Far-Infrared Spectra of Ring Compounds. VII. The Ring-Puckering Vibration in Chlorocyclobutane, Bromocyclobutane, and Cyanocyclobutane

The infrared spectra of chlorocyclobutane, bromocyclobutane, and cyanocyclobutane have been recorded in the range 200–33 cm−1. Under high resolution each of the spectra in this range show four or five distinct Q branches, which have been assigned to transitions between energy levels of the ring-puckering vibration for the equatorial isomer of each molecule. The transitional frequencies were fitted to asymmetric single-minimum potential functions of the form V(X)=aX4 − bX2 + cX3, where X is the ring-puckering coordinate. For V in cm−1 and X in angstroms, the respective values of a, b, and c are: chlorocyclobutane, 6.048·105, 2.039·104, 11.78·104; bromocyclobutane, 11.28·105, −7.247·104, 45.21·104; cyanocyclobutane, 10.04·105, −6.61·104, 42.63·104.