Knowledge Bank

The perpendicular band of the fourfold degenerate C-H stretching vibration of dimethylacetylene ( 2-butyne ) has been measured in the region $2955-3065cm-1$ with Doppler-limited resolution at $\sim$195 K on a diffrenece-frequency laser spectrometer, and on a high resolution Fourier transform instrument at $\sim$295 K s well as in a molecular jet at $\sim$20 K. Over 700 P- and R-branch lines from 37 diffrenet subbands of this band, plus 25 additional unresolved Q-branches were assigned and fitted to the model of rotational-torsional interactions described by Bunker and di Lauro to determine spectroscopic constants for the upper C-H stretching state and for the Fourier components of the torsional potential in the ground and excited states. The results yield the value $6.316\times 0.034$ $cm-1$ for the barrier to internal rotation in the ground state, and $6.643\times 0.006$ $cm-1$.for the C-H stretching state. This value for the ground state barrier is somewhat higher than that recently determined from an analysis of the $CH3$-rocking band by Bunker et al.