INTERMOLECULAR AND INTRAMOLECULAR INTERACTIONS AND ROTATIONAL TUNNELING OF METHYL-GROUPS IN TETRAMETHYLTIN

Rotational tunneling of methyl groups in partially deuterated tetramethyltin compounds, (CH3)x-Sn(CD3)4-x with x = 1 and 3, and in various isotopic mixtures, [(CH3)4Sn]x[Sn(CD3)4]1-x, x = 0.027, 0.20, and 0.50, and [CH3Sn(CD3)3]x[Sn(CD3)4]1-x, x = 0.10, respectively, has been studied using the inelastic neutron-scattering (INS) technique. Compared with the INS spectrum of fully protonated tetramethyltin, (CH3)4Sn, the spectra obtained in the present study show remarkable shifts and broadenings or splittings of tunnel lines. The interpretation of the INS results in based on the assumption that all partially deuterated compounds and isotopically mixed systems have the same crystal structure as pure (CH3)4Sn. Combining the INS results with a detailed discussion of the crystal structure, all features can be explained in terms of both the intra- and intermolecular interactions of methyl groups. The overall decrease of tunnel splittings with deuteration is explained by the reduction of the lattice parameter and the increased octopole moment of the CD3 groups. The interaction between different molecules is mediated by a subset of methyl group pairs only.