Nuclear quadrupole relaxation of atoms and ions in polar solvents

A simple continuum model for quadrupole relaxation rates of atoms and large ions in polar solvents is obtained by generalizing the recent Hynes-Wolynes model. Whereas the original model considers fluctuations in the electric field gradient at the quadrupolar nucleus to arise solely from rotational motions of solvent dipoles, the present model also allows for fluctuations due to translational solvent motions. It shows that when both translational and rotational motions of polar solvent molecules occur simultaneously the rate of spin-lattice relaxation is reduced relative to the case where relaxation is induced by rotational solvent motions alone. Predicted rates from an amended version of the original Hynes-Wolynes model and from the generalized model are compared with experimental rates for ¹³¹Xe, ¹²⁷I^- and ⁸¹Br^- in several solvents.