Structural Properties of Nickel Dimethylglyoxime at High Pressure: Single-Crystal X‑ray Diffraction and DFT Studies

Structural changes in nickel dimethylglyoxime (Ni­(dmg)₂) were followed by single-crystal X-ray diffraction in a diamond-anvil cell (DAC) at pressures up to 5.1 GPa, that is, in the pressure range through the major color change point (2 GPa), but before the phase transition at 7.4 GPa. Significant average compression (∼4%/GPa) was observed, with anisotropic, but continuous and monotonic lattice strain. The maximum compression was observed for the direction perpendicular to planar layers of Ni­(dmg)₂ and thus corresponds to decreasing the shortest contacts between nickel cations. Compression within the layers was not so pronounced as the compression between the layers. The structure and dynamics of the short O–H···O hydrogen bond connecting the adjacent dimethylglyoxime ligands were investigated by periodic DFT calculations and showed evidence of a flat, asymmetric single-well proton potential facilitating large-amplitude proton oscillations. The proton motion appears to be coupled to the dynamics of the adjacent methyl groups, resulting in the increased asymmetry of the hydrogen bond at higher pressures

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 (Δ<i>H</i>_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 Δ<i>H</i>_fus, respectively. The difference in experimental Δ<i>H</i>_fus for the α- and β-forms of RDX is 20.46 ± 0.92 kJ mol^–1. Ambient-pressure lattice energies (<i>E</i>_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 <i>E</i>_L (20.35 kJ mol^–1) between the α- and β-forms is in excellent agreement with the experimental difference in Δ<i>H</i>_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

High-Pressure Experimental and DFT‑D Structural Studies of the Energetic Material FOX‑7

This work reports the hydrostatic compression of the perdeuterated α-form of FOX-7 using neutron powder diffraction to follow the structural changes up to 4.58 GPa at room temperature. The equation of state for the hydrostatic compression of the α-form over the range 0–4.14 GPa has been determined, and a phase transition was observed over the pressure range 3.63–4.24 GPa. On the basis of dispersion-corrected density functional theory (DFT-D) calculations performed on the γ-form over a range of pressures, the high-pressure form observed in the neutron diffraction experiments can unambiguously be identified as being different from the γ-form and should therefore be denoted as the ε-form. Based on similarities between the simulated and experimental powder diffraction patterns of the γ- and ε-forms, it is suggested that the ε-form adopts a planar, layered structure. The structural responses to pressure of the α-form observed experimentally are reproduced by DFT-D calculations, but in-depth analysis of the bond lengths, angles, dihedrals, and vibrational frequencies calculated in the DFT-D simulations identified a very subtle second-order phase transition at 1.9 GPa. This corroborates results obtained from previous far- and mid-IR vibrational spectroscopic studies. These very small changes in molecular geometry do not manifest themselves in either the measured or calculated lattice parameters or unit-cell volumes and are much smaller than can be detected by diffraction experiments. The results of phonon calculations were compared with experimental inelastic neutron scattering measurements and were used to investigate the effect of pressure on the heat capacities of α-FOX-7. The simulations predict very weak pressure dependencies (approximately −1 J K^–1 mol^–1 GPa^–1), in accordance with the conclusions reached in our previous studies of the energetic material RDX