Site-site potentials in neopentane and tetramethylsilane

Neopentane and TMS are used as model M(CH(3))(4) systems to investigate intramolecular interactions. The nonbonded site-site potential between two proximal hydrogen atoms on different methyl groups, V(nb)(d(HH)), is not Lennard-Jones- or Morse-like but is found to be pseudolinear in hydrogen-hydrogen internuclear separation, d(HH), for both neopentane and TMS. The Morse potential is found to be a poor basis in which to expand V(nb)(d(HH)). The nonbonded site-site potential is conformation-dependent and not transferable between molecules. The individual contributions to V(nb)(d(HH)) are presented. The local mode parameters for neopentane and TMS are calculated ab initio for a variety of molecular conformations. The ab initio values of the local mode frequency and local mode anharmonicity are increasingly blue-shifted with increasing steric hindrance. Electron correlation is found to be increasingly important with decreasing internuclear separations, d(HH).