Je. Roberts et J. Schnitker, IONIC QUADRUPOLAR RELAXATION IN AQUEOUS-SOLUTION - DYNAMICS OF THE HYDRATION SPHERE, Journal of physical chemistry, 97(20), 1993, pp. 5410-5417
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.