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

The experimental and theoretical ¹³C-²⁹Si spin-spin coupling tensors, ¹J_Csi, are reported for methylsilane, ¹³CH₃ ²⁹SiH₃. The experiments are performed by applying the liquid crystal NMR (LC NMR) method. The data obtained by dissolving CH₃SiH₃ in nematic phases of two LC's is analyzed by taking into account harmonic and anharmonic vibrations, internal rotation, and solvent-induced anisotropic deformation of the molecule. The necessary parameters describing the relaxation of the molecular geometry during the internal rotation, as well as the harmonic force field, are produced theoretically with semiempirical (AM1 and PM3) and ab initio (MP2) calculations. A quantum mechanical approach has been taken to treat the effects arising from internal rotation. All the J tensors are determined theoretically by ab initio MCSCF linear response calculations. The theoretical and experimental J coupling anisotropies, Δ¹J_Csi = -59.3 Hz and -89 ± 10 Hz, respectively, are in fair mutual agreement. These results indicate that the indirect contribution has to be taken into account when experimental ¹D_Csi ^exp couplings are to be applied to the determination of molecular geometry and orientation. The theoretically determined J tensors are found to be qualitatively similar to what was found in our previous calculations for ethane, which suggests that the indirect contributions can be partially corrected for by transferring the corresponding J tensors from a model molecule to another.