Dimethylacetylene: Internal Rotation and the Analysis of the Methyl Rocking Infrared Fundamental Band

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We have obtained a very high-resolution infrared spectrum of the perpendicular methyl rocking fundamental band of dimethylacetylene (CH3CCCH3 or 2-butyne). which occurs in the region 1000 to 1100 cm−1, and have resolved and assigned much of the rotational structure. The spectrum was recorded at a temperature of −60°C with a pressure of 1.4 Torr, an absorption path of 8 m, and a resolution of 0.0014 cm−1, using a modified double-pass Bomem Fourier transform spectrometer and a cooled multiple-traversal absorption cell. We analyze the band allowing for the effects of internal rotation in the ground and excited vibrational states and for the effect of the dynamical end-to-end coupling of the methyl rocking motions through the acetylene bond. As well as determining the rotational constants for the states, the band origin, and the parameters necessary to characterize the dynamical coupling of the methyl rocking motions, we determine that the internal rotation barrier is 5.20 ± 0.08 cm−1 in the ground state and 5.77 ± 0.14 cm−1 in the methyl rocking excited state, where the quoted uncertainties are three times the standard errors.