HIGH RESOLUTION INFRARED SPECTRA OF DIMETHYLACETYLENE AND THE BARRIER TO INTERNAL ROTATION.

This research was supported, in part, by the Office of Naval Research through a contract with The Ohio State University Research Foundation. $^{1}$D. Papousek, J. Mol. Spectry. (in press).Author Institution: Laboratory of Molecular Spectroscopy and Infrared Studies, Department of Physics, The Ohio State UniversityThe fine structure of the perpendicular bands of an ethane-like molecule with nearly free internal rotation is discussed from the point of view of determining the barrier to internal rotation. The existing theory has been $extended^{1}$ to take into account the variation of the rotational constants with vibration state, the $(P_{xp_{x}}+P_{yp_{y}})$-Coriolis coupling and the Fermi resonance effects. The structure of ``hot' bands associated with the low frequency skeletal bending vibrations is also discussed. The perpendicular bands of the skeletal bending vibrations $\nu_{12}$ and of the C-H stretching and rocking vibrations $\nu_{2}$ and $\nu_{11}$ respectively, of dimethylacetylene were measured and the structure of the bunches of Q branches was resolved in $\nu_{9}$ and $\nu_{12}$ bands. The analysis is complicated especially by overlap with several ``hot'' bands but an upper limit of $4 cm^{-1}$ can be determined for the barrier to internal rotation. The value $(0.46 \pm 0.07) cm^{-1}$ was found as a probable value of the barrier from the splitting of the Q branches with the torsional quantum number $k_{i} = +3.