AN ABSOLUTE CHEMICAL SHIELDING SCALE FOR TIN FROM NMR RELAXATION STUDIES AND MOLECULAR-DYNAMICS SIMULATIONS

An approximate nuclear magnetic shielding scale for tin has been obtained from Sn-119 spin-rotation constants, C-eff(Sn-119), in a series of spherical top molecules (SnX(4), X = H, D, CH3, and Cl). The paramagnetic shielding at a nucleus induced by circulating charge is known to be intimately related to the nuclear spin-rotation constant. Tin-119 spin-rotation constants in stannane, SnH4, have been determined by two independent techniques. In the first, C-eff(Sn-119) has been obtained from the experimentally determined ratio of the H-1 and Sn-119 spin-lattice relaxation times of gaseous stannane, T-1(H-1)/T-1(Sn-119) = 6570 +/- 600, and the known effective H-1 spin-rotation constant for methane. The second technique involved combining experimental T-1(Sn-119) data for liquid SnH4, SnD4, Sn(CH3)(4), and SnCl4 with angular momentum correlation times, tau(J)'s, calculated from molecular dynamics (MD) simulations. Since the spin-rotation relaxation mechanism is the only important mechanism for Sn-119 T-1 relaxation in these compounds, values of C-eff(Sn-119) were readily determined from the experimental liquid-phase T-1(Sn-119) data and the MD simulations. The tin relaxation data for liquid tetramethyltin and tin tetrachloride were taken from the literature, while T-1 data for SnH4 and SnD4 were obtained in the present study. Consideration of all the spin-rotation constant data obtained in this study indicates that the tin shielding constant, sigma(Sn), in liquid tetramethyltin is 2180 +/- 200 ppm with respect to the bare nucleus. The absolute nuclear shielding values obtained for tin in this study are compared to values recently obtained from quantum mechanical calculations. On the basis of the results of the present study, the Sn-119 magnetogyric ratio, gamma(Sn.119), is -9.997 559(8) x 10(7) rad s(-1) T-1, approximately 3% less than the accepted value.