Microwave Spectrum of Propylene. II. Potential Function for the Internal Rotation of the Methyl Group

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Abstract
Rotational spectrum of propylene, the normal as well as some of the deuterated species, were observed in the first and second excited states of methyl torsion, to determine the shape of the potential function precisely. Two terms in the function, V3 and V6, are obtained to be 698.4±0.5 and −13±2 cm−1, respectively, where the moment of inertia of the methyl group about its symmetry axis is assumed to be 3.160 amu·Å2. The symmetry axis is nearly parallel to the CH3–CH bond, thus excluding the tilt of the methyl group in propylene. The theory of internal rotation is satisfactory in explaining the gross feature of the rotational spectra. Possible effects of other vibrations on the internal rotation is discussed briefly.