Despite long study, a molecular picture of the mechanism of water reorientation is still lacking. Using numerical simulations, we find support for a pathway in which the rotating water molecule breaks a hydrogen bond (H-bond) with an overcoordinated first-shell neighbor to form an H-bond with an undercoordinated second-shell neighbor. The H-bond cleavage and the molecular reorientation occur concertedly and not successively as usually considered. This water reorientation mechanism involves large-amplitude angular jumps, rather than the commonly accepted sequence of small diffusive steps, and therefore calls for reinterpretation of many experimental data wherein water rotational relaxation is assumed to be diffusive.

We detail and considerably extend the analysis recently presented in Science 2006, 311, 832- 835 of the molecular mechanism of water reorientation based on molecular dynamics simulations and the analytic framework of the extended jump model (EJM). The water reorientation is shown to occur through large-amplitude angular jumps due to the exchange of hydrogen (H)-bond acceptors, with a minor contribution from the diffusive H-bond frame reorientation between these exchanges. The robust character of this mechanism with respect to different water models is discussed. We fully characterize these jump events, including the distributions of trajectories around the average path. The average path values and the distributions of the jump time and the jump amplitude, the two key parameters in the Ivanov jump model component of the EJM, are determined. We also discuss the possibility of selectively exciting water molecules close to the jump event, of interest for ultrafast infrared experiments. In addition to a comparison of predicted reorientation times with experimental results, the reorientation time temperature dependence is discussed. A detailed description of the pathway free energetics for the water reorientation is presented; this is used to identify the jump rate-limiting step as the translational motion in which the initial H-bond of the reorientating water is elongated and the new H-bond acceptor water approaches.