Apparent contradiction between calculated kinetic and potential energy fractions of phonons in molecular solids with implications on condensed-phase chemistry

Abstract

Many computational and experimental analyses of molecular excitation rely on relative atomic velocities to identify the excitation of chemical bonds. These are often interpreted through the aid of normal mode calculations. We address two potential gaps in this approach: 1. Relative velocities may not reflect the total potential energy absorbed by the bond. 2. Normal mode calculations omit interactions between neighboring molecules, effectively assuming a similarity between condensed- and gas-phase chemistry. Phonon calculations implicitly include interactions between neighboring molecules and allow analyses of both relative velocities and potential energies in solids.In the present work, we compare kinetic and potential energy fractions of the phonon modes of the molecular solids nitromethane and β-HMX. We show that velocity-based methods are likely sufficient to analyze nitromethane. However, they cannot detect the majority of excitation ofthe N-N bonds in β-HMX, the scission of which appears to begin major decomposition pathways in the molecules.In the broader context of condensed-phase chemistry, this implies that important interactions may not all be identifiable through analyses that rely solely on relative atomic velocities.