IONIC QUADRUPOLAR RELAXATION IN AQUEOUS-SOLUTION - DYNAMICS OF THE HYDRATION SPHERE

A simple description of the quadrupole relaxation dynamics of atomic i ons in aqueous electrolyte solution is given. It is shown that the mol ecular electrostatic theories often used to interpret experimental dat a do not directly heed to some important characteristics of the dynami cs of the relaxation process. In particular, a prominent fast initial decay of the electric field gradient time correlation function is expe cted due to the interplay of static field gradient cancellations and c onservation of correlations in the hydrogen-bonded solvation sphere. T he rapid initial decay can easily lead to an order of magnitude reduct ion in the correlation time. This attribute of the dynamics is not spe cific for a particular microscopic model of solvation but should apply to any case of ionic quadrupole relaxation in a medium with strong so lvent-solvent interactions. The point is illustrated by analysis of mo lecular dynamics simulations of aqueous solutions, considering the NMR active solutes Li-7+, Na-23+, Mg-25(2+), Cl-35-, K-39+, Br-81, I-127- , Xe-131, and CS-133+ and the paramagnetic ion Ni2+. We analyze the el ectric field gradient fluctuation at the solute nucleus and in each ca se find clear evidence for the anticipated pronounced decrease in the overall correlation time. Generally, the simulated relaxation rates, 1 /T1, are in fairly good agreement with experiment. However, some inade quacies show that further refinements, such as the explicit inclusion of many-body effects, will be needed in order to achieve a universally accurate representation of ionic quadrupolar relaxation by simulation techniques.