Experimental and theoretical study of the spin-spin coupling tensors in methylsilane

The experimental and theoretical C-13-Si-29 spin-spin coupling tensors, (1) J(CSi), are reported for methylsilane, (CH3SiH3)-C-13-Si-29. The experiment s are performed by, applying the liquid crystal NMR (LC NMR) method. The da ta obtained by dissolving CH3SiH3 in nematic phases of two LC's is analyzed by taking into account harmonic and anharmonic vibrations, internal rotati on, and solvent-induced anisotropic deformation of the molecule. The necess ary parameters describing the relaxation of the molecular geometry during t he internal rotation, as well as the harmonic force field, are produced! th eoretically with semiempirical (AMI and PM3) and ab initio (MP2) calculatio ns. Pr quantum mechanical approach has been taken to treat the effects aris ing from internal rotation. All the J tensors are determined theoretically by ab initio MCSCF linear response calculations. The theoretical and experi mental J coupling anisotropies, Delta(1)J(CSi) = -59.3 Hz and -89 +/- 10 Hz , respectively, are in fair mutual agreement. These results indicate that t he indirect contribution has to be taken into account when experimental D-1 (CSi)exp couplings are to be applied to the determination of molecular geom etry and orientation. The theoretically determined J tensors are found to b e qualitatively similar to what was found in our previous calculations for ethane, which suggests that the indirect contributions can be partially cor rected for by transferring the corresponding J tensors from a model molecul e to another.