SHIELDING ANISOTROPIES OF ACETYLENIC CARBONS AND SOLUTION MOLECULAR-DYNAMICS OF ETHYNYLTRIMETHYLSILANE AS STUDIED BY C-13 NUCLEAR-SPIN RELAXATION AND THEORETICAL AB-INITIO CALCULATION

The C-13 longitudinal relaxation times and {H-1}-C-13 NOE factors for ethynyltrimethylsilane, (CH3)(3)SiC=CH, in CDCl3 solution at 302.8 K have been measured at four magnetic fields. The dipole-dipole relaxation mechanism dominates the relaxation of protonated carbons, though the shielding anisotropy mechanism contributes remarkably to the total relaxation rate of the acetylenic =C-H carbon at B-0 = 14.1 T. For the Si-bonded acetylenic carbon, the latter relaxation mechanism is the most important. Describing the molecular dynamics as the isotropic overall tumbling accompanied by the internal rotation of methyl groups, the relaxation data were analyzed to give diffusion constants for both types of motion and shielding anisotropy parameters for acetylenic carbons, The reliability of the above analysis has been confirmed by the theoretical ab initio calculation of shielding anisotropy parameters of carbon nuclei in ethynyltrimethylsilane. (C) 1994 Academic Press, Inc.