ROTATIONAL SPECTRUM, INTERNAL-ROTATION BARRIER AND AB-INITIO CALCULATIONS ON 1-CHLORO-1-FLUOROETHANE

The microwave spectrum of 1-chloro-1-fluoroethane (CFC 151-a) has been studied in the frequency region 8-250 GHz using waveguide Fourier tra nsform, Stark, and source modulation spectrometers. Accurate rotationa l, quartic, and sextic centrifugal distortion and quadrupole coupling constants have been obtained from a global fit for the ground, upsilon (17) = 1 (Cl-F Skeletal bending mode), and upsilon(18) = 1 (CH3 torsio nal) vibrational states of the Cl-35 isotopomer and for the ground sta te of the Cl-37 isotopomer. The larger off-diagonal element of the chi tensor was also determined for the Cl-35 isotopomer. Assignment of th e upsilon(17) = 1 and upsilon(18) = 1 states was confirmed by the pres ence of small A-E internal rotation splittings in the upsilon(18) = 1 state, in agreement with ab initio calculations, but in contradiction with previous assignment of the microwave spectrum by Thomas et al. [J . Chem. Phys. 61, 5072 (1974)]. The barrier height for the internal ro tation of the methyl group was determined to be 3814(11) cal/mol, and compared with the result of ab initio calculations made for 1-chloro-1 -fluoroethane and other related chlorine or fluorine substituted ethan es. (C) 1996 American Institute of Physics.