The microwave spectrum, barrier to internal rotation, and dipole moment of cyclopropyl methyl ketone

The microwave spectrum of cyclopropyl methyl ketone has been examined in the region 8-36 GHz. Transitions in the ground and first three excited states of the acetyl torsional mode have been assigned for the species in which the carbonyl group is cis to the ring. The observed transitions appear as doublets as a result of internal rotation of the methyl group. The hight of the barrier to the internal rotation is V₃ = 1180 ± 20 cal/mole if the moment of inertia of the methyl group is assumed to be 3.12u·Å ². The dipole moments are μ_a = 0.47 ± 0.17 D, μ_b = 2.58 ± 0.26 D, and μ = 2.62 ± 0.26 D. In spite of an intensive effort, no transitions belonging to another species could be identified.