BARRIER TO INTERNAL-ROTATION IN ETHANE FROM THE MICROWAVE-SPECTRUM OF CH3CHD2

Rotational spectra of CH3CHD2 were observed by using a source-modulation microwave spectrometer. Of 26 observed transitions, 16 showed splittings due to internal rotation. An analysis of the observed spectra, which utilized an internal axis method modified to include centrifugal distortion effects, gave the following results: A = 60809.592 (59) MHz, B = 17770.87 (16) MHz, C = 17084.84 (16) MHz, F = 257.70 (35) GHz, and V3 = 2.8783 (28) kcal/mole, with 2.5 times standard errors in parentheses. In the analysis V 6 = 0 was assumed, and D, a constant associated with a cross product of inertia, was fixed to a calculated value. When the uncertainty in D is taken into account, the error of V3 increases to 0.0029 kcal/mole. Effects of the molecular vibrations on the barrier are discussed briefly. © 1979 American Institute of Physics.