An intermolecular H–O potential for methyl rotations in solid nitromethane

Abstract

A reliable determination of the (H–O) intermolecular potential by which molecules of nitromethane interact with each other is presented. This effort is based upon a very complete body of experimental information which is available on the crystal structure and the rotational properties of the methyl group of the molecule in the solid state. The crystal structure is known in the temperature range of 4–233 K. The rotational properties of the methyl group of the molecule have been studied by inelastic neutron scattering with which the energy levels, including the ground state tunnel splitting, have been measured in both the protonated and deuterated samples. These studies provide a most comprehensive characterization of any such similar molecular crystal. The measured properties are directly related to the intermolecular potential but the inversion of the underlying intermolecular potential from the measured properties has not been straight forward. We describe the application of the principle of maximum entropy in the determination of an intermolecular H–O potential. The resulting potential is of a novel character. At 3.5 GPa x-ray diffraction studies indicate that the equilibrium orientation of the methyl group in crystalline nitromethane is rotated 45° from the position in the low-temperature ambient pressure form. Calculations of the potential energy as a function of methyl group orientation in crystalline nitromethane at a pressure of 3.5 GPa using the intermolecular potential described herein has reproduced this experimental observation.