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

摘要

1,1hyphen;Difluoropropene has been synthesized and characterized by study of the19F 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 theAhyphen;type torsional levels: Ground state,A= 10 154.30,B= 3742.19, andC= 2779.33; first excited torsional state,A= 10 098.00,B= 3741.80, andC= 2781.80 (all in megacycles per second). The barrier to internal rotation of the methyl group was determined from the rotational spectra to beV3= 1252plusmn;20 cal/mole. The molecularhyphen;dipole components along the principal inertial axes are determined by the Stark effect to beua= 0.836plusmn;0.005 andub= 0.309plusmn;0.005 D with the total dipole beingu= 0.889plusmn;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 thecissubstituent. It is concluded from the difference in the barriers betweencishyphen;fluoropropene and 1,1hyphen;difluoropropene that the barrier is insensitive to small changes in the distance between thecishyphen;fluorine atom and the methyl group.