Far-Infrared and Raman Spectra of Solid Methanol and Methanol-d4

Abstract

The far‐infrared and Raman spectra were obtained of methanol and methanol‐d₄ at − 185°C, and a temperature‐dependence study of the Raman spectrum was carried out. In the low‐temperature $\alpha$ phase, splitting of the C–O stretching mode was observed in the Raman spectrum but disappeared when the sample was warmed to the high‐temperature $\beta$ phase. The coalescence of this band is attributed to the disorder caused by the interchange of the methyl groups above and below the plane of the chain. Four bands were observed at 350, 167, 109, and 57 cm⁻¹ in the low‐frequency infrared spectrum of methanol which shifted to 325, 161, 97, and 61 cm⁻¹ with deuteration. One skeletal mode was observed at 196 cm⁻¹ in the Raman effect and shifted to 180 cm⁻¹ with deuteration. It is shown that the spectra arising from the skeletal motions can be interpreted on the basis of line‐group theory in which the line‐group factor group is proposed to be $C_2$ for both phases. Also the nature of the molecular motions giving rise to the low‐frequency bands is discussed in relation to the observed shift factors with deuteration. Possible assignments for the skeletal modes is also given in terms of the factor group of the $C_2^2{\mathrm{–P}}_{2_1}$ space group, but the other suggested space group $C_{\text{2h}}^2{\mathrm{–P}}_{2_{\text{1/m}}}$ is not consistent with the number of observed far‐infrared bands.