Experimental and DFT-D Studies of the Molecular Organic Energetic Material RDX

We have performed simulations utilizing the dispersion-corrected density functional theory method (DFT-D) as parametrized by Grimme on selected polymorphs of RDX (cyclotrimethylenetrinitramine). Additionally, we present the first experimental determination of the enthalpy of fusion (Delta H-fus) of the highly metastable beta-form of RDX. The characteristics of fusion for beta-RDX were determined to be 186.7 +/- 0.8 degrees C, 188.5 +/- 0.4 degrees C, and 12.63 +/- 0.28 kJ mol(-1) for the onset temperature, peak temperature (or melting point), and Delta H-fus, respectively. The difference in experimental Delta Hfus for the alpha- and beta-forms of RDX is 20.46 +/- 0.92 kJ mol(-1). Ambientpressure lattice energies (E-L) of the alpha- and beta-forms of RDX have been calculated and are in excellent agreement with experiment. In addition the computationally predicted difference in E-L (20.35 kJ mol(-1)) between the alpha- and beta forms is in excellent agreement with the experimental difference in Delta H-fus. The response of the lattice parameters and unit cell volumes to pressure for the alpha- and gamma-forms have been investigated, in addition to the first high-pressure computational study of the e-form of RDX- these results are in very good agreement with experimental data Phonon calculations provide good agreement for vibrational frequencies obtained from Raman spectroscopy, and a predicted inelastic neutron scattering (INS) spectrum of alpha-RDX shows excellent agreement with experimental INS data determined in this study. The transition energies and intensities are reproduced, confirming that both the eigenvalues and the eigenvectors of the vibrations are correctly described by the DFT-D method. The results of the high-pressure phonon calculations have been used to show that the heat capacities of the alpha-, gamma-, and epsilon -forms of RDX are only weakly affected by pressure.