Knowledge Bank

The 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 $extended1$ to take into account the variation of the rotational constants with vibration state, the $(Pxpx+Pypy)$-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 $4cm-1$ 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 $ki=+3.$