Microwave spectrum and ring-puckering vibration in 1,1-difluorocyclobutane

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 Δ01=0.0620975 cm-1 and Δ23=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 4-77.30Q2 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.