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

Abstract

EPSRC's High End Computing Programme. We acknowledge the support of the Diamond Light Source, STFC, and the ISIS Neutron and Muon Facility. Supporting information:The remaining DSC thermograms for β-RDX (Figures S1−S4); a comparison between the calculated lattice parameters and unit cell volumes for α-and γ-RDX, compared to previous DFT-D studies as well as relevant experimental data ( Figures S5 and S6); comparison of predicted INS spectra for α-RDX to those for γ-and ε-RDX at similar pressures ( Figures S7 and S8); a comparison of the DFT-D predicted crystallographic parameters with corresponding experimental data for α-, γ-, and ε-RDX (Tables S1, S3, and S4, respectively); full lists of calculated phonon modes as a function of pressure for α-, γ-, and ε-RDX compared to experimental Raman data, including pressure-induced shift values (Tables S2, S5, and S6). This material is available free of charge via the Internet at http://pubs.acs.org Graphical abstract: Abstract: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 (ΔH fus ) of the highly metastable β-form of RDX. The characteristics of fusion for β-RDX were determined to be 186.7 ± 0.8 °C, 188.5 ± 0.4 °C, and 12.63 ± 0.28 kJ mol -1 for the onset temperature, peak temperature (or melting point), and ΔH fus , respectively. The difference in experimental ΔH fus for the α-and β-forms of RDX is 20.46 ± 0.92 kJ mol -1 . Ambient-pressure lattice energies (E L ) of the α-and β-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 α-and β-forms is in excellent agreement with the experimental difference in ΔH fus . The response of the lattice parameters and unit-cell volumes to pressure for the α-and γ-forms have been investigated, in addition to the first high-pressure computational study of the ε-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 α-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 α-, γ-, and ε-forms of RDX are only weakly affected by pressure. 4

High-Pressure Stability of Energetic Crystal of Dihydroxylammonium 5,5′-Bistetrazole-1,1′-diolate: Raman Spectroscopy and DFT Calculations

“…The last value is likely affected by the presence of mode ν44 . These values are somewhat higher than the typical value reported for FOX-7 (4.65 cm -1 ) 12. This suggests that the modes coupling in TKX-50 is somewhat stronger than in FOX-7.…”

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“…5 The importance of molecular crystals has lead to various computational approaches being employed to understand and elucidate their properties. While crystal-structure prediction and pharmaceuticals are very active and important areas of research, 6,7 many computational studies have also focused on energetic materials, 8,9 proton-transfer systems, 4,10 vibrational properties such as phonons and thermal displacement parameters, 11,12 as well as electronic properties. 13 The methods employed range from tailor-made force fields 8 to generic force fields such as OPLS 14 and CHARMM 15 to firstprinciples methods such as density-functional theory (DFT).…”

Crystal Structure and Properties of 7‐Imino‐3‐Nitroimino‐2,4,6,8‐Tetraazabicyclo[3.3.0]Octane Hydrochloride

“…The critical temperature of thermal explosion (T b ) is an important parameter to evaluate the thermal stability of an energetic compound because it is essential to insure the safe storage and process operations involving explosives, propellants, and pyrotechnics. [20,21] The value of T b of 558.5 K can be calculated by the following Equation (8): [22] T b = (E 0 -ΊE 0 2 -4E 0 RT p0 ) / 2R (8) where E 0 is the apparent activation energy obtained by Ozawa's method, R is the gas constant, and T p0 is the peak temperature corresponding to βǞ0, 551.3 K. The distribution of electronic density is an important indicator to express the chemical and physical properties of a molecule, [23] and the contour line map of the electronic density on compound A was drawn in Figure 4 using the Multiwfn program. [24] It is reported that heavy nucleus always possess high peaks due to the nuclear charge improving electron aggregation and then displays integral exponential attenuation towards all surrounding atoms.…”