Methyl group tunnelling and torsion in 2,4-hexadiyne

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Abstract
The tunnelling splitting of the ground torsional level of solid 2,4-hexadiyne and transitions to excited torsional states have been measured at low temperatures using neutron inelastic scattering. At 4 K the tunnelling splitting is 1·060 μeV (0·0086 cm-1). It decreases as the temperature is raised, to 0·834 μeV (0·0067 cm-1) at 35 K, and to less than 0·6 μeV at 50 K. A V-2←V=0 transition in the torsional vibration has been observed at 222 cm-1 which shifts to 160 cm-1 in the fully deuterated compound. The values of the torsional frequencies, tunnelling frequency, and the change of tunnelling splitting with temperature have been fitted exactly to a potential energy for rotation of a methyl group given by with a barrier to rotation of 432 cm-1. Changes in the tunnelling transitions as the temperature increases are compared with existing theories of the mechanism.