Intramolecular rotational coupling of inequivalent CH3 groups

The two methyl tunnelling-frequencies of a single crystal of 2,6 dimethyl pyrazine have been measured and assigned to separate CH3 groups on the basis of the low-temperature crystal structure. Ab initio and molecular-mechanics calculations have been applied to account for the anti-parallel orientation of the two CH3 groups in the crystal. For the isolated molecule the rotational potential of one group depends on the orientation of the other.