Reorientional dynamics of water molecules in anionic hydration shells

doi:10.1073/pnas.0701699104

Reorientional dynamics of water molecules in anionic hydration shells

Damien Laage^†,^‡ and
James T. Hynes^†,^§

^†Département de Chimie, Unité Mixte de Recherche 8640 PASTEUR, Ecole Normale Supérieure, 24 Rue Lhomond, 75231 Paris Cedex 05, France; and
^§Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215

Edited by F. Fleming Crim, University of Wisconsin, Madison, WI, and approved May 15, 2007 (received for review February 23, 2007)

Abstract

Water molecule rotational dynamics within a chloride anion's first hydration shell are investigated through simulations. In contrast to recent suggestions that the ion's hydration shell is rigid during a water's reorientation, we find a labile hydration sphere, consistent with previous assessments of chloride as a weak structure breaker. The nondiffusive reorientation mechanism found involves a hydrogen-bond partner switch with a large amplitude angular jump and the water's departure from the anion's shell. An analytic extended jump model accounts for the simulation results, as well as available NMR and ultrafast spectroscopic data, and resolves the discrepancy between them.

Footnotes

^‡To whom correspondence should be addressed. E-mail: damien.laage{at}ens.fr
Author contributions: D.L. and J.T.H. designed research; D.L. performed research; and D.L. and J.T.H. wrote the paper.
The authors declare no conflict of interest.
↵ ¶ The exchanges between the two states should be measurable through two-dimensional IR spectroscopy (see, e.g., ref. 20).
↵ ‖ The longer timescale associated with the dephasing of an OH vibration in liquid D₂O was identified as being associated with the analogous escape of a water from an anionic I⁻ hydration shell in ref. 28. This suggests that for the Cl⁻ case, there should be a close connection between the jump time τ₀ ^I→W and the vibrational dephasing of the OH in the Cl⁻ hydration shell, a topic to be pursued elsewhere.
↵ †† We have shown here that NMR and fsIRS probe the different components of the reorientation for water in the first hydration shell of a single ion (Cl⁻). For ionic mobility, a transition between the dominance of different components would be reflected in the differing behavior of the mobilities of different ions (3, 4, 29, 30), a feature that may also be found for hydration shell water reorientation for different ions.
Abbreviations:

fsIRS,

femtosecond infrared spectroscopy;

tcf,

time correlation function;

TS,

transition state.