ROTATIONAL QUANTIZATION OF METHYL-GROUPS IN A ROTATING-FRAME

The rotational tunnel spectrum of pairs of coupled methyl groups in dimethylsulphide was studied at low temperatures using low field NMR. Two tunnel frequencies of 750 and 100 kHz give rise to sidebands of the NMR spectrum. The sidebands change to a broad asymmetric spectrum near internal resonances where nuclear Larmor and methyl tunnel frequencies coincide. An oscillating field at 100 kHz restores the sidebands and removes the asymmetry. It is proposed that the changes are due to methyl rotation induced by the applied oscillating field modulating the dipole-dipole terms, which become secular at the resonances. The mechanism extends to molecular dynamics the concepts of spin thermodynamics, and is also similar to the electromagnetic Aharonov-Bohm effect.