted to more negative values to allow for the apparent loss there of a thermal excitation energyof about k_BT. It is suggested that a time sequence of V structures can be a valuable source of information for the analysis of longer time scale dynamics, since the rapid and large amplitude librational motions are effectively averaged out. By the same token, a study of the temperature variation of the V structure should be able to separate the variation in structure arising from changes in the underlying hydrogen bond network from that due to changes in the thermal vibrational excitation of that network.