Complete Raman spectral assignment of methanol in the C-H stretching region.

In this work, the Raman spectrum of gaseous methanol in the C-H stretching region was investigated by polarized Photoacoustic Raman spectroscopy (PARS). On the basis of the depolarization ratio measurement and density functional theory (DFT) calculations, a complete spectral assignment has been presented. The band at ~2845 cm(-1) was assigned to CH3 symmetric stretching, the bands at ~2925 and ~2955 cm(-1) were assigned to two Fermi resonance modes of CH3 bending overtones, and the bands at ~2961 and ~3000 cm(-1) were assigned to out-of-plane and in-plane vibrations of splitting CH3 antisymmetric stretching. Such assignments can clarify the confusions among the previous spectral studies from the different experimental methods and be confirmed by the Raman spectrum of liquid methanol. Furthermore, the large splitting of 39 cm(-1) between two antisymmetric stretching in gaseous methanol was ascribed to the strong coupling between CH3 and OH groups within methanol molecule because it decreased rapidly in other long-chain alcohol, such as CH3CD2OH.