Field dependence of nuclear magnetic relaxation of 119Sn in SnCl4, SnBr4, and SnI4

Abstract

Longitudinal and transverse relaxation times of 119Sn have been measured as a function of temperature at several field strengths in SnCl4, SnBr4, and SnI4. T2 in all three liquids is field independent and is governed by scalar coupling to the halogen. T1 is strongly field dependent in SnBr4 and SnI4 and exhibits a minimum with increasing temperature due to competing scalar and spin‐rotation interactions. Coupling constants and correlation times previously computed for SnCl4 and SnI4 have been confirmed by measurements at 3.3 kg. Analysis of the data for SnBr4 yield J(119Sn☒81Br)=920 Hz, T2(81Br)=0.748×10−6 sec at 294°K, and τθ(2)τθ(2)=3.1×10−12 sec at 294°K. Molecular reorientation in SnBr4 is highly unusual in that the reduced angular correlation time τθ* reaches a minimum value owing to the onset of dynamically coherent reorientation at a temperature (80°C) that is not far above the melting point (30°C), and τθ* remains in the inertial rotation region over much of the liquid range. Various other indications of significant dissimilarity of microdynamical behavior in SnCl4, SnBr4, and SnI4 are pointed out. Assuming the validity of the J‐diffusion model, we complete the spin‐rotation and magnetic shielding constants for SnBr4, but these results are not consistent with the chemical shifts and shielding constants previously inferred for SnCl4. The inconsistency is believed to be associated with the extremely small reduced frictional constant of SnBr4 and with the possibly inappropriate use of extended diffusion theory to describe reorientation in this liquid.