Rotation–Vibration Interaction and Barrier to Ring Inversion in Cyclopentene

The contributions to the effective rotational constants of cyclopentene originating in the term in the Hamiltonian which couples the ring-bending vibration with rotation are examined. After corrections which depend only on experimentally determined parameters are applied to the constants for υ = 0 and υ = 1, the ring-bending vibrational dependence of the constants is expressible as an expansion in expectation values of even powers of the bending coordinate. Analysis of the rotation–vibration interaction yields a ring-bending potential function which is in excellent agreement with results of far-infrared studies of other workers; both sets of data are consistent with a barrier to ring inversion of 232 ± 5 cm−1. The results given in this paper are also necessary for a correct interpretation of the dependence of rotational constants in methylenecyclobutane and trimethylene sulfide on the ring-puckering vibrational state.