Spectra and structure of small ring compounds LIII. Rotational and vibrational spectra and conformational stability of cyclobutylcarboxylic acid chloride

Abstract

The microwave spectrum of cyclobutylcarboxylic acid chloride, c-C 4H 7C(O)Cl, has been recorded from 26.5 to 39.0 GHz and only a-type transitions were observed. The R-branch assignments have been made for the ground and first two excited states of the asymmetric torsional mode of the 35Cl species. The ground state rotational constants determined from these assignments are consistent with an equatorial- gauche conformation and the values are: A = 4349.86 ± 0.17, B = 1414.78 ± 0.01, and C = 1148.24 ± 0.01 MHz. These values when used with a set of reasonably assumed heavy atom structural parameters are consistent with a dihedral angle of puckering of 20°. From the relative intensity measurements of the excited state transitions, the frequency of the asymmetric torsion was estimated to be 54 ± 10 cm −1. From a comparison of the infrared spectrum of the gas (3500 to 30 cm −1) and the Raman spectrum of the liquid (3200 to 100 cm −1) to the corresponding spectra obtained for the solid a second conformation can be shown to be present in the fluid phases at ambient temperature. From variable temperature studies of the Raman spectrum of the liquid the energy difference between the preferred equatorial- gauche and high energy (assumed to be equatorial- trans) conformations is determined to be 1.4 kcal mol −1. The ring puckering fundamental has been assigned to a weak band observed in the far infrared spectrum of the solid at 153 cm −1 and this assignment in addition to the assignments made for the remainder of the vibrational spectra are compared to those for some similar molecules.