Barrier Studies in the Halopropenes. II. Microwave Spectra, Molecular Structure, Dipole Moment, and the Barrier to Internal Rotation of the Methyl Group in 1,1-Difluoropropene

1,1-Difluoropropene has been synthesized and characterized by study of the 19F and H nuclear magnetic resonance spectra, infrared spectra, and mass spectra. The microwave spectra in the ground and first excited torsional states are assigned giving the following rotational constants for the A-type torsional levels: Ground state, A = 10 154.30, B = 3742.19, and C = 2779.33; first excited torsional state, A = 10 098.00, B = 3741.80, and C = 2781.80 (all in megacycles per second). The barrier to internal rotation of the methyl group was determined from the rotational spectra to be V3 = 1252±20 cal/mole. The molecular-dipole components along the principal inertial axes are determined by the Stark effect to be ua = 0.836±0.005 and ub = 0.309±0.005 D with the total dipole being u = 0.889±0.007 D. The barrier to the internal rotation of the methyl group is discussed in terms of the interaction of the methyl group and the cis substituent. It is concluded from the difference in the barriers between cis-fluoropropene and 1,1-difluoropropene that the barrier is insensitive to small changes in the distance between the cis-fluorine atom and the methyl group.