Low-temperature Raman spectra of nitromethane single crystals. Lattice dynamics and Davydov splittings

By means of Raman scattering experiment performed on single crystals of solid nitromethane in various polarization geometries at normal pressure and low temperatures, the Davydov splitting of the vibrational levels in the solid phase of hydrogenated CH3NO2 and perdeuterated CD3NO2 nitromethane is reported for the first time. A re-examination of the lattice vibrations in their various symmetry species is also given. These experimental data are discussed on the basis of calculations using harmonic models of stretching, bending, and torsional motions of an isolated molecule and using density functional theory and second-order Moller–Plesset perturbation theory. Then, following Sorescu et al. [J. Phys. Chem. B 104, 8406 (2000).], the intermolecular interactions (Buckingham 6-exp form plus charge–charge Coulombic interactions) were added to the intramolecular potential. The frequencies of the q≈0 phonons or vibrons of different symmetry species were calculated paying a special attention to the symmetric and an...