Quadrupolar spin relaxation due to electric field gradients induced by vibrations and collisions

Abstract

The spin relaxation of quadrupolar nuclei in highly symmetric electronic environments via vibrationally-induced electric field gradients is considered. A model is presented for tetrahedral molecules which yields a nuclear quadrupole coupling constant for ¹⁸⁹Os in an excited vibrational state of OsO₄ which is in reasonable agreement with experimentally observed values. The nuclear quadrupole coupling constants for the central nucleus in excited E and F₂ vibrational states of GeCl₄, GeBr₄, RuO₄, OsO₄ molecules as well as the MO ^n- ₄ ions (M = V, Cr, Mn, Mo, Tc, Re) are calculated using this model. These coupling constants lead to quadrupolar relaxation rates which are orders of magnitude too small compared to experiment. Alternate mechanisms, collisional-deformation by long-range van der Waals interactions and fields induced by octopole moments, are proposed. A binary collision model is used in which the fluctuating electric fields associated with London dispersion forces during a collision create electric field gradients at the quadrupolar nucleus. Parallel development of vibrational and intermolecular effects on nuclear shielding with vibrational and collisional-deformation-induced electric field gradients is shown. The latter mechanism and the octopole-induced fields are capable of giving relaxation rates of the right order of magnitude.