The Interplay of Molecular Conformation and Crystal Packing in Pressure-Frozen Tetramethylsilane

In crystalline tetramethylsilane [TMS, Si(CH 3 ) 4 ] C 3 -symmetric molecules have been observed with one methyl group rotated by 18(4)° and three others by 4(2)° from the fully staggered conformation. The structure of TMS, in situ pressure-frozen in a diamond anvil-cell, has been determined by single-crystal X-ray diffraction at 0.58 GPa, 296 K. The densely packed molecules occupy C 3 -symmetric sites in cubic crystals, space group Pa 3̅, but their coordination scheme and unusual coordination number of 13 are considerably different than those in polymorphs α and β of analogous chlorotrimethylsilane Si(CH 3 ) 3 Cl. The observed rotations of methyl groups, distorting the molecular conformation from ideal symmetry T d, are consistent with the electron-diffraction results for the gaseous phase.