Far-Ir Spectrum of Dimethylacetylene: Internal Rotation and Evidence for a D6h Effective Symmetry

The spectrum of dimethylacetylene vapor, liquid, and solid phases has been measured in the region 50 to 450 cm—1. Therefrom, together with previous work, a unique and ``complete'' vibrational assignment is derived. Rotational fine structure seems to confirm ``free internal rotation,'' i.e., a barrier below 100 cal/mole. Dimethylacetylene is an ethane-type molecule with practically free internal rotation. The new data strongly confirm the previous conclusion that neither D3, D3h, or D3d are appropriate symmetry groups. The selection rules are consistent with a D6h effective symmetry. An appendix deals with the relative intensities in degenerate vibrational transitions.