Rotational Diffusion and Magnetic Relaxation of 119Sn in Liquid SnCl4 and SnI4

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. $T_2{\mathrm{\ll \; T}}_1$ 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 ${\mathrm{[\tau }}_{\theta }{\text{=3.67(10}}^{\text{−12}}\mathrm{)\hspace{0.17em}}\sec$ 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±0.00036μ_N). 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 ${\mathrm{SnI}}_4{\mathrm{(\tau }}_J\text{*∼ 2\hspace{0.17em}}\mathrm{at}\text{\hspace{0.17em}220°}\mathrm{C}\mathrm{),}$ 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.