“…data of Si(CH3) 4 (Brockway & Jenkins, 1936;Sheehan & Schomaker, 1952;Beagley, Monaghan & Hewitt, 1971), Ge(CH3) 4 (Brockway & Jenkins, 1936;Hencher & Mustoe, 1975), Sn(CH3) 4 (Brockway & Jenkins, 1936;Nagashima, Fujii & Kimura, 1973) and Pb(CH3) 4 (Brockway & Jenkins, 1936;Wong & Schomaker, 1958;Oyamada, Iijima & Kimura, 1971) show the molecules to be undistorted tetrahedra in the gaseous state. In the course of investigations on low-temperature rotational tunnelling in the solid state of methyl groups in the series M(CH3) 4 with M = Si, Ge, Sn and Pb, more than one tunnel splitting and other anomalies in the nuclear magnetic resonance and inelastic neutron scattering experiments suggested non-equivalent methyl groups in Sn(CH3) 4 (M/illerWarmuth, Dupr~e & Prager, 1984;Prager, Dupr~e & M/iller-Warmuth, 1983).…”