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Explosives under pressurethe crystal structure of γ-RDX as determined by high-pressure X-ray and neutron di raction COMMUNICATION Swift et al. Structure of a lead urate complex and its e ect on the nucleation of monosodium urate monohydrate CrystEngComm www.rsc.org/crystengcomm

The structural response of the nootropic drug piracetam (2-oxo-pyrrolidineacetamide) to both direct compression and high-pressure recrystallization from aqueous solution is reported. Crystals obtained by these methods have been characterized in situ by single-crystal X-ray diffraction. Compression of form II between pressures of 0.45-0.70 GPa caused a reversible, singlecrystal to single-crystal transition to give a new polymorph, form V. Crystallization from a dilute aqueous solution of piracetam at a pressure of 0.6 GPa via crystallization of high-pressure ice-VI resulted in the formation of a previously unreported dihydrate. The molecular packing arrangements of these new structures are compared with the known polymorphs and hydrates of piracetam. This study highlights how the systematic variation of pressure is a powerful method for the exploration of polymorphism and solvate formation and has the potential to add a further dimension to polymorph screening of pharmaceuticals.

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.

Details of the 0.0 and 3.5 GPa experimental and computationally calculated structures, given as fractional coordinates of the unit cell for both OTF and 00PBE types of pseudopotential. Figures displaying the pressure dependence of lattice parameter and unit cell volume calculated using 00PBE pseudopotentials. Calculated equation of state parameters for 00PBE pseudopotential calculations. This material is available free of charge via the Internet at http://pubs.acs.org