Vibrational Spectra and Structure of Four-Membered Ring Molecules. III. Bromocyclobutane, Bromocyclobutane-d1, Bromocyclobutane-d4, and Bromocyclobutane-d5

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Abstract
The infrared spectra of bromocyclobutane, α‐bromocyclobutane‐d1, β,β,β′,β′‐bromocyclobutane‐d4, and α,β,β,β′,β′‐bromocyclobutane‐d5 in both the liquid and vapor phases have been recorded from 4000 to 250 cm−1. The far‐infrared spectra of bromocyclobutane and β,β,β′,β′‐bromocyclobutane‐d4 have been recorded to 33 cm−1. The Raman spectra of the four isotopic species of bromocyclobutane in the liquid state have also been recorded and depolarization values have been measured; the carbon—bromine stretching region of bromocyclobutane‐d1 has been scanned at temperatures varying from 35° to −83°C. The spectra have been interpreted by considering the molecules as having one plane of symmetry and belonging to point group Cs. Frequency assignments which are based on group‐frequency correlations, isotopic shifts, depolarization values, band contours, and relative intensities where applicable are given for the normal vibrations. The assignments are tabulated and discussed according to symmetry species. The regimens of the product and noncrossing rules have been followed. Support for the existence of ``equatorial'' and ``planar'' conformers is presented and the nature of the potential function governing the ring‐puckering vibration for bromocyclobutane is discussed.