Cryogenic Raman spectroscopy of glycerol

Abstract

In situ Raman spectra of glycerol in the spectral range 3700-300 cm were obtained at 298, 223 and 77 K using a Raman microprobe. Comparative polarization Raman spectroscopy followed by band component analysis of glycerol at 298 both and 223 K were employed for band assignment. In addition to the documentary information from glycerol-related compounds, the polarization analysis (interpreted mostly from intensity changes) and the Raman wavenumber shifts registered upon cooling glycerol were the criteria used for these assignments. In the high-wavenumber region (3700-2500 cm) the bands at 3407, 3240, 2946, 2886 and 2763 cm undergo a significant cryogenic shift to lower wavenumbers. The first two bands are assigned to antisymmetric and symmetric OH stretches, respectively, and the last three bands to CH stretching vibrations. These Raman shifts are related to intermolecular hydrogen-bond displacements upon cooling glycerol from room temperature. Below 1500 cm, the assignment of Raman bands is more complicated because many vibrations are coupled. Cryogenic shifts to higher wavenumbers are observed mainly for the lines attributed to the COH deformation (1342 cm), CCO vibration modes (1054 and 1115 cm) and CH rocks (976 and 925 cm). The peak at 673 cm arising from a bending vibration also shifts to a higher wavenumbers on cooling. The Raman shifts are well interpreted as an effect of cooling on H-bond strengthening in glycerol.