Microwave spectrum and internal rotation of 2-butyne-1, 1, 1-d3 (dimethylacetylene), CH3C≡CCD3

Abstract
The rotational transitions of CH3C≡CCD3 have been observed for J=13←12, 15←14, 18←17, 19←18, 20←19, 22←21, 23←22, and 26←25 using a source‐frequency modulation microwave spectrometer with a 3.7 m long free space absorption cell maintained at −50 to −60 °C. The observed spectrum clearly shows the effect of internal rotation with a small potential barrier. The expression for the rotational transition frequency derived by Kirchhoff and Lide [J. Chem. Phys. 4 3, 2303 (1965)] using a second‐order perturbation theory for the contributions of the internal rotation was employed in analyzing the observed spectrum, and the least‐squares analysis has yielded the rotational constant B=2982.4980±0.0011 MHz and a few centrifugal distortion constants. The barrier height to internal rotation has been estimated to be 5.62±0.16 cm1.

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