Microwave Spectrum and Ring-Puckering Vibration in 1,1-Difluorocyclobutane

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Abstract
The microwave spectrum of 1,1‐difluorocyclobutane has been studied in the ground vibrational state and in six excited satellites of the ring‐puckering vibration. Large perturbations to the rotational spectra were observed for the lowest two pairs of vibrational levels. Double‐resonance experiments were used to help assign these nonrigid‐rotor spectra. These perturbations resulted from the coupling of vibrational and rotational angular momentum and gave the separations between the pairs of levels as Δ 01 = 0.0620975 cm −1 and Δ 23 = 3.87 cm −1 . From these splittings and the variation of effective rotational constants with vibrational state, the ring‐puckering potential was determined as V(Q) = 6.184Q 4 − 77.30Q 2 cm −1 , where Q is a reduced hermonic‐oscillator coordinate. This corresponds to a barrier of 241 ± 5 cm−1 to the planar configuration. Calculations of the ring‐puckering potential energy and of the vibration‐rotation kinetic energy were made from a computer program for a dynamical model of the vibration. The excellent agreement between the calculated and experimental results provides good confirmation for the description of the vibration given by the model.