A Molecular Dynamics Study of Classical Vibrational Spectra in Hydrostatically Compressed Crystalline Nitromethane

Abstract

The effects of pressure on the vibrational spectra of crystalline nitromethane have been studied by computing normal-mode frequencies and eigenvectors and classical power spectra at several hydrostatic pressures between 0 and 27.3 GPa using the full-dimensional Sorescu-Rice-Thompson (J. Phys. Chem. B 2000, 104, 8406) (SRT) valence force field. The purpose of the study was to determine the limits within which the SRT force field, and classical mechanics more generally, captures the qualitative pressure effects observed experimentally. The current results exhibit good overall agreement between the calculated normal-mode frequencies (and especially their pressure-dependent shifts) and those obtained in published experimental and theoretical studies. Comparisons of the pressure dependencies near room temperature of classical power spectra to experimental pressure-dependent infrared (IR) spectra for particular vibrational modes yield, in the case of the CN stretch, a CH(3) deformation, and the NO(2) asymmetric stretch, intriguingly similar evolution of spectral intensity with respect to pressure, whereas for the case of the NO(2) symmetric stretch mode the classical result bears little similarity to the experimental result.