Identification of Alcohol Conformers by Raman Spectra in the C-H Stretching Region

The spontaneous polarized Raman spectra of normal and deuterated alcohols (C-2-C-5) have been recorded in the C-H stretching region. In the isotropic Raman spectra, a doublet of -C alpha H stretching vibration is found for all alcohols at below 2900 cm(-1) and above 2950 cm(-1). By comparing the experimental and calculated spectra of various deuterated alcohols, the doublets are attributed to the -C alpha H stretching vibration of different conformers. For ethanol, the band observed at 2970 cm(-1) is assigned as the stretching vibration of -C alpha H in the C-alpha-O-H plane of the gauche-conformer, while the band at 2895 cm(-1) is contributed from both the -C alpha H2 symmetrical stretching vibration of the trans-conformer and the -C alpha H stretching vibration out of the C-alpha-O-H plane of the gauche-conformer. The population of gauche-conformer is estimated to be 54% in liquid ethanol. For the larger alcohols, the same assignments for the doublet are obtained, and the populations of gauche-conformers with plane carbon skeleton are found to be slightly larger than that of ethanol, which is consistent with results from molecular dynamics simulations.