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Quantum-mechanical tunnelling and thermally activated reorientation of methyl groups have been studied in the series compounds X(CH 3 ) 4  with X = Si, Ge, Sn, Pb and LiCH 3  using NMR and. if possible, neutron scattering (INS and QNS) techniques. The temperature and frequency dependence of the relaxation rates 1/T 1  and 1/T 1Q  mirror the transition from small to large tunnel splittings as the barrier to rotation decreases in the succession X = Si. LiCH 3 , X = Ge, Sn, Pb. The various relaxation maxima, correlation times and apparent activation energies have been discussed in terms of current theories. Using the NMR, INS, QNS results and available literature data, relationships between tunnelling and temperature, activation, molecular structure and potential functions have been derived. Potential barriers decrease from about 7 to 0.2 kJ/mol and the symmetry is not far from threefold.

Methyl Tunnelling and Reorientation in the Solid Tetramethyl Compounds of Silicon, Germanium Tin and Lead, and in Methyl Lithium