Vibrational spectra of the terminal cis and methyl C-H stretches of cis-propene-1,2-d2 were obtained for vibrational levels (v) one through seven. The C-D stretches were observed up to v = 3. For comparison, spectra for all the C-H stretching modes of propene were obtained from the v = 1 to the v = 7 level. Lower level overtones were obtained by standard infrared techniques, and higher absorptions (>12000 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 cis-propene-1,2-d2 these values are (cis terminal olefinic C-H stretch) ωc = 3076 ± 28 cm-1 and Xcc = -56 ± 5 cm-1, (in-plane methyl C-H stretch) ωa = 3000 ± 17 cm-1 and Xaa = -58 ± 4 cm-1, and (out-of-plane methyl C-H stretch) ωb = 2975 ± 17 cm-1 and Xbb = -60 ± 4 cm-1. The ωi's and Xii's for propene are in good agreement with prior work of Fang et al. (J. Phys. Chem. 1985, 89, 225). Behavior characteristic of the normal-mode description of vibrations is observed for all C-H stretching modes at v = 1 and 2, whereas local-mode behavior prevails at v = 3 and higher. Line widths were measured for all C-H stretching modes for v = 3-7. Broadening of the methyl C-H absorptions in propene and cis-propene-1,2-d2 is observed at v = 6 and 7. Possible resonances that lead to efficient intramolecular energy transfer and the observed line broadening in both molecules are discussed. The role of molecular geometry in the difference in behavior of the olefinic versus methyl C-H line widths is considered. Peak absorption cross sections have been measured in both molecules.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry