Nuclear magnetic relaxation of 119Sn, 35Cl, and 127I in two symmetric top molecules, SnCl3I and SnI3Cl, in liquid mixtures

Abstract

119Sn nuclear magnetic relaxation times were measured for the two symmetric top chloroiodides of tin in the liquid state using pulsed nuclear magnetic resonance. The temperature and magnetic field dependence of T1 and T2 of 119Sn in SnCl3I and SnI3Cl were studied to identify and separate relaxation mechanisms. The longitudinal relaxation rates were found to arise from competing scalar and spin–rotation interactions, while the transverse rates are completely scalar dominated. T1’s of both SnICl3 and SnCl3I show evidence of a maximum in (T1)−1scalar when T2(127I) equals the difference of the 119Sn and 127I Larmor frequencies. This behavior is predicted by Abragam’s ’’scalar relaxation of the second kind.’’ The maximum occurs near the melting points in a 5 kG field and was used indirectly to determine 127I relaxation times. A least-squares analysis of these results permitted direct determination of three tin–halogen scalar coupling constants: J (119Sn–127I) =1638 Hz for SnCl3I, J (119Sn–127I) =1097 Hz for SnI3Cl, and J (119Sn–35Cl) =421 Hz for SnI3Cl. Rotational correlation times and halogen relaxation times were also measured as a function of temperature. The analysis failed to reveal a T1 component due to chemical shift anisotropy for any of the compounds at or below 13.3 kG. A comparison of molecular rotational correlation times with those of the unmixed tetrahalides, SnCl4 and SnI4, indicates that electric dipole forces and shape effects associated with deviations from tetrahedral symmetry are less important determinants of the rotational diffusion tensor than is intermolecular rotational friction resulting from London dispersion forces. The effect on rotational diffusion constants of varying the composition of the medium was examined and found to be small.