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

Abstract

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(-)···H(2)O 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(-)···H(2)O pair. The dominance of this effect for e.g. multiply charged ions is suggested.

Effect of Ions on Water Dynamics in Dilute and Concentrated Aqueous Salt Solutions

“…Previous studies 5,7 had suggested that the average jump angle ∆θ varies very little (±5 • ) among the halide series, and for simplicity, we assume a constant ∆θ = 68 • . 5,7 The induced error on the jump contribution to the overall reorientation is expected to be ≈ ±10%, and that on the total reorientation time estimated from the jump model ≈ ±5%. As shown below, the very good agreement between the jump model predictions and the direct calculation of the reorientation times in the simulation suggests that this approximation does not significantly affect our results.…”

“…33 While water reorientation is not strictly isotropic, especially in the presence of ions, very little decoupling was observed between the reorientation of various molecular axes next to halide anions. 5 We thus compare the slowdown of the (simulated) OH bond reorientation time and the experimentally-determined value for the HH axis. We stress that this slowdown calculation is averaged over all water OH groups in the solution, and includes water molecules in different environments, ranging from a bulk-like environment to a direct contact with an ion.…”

“…7 Similar observations had been made on other, non-ionic solutions. 11 Our previous studies on ions have been limited to dilute halides, [5][6][7] and to dilute and concentrated sodium sulfate and sodium perchlorate solutions. 7 Very recently, simulations of sodium and potassium thiocyanate solutions from dilute conditions up to very high concentrations were analyzed using the same approach we originally applied to other solutions.…”

Reorientation dynamics of nanoconfined water: Power-law decay, hydrogen-bond jumps, and test of a two-state model

“…24,26 This model was shown to describe successfully the water reorientation dynamics next to a broad range of solutes and interfaces, 26 including hydrophobic 47 and hydrophilic interfaces. 48 Since jumps represent a dominant contribution to the reorientation kinetics (except next to strong H-bond acceptors 49,50 ), differences in the reorientation time usually result directly from changes in the jump time, τ 0 , which is the inverse rate constant for exchanging H-bonding partners. It can be obtained from the side-side correlation function,…”

A mode-coupling theory analysis of the observed diffusion anomaly in aqueous polyatomic ions

“…Diffusion of ions is of great interest because of their importance in physical and chemical industry, biology and other areas [1][2][3]. Although many interesting past theories and simulations have focused on structure of solvation shell and dynamics of rigid monatomic alkali cations and halide ions [4][5][6][7][8][9][10][11][12], a large number of chemically and industrially important electrolytes are made of polyatomic ions such as NO 3 -, SO 4 2-etc. that has not received much attention.…”