Molecular Structure and Molecular Vibrations of 1,3,5,7,-Tetramethyl-2,4,6,8,9,10- Hexathiaadamantane

The molecular structure and vibrations of 1,3,5,7,-tetramethyl-2,4,6,8,9,10-hexathiaadamantane have been determined by a joint computational, gas-phase electron diffraction, and spectroscopic investigation. The geometry and harmonic force field of the molecule was calculated at the Becke3-Lee-Yang-Parr/6-31G* level. Vibrational analysis was performed using FT-IR and FT-Raman spectra recorded in the 4000−150 cm-1 range and utilizing Pulay's DFT-based scaled quantum mechanical (SQM) method (DFT: density-functional theory). This SQM method was extended to molecules containing C−S structural motifs. The joint computational and electron diffraction analysis resulted in an equilibrium geometry of T d symmetry characterized by staggered orientation of the methyl groups with respect to their adjacent C−S bonds. The electron diffraction study yielded the following bond lengths (r g) and bond angles (with estimated total errors): C−S, 1.820 ± 0.004 Å; C−C, 1.536 ± 0.004 Å; C−H, 1.119 ± 0.005 Å; C−S−C, 102.2 ± 0.2°; H−C−H, 109.9 ± 0.7°. The barrier to methyl rotation was computed to be 17 kJ/mol in good agreement with that estimated from the average methyl torsion (with respect to the staggered form) of 10 ± 3° from the electron diffraction analysis.