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 h ave been measured at four magnetic fields. The dipole-dipole relaxatio n mechanism dominates the relaxation of protonated carbons, though the shielding anisotropy mechanism contributes remarkably to the total re laxation rate of the acetylenic =C-H carbon at B-0 = 14.1 T. For the S i-bonded acetylenic carbon, the latter relaxation mechanism is the mos t important. Describing the molecular dynamics as the isotropic overal l tumbling accompanied by the internal rotation of methyl groups, the relaxation data were analyzed to give diffusion constants for both typ es of motion and shielding anisotropy parameters for acetylenic carbon s, The reliability of the above analysis has been confirmed by the the oretical ab initio calculation of shielding anisotropy parameters of c arbon nuclei in ethynyltrimethylsilane. (C) 1994 Academic Press, Inc.