The Rotation-Torsion Structure in the ν 11/ν 15( G s) Methyl Rocking Fundamental Band of Dimethylacetylene

We have extended to higher frequency our earlier assignments (J. Mol. Spectrosc.162,142-151, 1993) of the dense rotation-torsion structure in theG_smethyl rocking infrared fundamental band of dimethylacetylene (CH₃CCCH₃or 2-butyne) centered at about 1036 cm^-1. TheE_2dcombination band ν₄+ ν₁₆at 1057 cm^-1significantly perturbs this high-frequency structure, and we include its effect explicitly in the Hamiltonian matrix. We can now include considerably more of the high-frequency data in the analysis, and the new fitting leads to a determination of the ground state torsional potential parameterV₃as 1.517 cm^-1, and a torsional barrier height (4V₃+ 4V₉) of 5.98 ± 0.03 cm^-1. We cannot reproduce satisfactorily the structure in the immediate vicinity of the perturbation because of our lack of data for the ν₄+ ν₁₆perturbing state.