Microwave and Infrared Spectra, Adjusted r sub(0) Structural Parameters, Conformational Stabilities, Vibrational Assignments, and Theoretical Calculations of Cyclobutylcarboxylic Acid Chloride

The FT-microwave spectrum of cyclobutylcarboxylic acid chloride, c-C sub(4)H sub(7)C(O)Cl, has been recorded and 153 transitions for the super(35)Cl and super(37)Cl isotopologues have been assigned for the gauche-equatorial (g-Eq) conformation. The ground state rotational constants were determined for super(35)Cl [ super(37)Cl]: A = 4349.8429(25) [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 sub(0) parameters are reported with distances (Aa) rC sub( alpha )-C = 1.491(4), rC=O = 1.193(3), rC sub( alpha )-C sub( beta ) = 1.553(4), rC sub( alpha )-C sub( beta ' ) = 1.540(4), rC sub( gamma )-C sub( beta ) = 1.547(4), rC sub( gamma )-C sub( beta ' ) = 1.546(4), rC-Cl = 1.801(3) and angles (deg) tau C sub( gamma )C sub( beta )C sub( beta ')C sub( alpha ) = 30.9(5). Variable temperature (-70 to -100 degree C) infrared spectra (4000 to 400 cm super(-1)) were recorded in liquid xenon and the g-Eq conformer was determined the most stable form, with enthalpy differences of 91 plus or minus 9 cm super(-1) (1.09 plus or minus 0.11 kJ/mol) for the gauche-axial (g-Ax) form and 173 plus or minus 17 cm super(-1) (2.07 plus or minus 0.20 kJ/mol) for the trans-equatorial (t-Eq) conformer. The relative amounts at ambient temperature are 54% g-Eq, 35 plus or minus 1% g-Ax, and 12 plus or minus 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.