Rotation–Vibration Interaction and Barrier to Ring Inversion in Cyclopentene

Abstract

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 $\text{υ\hspace{0.17em}=\hspace{0.17em}0}$ and $\text{υ\hspace{0.17em}=\hspace{0.17em}1}$ , 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.