Raman spectroscopy study of acetonitrile at low temperature.

We report the low-temperature studies of liquid CHCN by Raman spectral measurements at ambient pressure with decreasing the temperature from 20 to -196 °C. Detailed internal modes especially the lattice modes analysis revealed that the structural phase transitions of acetonitrile from liquid to solid phase β and solid phase β to solid phase α were occurring at -50 and -60 °C, respectively. Further, the Fermi resonance parameters between the fundamental ν and combination (ν + ν) of CHCN at different temperatures were calculated based on the Bertran's equations. It is found that the Fermi resonance parameters as a function of temperature become discontinued at -50 and -60 °C, which coincides with discontinuities observed in the Raman shifts of CHCN at -50 and -60 °C. The results suggest that the Fermi resonance parameters could be used as an indicator to assess the structural phase transition for CHCN under low temperature.