The vibrational overtone spectra of the acetylenic and methyl C-H stretches of propyne were obtained for the v = 1 to v = 6 and v = 1 to v = 7 levels, respectively. Propyne-d1 was also studied and the methyl C-H stretching overtones were measured from v = 1 to the v = 7 level. The C-D stretch was observed only in the fundamental and first overtone regions. Lower level overtones were obtained by standard infrared techniques, while higher absorptions (> 12 000 cm-1) were obtained by intracavity dye laser photoacoustic spectroscopy. The C-H stretches in both molecules were analyzed in terms of the local-mode model, and harmonic frequencies (ωi) and anharmonicities (Xii) were calculated. In propyne these values were (acetylenic C-H stretch) ω1 = 3384 ± 5 cm -1 and Xii = -50 ± 1 cm-1 and (methyl C-H stretch) ωm = 3037 ± 5 cm-1 and X mm = -65 ± 2 cm-1. In propyne-d1 the methyl C-H stretch parameters were ωm = 3034 ± 5 cm-1 and Xmm = -64 ± 2 cm-1. For propyne, a hot band (v9 → v9 + uv1) accompanying the acetylenic C-H stretch was observed for v = 1-6 and the anharmonic interaction constant (X19 = -23 ± 7 cm-1) was calculated. A crossover from normal- to local-mode behavior has been observed for the methyl C-H stretches in propyne and propyne-d1 at the v = 3 and 4 levels. Below v = 3 the symmetric and antisymmetric methyl C-H stretches are designated by the usual normal-mode notation (v2 and V6 in propyne; v1 and V6 in propyne-d1), while for v≥3 the single observed band is designated as a "methyl" C-H stretch, uvm. Peak absorption cross sections have been measured for Δv1 = 1-5, Δv m = 3-5, Δv2 = 1 and 2, and the parallel component of 2v6 in propyne, and for Δv2 = 1 and 2, Δv1 = 1 and 2, Δvm = 3 and 4, and the parallel component of 2v6 in propyne-d1. During the course of this work the spectral constants of 3,3,3-trifluoropropyne were redetermined. The harmonic frequency ω1 is 3376 ± 6 cm-1, the anharmonicity X11 is -49 ± 1 cm-1, and the anharmonic interaction constant X17 is -17 ± 6 cm -1.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry