Water reorientation dynamics in the first hydration shells of F⁻ and I⁻

Molecular dynamics and analytic theory results are presented for the reorientation dynamics of first hydration shell water molecules around fluoride and iodide anions. These ions represent the extremes of the (normal) halide series in terms of their size and conventional structure-making and -breaking categorizations. The simulated reorientation times are consistent with NMR and ultrafast IR experimental results. They are also in good agreement with the theoretical predictions of the analytic Extended Jump Model. Analysis through this model shows that while sudden, large amplitude jumps (in which the reorienting water exchanges hydrogen-bond partners) are the dominant reorientation pathway for the I⁻ case, they are comparatively less important for the F⁻ case. In particular, the diffusive reorientation of an intact F⁻…H₂O hydrogen-bonded pair is found to be most important for the reorientation time, a feature related to the greater hydrogen-bond strength for the F⁻…H₂O pair. The dominance of this effect for e.g. multiply charged ions is suggested.