Reinvestigation of the Gas-Phase Structure of RDX Using Density Functional Theory Predictions of Electron-Scattering Intensities

Density functional calculations at the B3LYP/6-31G* level are used to generate electron-diffraction intensity curves for six conformers of hexahydro-1,3,5-trinitro-s-triazine (RDX). These are compared with an experimental curve (Shishkov, I. F.; Vilkov, L. V.; Lolonits, M.; Rozsondai, B. Struct. Chem. <BO>1991</BO>, 2, 57) for which a structural model of gas-phase RDX was proposed. The calculations were done to investigate possible causes of the discrepancies between the theoretical structures and the molecular model proposed by Shishkov et al. The results show that alternative structures to that proposed from experiment can reproduce the measured intensities. Also, barriers to interconversion between the conformers were calculated to investigate the possibility of rapid interconversion at the temperature of the experiment (433 K). Barriers range from 1.5 to 5 kcal/mol, suggesting that measurements of RDX in the gas phase might reflect a dynamically averaged structure that represents contributions from the individual conformers.