THE PERPENDICULAR C-H STRETCHING BAND NU(G) NU(13) AND THE TORSIONAL POTENTIAL OF DIMETHYLACETYLENE/

The perpendicular band of the fourfold degenerate C-H stretching vibra tion of dimethylacetylene (2-butyne) occurring in the region 2955-3065 cm(-1) has been measured with Doppler-limited resolution at a tempera ture of similar to 195 K on a difference-frequency laser spectrometer, and on a high-resolution Fourier transform instrument at room tempera ture as well as in a molecular jet at similar to 20 K. From a detailed rotational analysis of this complex band, based on an extension of th e model of the rotational-torsional interactions described by P. R. Bu nker and C. di Lauro (Chern. Phys. 190, 159-169, 1995), accurate spect roscopic constants were determined for the upper CH stretching state, along with values for the first three components of the Fourier expans ion for the torsional potential in the ground state, and for six compo nents of this potential in the upper state. Over 700 lines assigned to 37 well resolved subbands plus 25 additional unresolved line-like Q-b ranches were fitted with an overall standard deviation of 0.0024 cm(-1 ). The results yield a value of 6.316 +/- 0.034 cm(-1) for the barrier to internal rotation in the ground state, and 6.643 +/- 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 t he CH3 rocking band by P. R. Bunker et al. (J. Mol. Spectrosc. 162, 14 2-151, 1993). (C) 1996 Academic Press, Inc.