Variable temperature (-55 to -100°C) studies of the infrared spectra (4,000-400 cm -1 ) of chlorocyclobutane, c-C 4 H 7 Cl, 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 DH 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 r 0 parameters have been obtained. The determined heavy atom structural parameters for the equatorial conformer are: the distances C-Cl = 1.783(5), C 1 -C 4 = 1.539(3), C 4 -C 6 = 1.558(3) Å , and angles \C 6 C 4 C 1 = 86.9(5), \C 4 C 1 C 5 = 89.7(5)°, and for the axial conformer are: the distances C-Cl = 1.803(5), C 1 -C 4 = 1.547(3), C 4 -C 6 = 1.557(3) Å , and angles \C 6 C 4 C 1 = 86.3(5), \C 4 C 1 C 5 = 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.
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