Effect of the internal rotation of the CHD2 group on the CH stretching mode of nitromethane NO2CHD2 in crystalline phase

Abstract

The infrared and Raman spectra of nitromethane NO2CHD2 in crystalline state have been recorded in the CH stretching mode range in a large temperature range (17 to 185 K). At very low temperatures, three bands are observed; each of them is assigned to the vibration of a CH oscillator localized in a different site. A quantum theory of these spectra shows that no tunnelling effect appears; this is in agreement with the classical interpretation. Furthermore, this theory ascribes the temperature dependence of the relative intensities of the v(CH) bands to the population density of the first torsional levels in the vibrational ground state and suggests a very simple ordering of the isotopic system CHD2 at 4 K, the CH vibrator being localized in the plane perpendicular to the molecular plane. At higher temperatures, the strong relaxation observed on these v(CH) bands is due to the CHD2 group reorientation and can be described, mostly, by a great amplitude random jump process