Internal rotation of molecules with two inequivalent methyl groups: The microwave spectrum of methyl acetate
The microwave spectrum of methyl acetate has been investigated in the frequency range from 8 to 40 GHz. Internal rotation of the two methyl groups splits each overall rotational transition into a quintet. Microwave-microwave double resonance has been used extensively in order to assign these quintets. The dynamical constants for overall and internal rotations have been adjusted from the measured transition frequencies. Accurate transition frequencies have been calculated by diagonalizing large sections of the energy matrices directly. The potential barrier to internal rotation of the acetate CH 3 group has been determined to be 99.56(8) cm -1 whereas that of the methoxy CH 3 group has been determined as 425(31) cm -1.