The r0 structural parameters, conformational stability, and vibrational assignment of equatorial and axial bromocyclobutane

Variable temperature (−55 to −100 °C) studies of the infrared spectra (3500–400 cm−1) of bromocyclobutane, c-C4H7Br, dissolved in liquid xenon have been carried out as well as the infrared spectra of the gas and solid and Raman spectrum of the liquid. By utilizing eight pairs of conformers at ten different temperatures, the enthalpy difference between the more stable equatorial conformer and the axial form has been determined to be 291 ± 22 cm−1 (3.48 ± 0.26 kJ/mol). The percentage of the axial conformer present at ambient temperature is estimated to be 20 ± 1%. The ab initio MP2(full) average predicted energy difference from a variety of basis sets is 560 ± 46 cm−1 (6.70 ± 0.55 kJ/mol) and the average value of 569 ± 47 cm−1 from density functional theory predictions by the B3LYP method are significantly larger than the experimentally determined enthalpy differences. By utilizing previously reported microwave rotational constants for the equatorial conformer combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained. The determined heavy atom structural parameters for the equatorial conformer are: distances (Å) C–Br = 1.942(3), Cα–Cβ = 1.541(3), Cβ–Cγ = 1.552(3) and angles (°) ∠CαCβCγ = 86.8(5), ∠CβCαCβ = 89.7(5), ∠Br–(CβCαCβ) = 132.1(5) and a puckering angle of 29.8(5). The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations and compared to experimental values where available. The results are discussed and compared to the corresponding properties of some similar molecules.