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The microwave spectrum of fluoroacetylene (HCCF) has been measured in the frequency region 35 -260 GHz. Rotational constants of the ground state and rotational as well as rotation-vibrational interaction constants of several excited states of the C - C - H (ν4) and the C - C - F (ν5) bending vibrations have been obtained.

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The microwave spectrum of 1-chloro-1-fluoroethane (CFC 151-a) has been studied in the frequency region 8–250 GHz using waveguide Fourier transform, Stark, and source modulation spectrometers. Accurate rotational, quartic, and sextic centrifugal distortion and quadrupole coupling constants have been obtained from a global fit for the ground, v17=1 (Cl–F skeletal bending mode), and v18=1 (CH3 torsional) vibrational states of the 35Cl isotopomer and for the ground state of the 37Cl isotopomer. The larger off-diagonal element of the χ tensor was also determined for the 35Cl isotopomer. Assignment of the v17=1 and v18=1 states was confirmed by the presence of small A–E internal rotation splittings in the v18=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 rotation 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 ethanes.