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

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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.