Abstract
Longitudinal and transverse relaxation times of 119Sn have been measured as a function of temperature in liquid SnCl4 and SnI4. For SnCl4, T1 is a monotonically decreasing function of temperature and is dominated mechanistically by spin-rotation over the entire liquid range. In SnI4, T1 passes through a maximum near 190°C. Spin-rotation and scalar coupling to 127I, respectively govern the relaxation above and below this temperature. T2≪ T1 in both liquids; scalar coupling, modulated by relaxation of the halogen isotopes, dominates the transverse relaxation. Dipolar coupling does not contribute appreciably to relaxation in either compound. Knowledge of the scalar contributions to T1 and T2 in SnI4 permit calculation of the 127I relaxation time [τ127=1.5(10−7) sec at 150°C] and the angular correlation time [τθ=3.67(10−12) sec at 150°C]. These values, and the published τ35 for SnCl4, give the tin-halogen scalar coupling constants: J(119Sn–35Cl)=470 Hz and J(119Sn–127I)=940 Hz. Spin-rotation constants are obtained using Steele's rotational diffusion theory and are used to calculate an absolute shielding scale for 119Sn. From the shielding constants and the relative resonance frequencies of 119Sn and 1H, the 119Sn magnetic moment is calculated to be (−1.04347±0.00036μN). Rotational correlation times in SnCl4 and SnI4 have been compared with theoretical predictions of the J diffusion and damped diffusion models. SnCl4 shows significant deviations from Hubbard's relation although the motion is diffusive according to both theories. Inertial effects are important for reorientation in SnI4(τJ*∼ 2 at 220°C), and damped diffusion appears to describe reorientation more accurately at these temperatures than does J diffusion. The relative roles of spin-rotation, scalar coupling, and dipolar coupling as relaxation pathways in other tin-containing systems are considered. It is concluded that the first two interactions usually dominate dipolar coupling for tin and for other heavy metals.
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