A molecular jump mechanism of water reorientation.

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.