Microwave and Infrared Spectra, Adjusted r0 Structural Parameters, Conformational Stabilities, Vibrational Assignments, and Theoretical Calculations of Cyclobutylcarboxylic Acid Chloride

The FT-microwave spectrum of cyclobutylcarboxylic acid chloride, c-C4H7C(O)Cl, has been recorded and 153 transitions for the Cl-33 and Cl-37 isotopologues have been assigned for the gauche-equatorial (g-Eq) conformation. The ground state rotational constants were determined for Cl-35 [Cl-37]: A = 4349.8429(2S) [4322.0555(56)] MHz, B = 1414.8032(25) [1384.5058(25)] MHz, and C = 1148.2411(25) [1126.3546(25)] MHz. From these rotational constants and ab initio predicted parameters, adjusted r(0) parameters are reported with distances (A) rca c 1.491(4), rC=O = 1.193(3), rC(alpha)-C-beta = 1.553(4), rC(alpha)-C-beta, = 1.540(4), rC(gamma)-C-beta = 1.547(4), rC(gamma)-C-beta, = 1.546(4), rC-Cl = 1.801(3) and angles (deg) tau C gamma C beta C beta,C alpha = 30.9(5). Variable temperature (-70 to -100 degrees C) infrared spectra (4000 to 400 cm(-1)) were recorded in liquid xenon and the g-Eq conformer was determined the most stable form, with enthalpy differences of 91 +/- 9 cm(-1) (1.09 +/- 0.11 kJ/mol) for the gauche-axial (g-Ax) form and 173 +/- 17 cm(-1) (2.07 +/- 0.20 kJ/mol) for the trans-equatorial (t-Eq) conformer. The relative amounts at ambient temperature are 54% g-Eq, 35 +/- 1% g-Ax, and 12 +/- 1% t-Eq forms. Vibrational assignments have been provided for the three conformers and theoretical calculations were carried out. The results are discussed and compared to corresponding properties of related molecules.