High resolution Raman spectroscopy of gases with laser sources. XIV—The pure rotational spectra of dimethylacetylene and dimethyldiacetylene

Abstract

The pure rotational Raman spectra of dimethylacetylene (DMA), dimethylaCetyh1e-d6~ (DMA-d,) and dimethyldiacetylene (DMDA) vapor excited by the A, = 4579 A, 4765 A, 4880 A and 5145 A lines of a single mode argon ion laser have been photographed under high resolution. The spectra were recorded on Kodak IIIa-9 spectroscopic plates hypersensitized by baking in nitrogen. The spectra of DMA and DMA-d, are those of symmetric top rotors showing the R-and S-branches, whereas in the spectrum of DMDA only the S-branch is observed. The spectra of DMA and DMA-d, were analyzed by including an H,(J+3/2)5 term in the frequency formula for the S-branch, while for DMDA the conventional expression was employed. The results for the molecular constants are, in at-', DMA (H3C-CEC-CH3): DMA-d6 (D3C -CEC-CD,): DMDA (H3C-CEC-CEC-CH,): B = 0.112183 (13), B = 0.088879 (21), B = 0.037270 (7), Dj = 3.70 (51) X lo-' Dj = 1.69 (61) X lo-' DJ = 0.86 (6) x lo-'. These constants are averages over the ground and excited vibrational states. The quoted errors are five standard deviations. The rcEC and rc-c bond lengths in dimethylacetylene are related by the expression + [$rcEc+ rc-c]2 = 4.617134 A'