Electric field Gradient Modulation by Motion of Lone-Pair Electrons in [(CH3)3S]2MC16 (M = Pt, Sn) as a Possible Relaxation Mechanism of Chlorine Nuclear Quadrupole Resonance

The temperature dependence of the chlorine NQR spin-lattice relaxation time T ^ w a s observed for trimethylsulfonium hexachlorometallates(IV), [(CH3)3S]2MC16 (M = Pt, Sn), trimethylammonium hexachlorostannate(IV), [(CH3)3NH]2SnCl6, and tetramethylphosphonium hexachlorostannate(IV), [(CH3)4P]2SnCl6. The quadrupolar relaxation in [(CH3)3NH]2SnCl6 and [(CH3)4P]2SnCl6 can be described by the usual librational and reorientational motion of the complex anion at lower and higher temperatures, respectively. Besides T1Q due to these two mechanisms, T1Q minimum was observed at ca. 90 and 120 K for [(CH3)3S]2PtCl6 and [(CH3)3S]2SnCl6, respectively, which is attributable to the electric field gradient modulation by the motion of the [(CH3)3S] cation having lone-pair electrons. The activation energies for the isotropic reorientation of the complex anion were determined as46,65, 59, and 60 kJ m o l ^ for [(CH3)4P]2SnCl6, [(CH3)3NH]2SnCl6, [(CH3)3S]2PtCl6, and [(CH3)3S]2SnCl6, respectively.