Infrared Spectra and Intermolecular Potentials of Matrix-Isolated Nitromethane and Nitromethane-d3

Abstract

The infrared spectra of protonated and deuterated nitromethane isolated in a nitrogen matrix have been recorded from 4000 to 250 cm−1. Peaks have been assigned to monomer and dimer species by varying the concentration of nitromethane in the matrix. The frequencies of the monomer have been used in a normal coordinate analysis and the force constants evaluated agree well with those of the gas phase. The Cartesian displacements of the atoms for each normal mode are reported. These displacements showed that for the ``NO2 antisymmetric vibration'' there was considerably more motion of the hydrogen atoms. Lennard-Jones-type potentials were used to calculate dimer configurations with a minimum intermolecular potential energy, taking into account both dispersion and electrostatic forces. The charges on the atoms needed for the calculation of electrostatic forces were calculated from molecular orbital theory using both the MWH method and the INDO method. The position of minimum potential energy is a function of distance between a pair of nitromethane molecules and the angular rotation of two molecules with respect to each other. With the configuration of minimum potential energy and the Cartesian displacements in hand, the shifts and splittings due to intermolecular coupling could be evaluated by applying two-site, molecular exciton theory. The calculated shifts and splittings agree well with those observed in the spectra giving credibility to our calculations. The results will make it possible to estimate the moments of inertia of the dimer and force constants for the pair necessary for assessing partition functions for dimer formation under all conditions of temperature and pressure.