Microwave Studies of Butadiene Derivatives. I. Spectrum of Fluoroprene

Abstract

The microwave spectrum of fluoroprene (CH₂=CHCF=CH₂) has been studied in the 8–35 kMc region. The molecule is shown to have a planar trans configuration with ground‐state rotational constants a=10006.26, b=4264.35, c=2989.21 Mc. The spectra of five excited vibrational states, including the first three excited torsional states, have been analyzed in detail. Relative‐intensity measurements give a torsional frequency of 156±9 cm^—1 with no detectable anharmonicity. Negative inertial defects are observed in the excited torsional states; this is explained by a simple model of the torsional oscillation. The dipole moment in the ground state has a magnitude of 1.417±0.010 D and makes an angle of 10°—15° with the C–F bond. Possible forms of the torsional potential function which are consistent with the experimental results are discussed. It is suggested that a function which allows for a nonplanar isomer in butadiene‐type molecules (so far unobserved) gives a reasonable explanation of all the present data.