On the Existence of Conformers of Cyclobutyl Monohalides. II. Temperature Dependence of the Infrared Spectra of Bromocyclobutane and Chlorocyclobutane

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Abstract
The infrared spectra of bromocyclobutane and chlorocyclobutane vapor between 250 and 3100 cm−1 are reported as a function of temperature between 30° and 172°C. The spectra of the low‐temperature (−185°C) solids are also given. The data are described in terms of two conformers which are present in each halide. The energy difference between the two conformers of bromocyclobutane was measured to be about 1 kcal/mole. The conformers differ by their average dihedral angle: The more stable conformer is in a bent ring conformation (``equatorial''), the less stable one is in an essentially planar ring conformation. The sets of energy levels of the ring‐puckering motion of the two conformers are contiguous, there is no tunneling between the conformers. The conformations are sufficiently different as to lead to two widely separated carbon—halogen stretching fundamentals for each cyclobutyl halide. One stretching fundamental is based on the equatorial, the other (towards higher wavenumbers) is based on the planar ring conformation. The data were evaluated with the help of (1) calculations of the dipole moment as a function of the ring conformation, (2) computer calculations of vapor band envelopes including rotation—vibration interactions, (3) quantum‐mechanical computer calculations of the energy levels, probability distribution, transition moments, infrared intensities, and average dihedral angles of the ring‐puckering mode, and (4) some simple, qualitative considerations of the contributions of exchange interactions to the measured and calculated energy differences between the two conformers.