Internal rotation dynamics of nitromethane at low temperatures

A detailed Raman spectroscopic study of nitromethane in the temperature int erval from room temperature to 15 K was undertaken. The crystal structure w as stable from 15 to 242 K. In the interval 243-244 K a new phase was obser ved before the transition into the liquid state took place. These observati ons were supported by DSC measurements which indicated the existence of an intermediate phase between the stable crystal form and the liquid phase. Th e activation energy for the internal rotation of the methyl group was deter mined by measuring the FWHM of the symmetric CH3 stretching band from 15 to 235 K. The activation energy found was 25 +/- 1 meV, which was in very goo d agreement with the energy difference between the ground energy level and the barrier top as determined by previous neutron and Raman scattering expe riments [D. Cavagnat, J. Chim. Phys. Phys. Chim. Biol. 82(2-3) (1985) 239]. (C) 1999 Elsevier Science B.V. All rights reserved.