INTRAMOLECULAR ROTATIONAL COUPLING OF INEQUIVALENT CH3 GROUPS

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