The r0 structural parameters of equatorial and axial chlorocyclobutane, conformational stability from temperature dependent infrared spectra of xenon solutions, and vibrational assignments

Variable temperature (−55 to −100 °C) studies of the infrared spectra (4,000–400 cm−1) of chlorocyclobutane, c-C4H7Cl, dissolved in liquid xenon have been carried out. The infrared spectrum (4,000–100 cm–1) of the gas has also been recorded. For this puckered ring molecule the enthalpy difference between the more stable equatorial conformer and the axial form, has been determined to be 361 ± 17 cm−1 (4.32 ± 0.20 kJ/mol). This stability order is consistent with that predicted by ab initio calculations but the ∆H is much lower than the average energy value of 646 ± 73 cm−1 obtained from the MP2 ab initio calculations or 611 ± 28 cm−1 from the B3LYP density functional theory calculations. The percentage of the axial conformer present at ambient temperature is estimated to be 15 ± 1%. By utilizing previously reported microwave rotational constants for both conformers 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: the distances C–Cl = 1.783(5), C1–C4 = 1.539(3), C4–C6 = 1.558(3) Å, and angles ∠C6C4C1 = 86.9(5), ∠C4C1C5 = 89.7(5)°, and for the axial conformer are: the distances C–Cl = 1.803(5), C1–C4 = 1.547(3), C4–C6 = 1.557(3) Å, and angles ∠C6C4C1 = 86.3(5), ∠C4C1C5 = 88.9(5) and the puckering angles for the equatorial and axial conformers are 30.7(5)° and 22.3(5)°, respectively. 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.