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l-Alanine is the smallest chiral amino acid, crystallising in the space group P212121. By comparison with other amino acids its crystal structure is extremely resilient to pressure, the ambient-pressure crystalline phase persisting until 13.6 GPa. At 15.46 GPa, l-alanine undergoes a reversible transformation to an amorphous phase. Hirshfeld fingerprint analysis and PIXEL calculations show that there is no development of destabilising contacts as pressure increases, so minimisation of volume is the likely driving force for this transition. Void-space analysis suggests that interstitial voids are all but absent in alanine at 13.6 GPa, and further compression is only possible with a change of phase. With the exception of benzene, 15.46 GPa is the highest pressure for which crystal structure data have been obtained for an organic system. [ABSTRACT FROM AUTHOR]

The crystal structure of alloxan, C4D2N2O4, is shown by neutron powder diffraction to transform to a new orthorhombic phase below 35 K. [ABSTRACT FROM AUTHOR]

The high pressure structure of α-oxalic acid di-hydrate shows a prototypical behaviour of acid to base proton transfer reaction. The solid state transformation has been characterized by neutron and X-ray diffraction as well as by quantum chemical calculations with periodic boundary conditions. The two resonant configurations playing the most important role in the hydrogen bond mechanism, i.e.a neutral and a charge transfer configuration, are interchanged by modifying the pressure on the system. A different behaviour is predicted for the less common β form, despite an apparently similar packing motif. Implications for the characterization of hydrogen bonds in the solid state are discussed as well as the possibility of estimating internal energy variations from experimental measurements. [ABSTRACT FROM AUTHOR]

The high pressure structure of α-oxalic acid di-hydrate shows a prototypical behaviour of acid to base proton transfer reaction. The solid state transformation has been characterized by neutron and X-ray diffraction as well as by quantum chemical calculations with periodic boundary conditions. The two resonant configurations playing the most important role in the hydrogen bond mechanism, i.e.a neutral and a charge transfer configuration, are interchanged by modifying the pressure on the system. A different behaviour is predicted for the less common β form, despite an apparently similar packing motif. Implications for the characterization of hydrogen bonds in the solid state are discussed as well as the possibility of estimating internal energy variations from experimental measurements. [ABSTRACT FROM AUTHOR]

The high pressure structure of α-oxalic acid di-hydrate shows a prototypical behaviour of acid to base proton transfer reaction. The solid state transformation has been characterized by neutron and X-ray diffraction as well as by quantum chemical calculations with periodic boundary conditions. The two resonant configurations playing the most important role in the hydrogen bond mechanism, i.e.a neutral and a charge transfer configuration, are interchanged by modifying the pressure on the system. A different behaviour is predicted for the less common β form, despite an apparently similar packing motif. Implications for the characterization of hydrogen bonds in the solid state are discussed as well as the possibility of estimating internal energy variations from experimental measurements. [ABSTRACT FROM AUTHOR]

The high pressure structure of α-oxalic acid di-hydrate shows a prototypical behaviour of acid to base proton transfer reaction. The solid state transformation has been characterized by neutron and X-ray diffraction as well as by quantum chemical calculations with periodic boundary conditions. The two resonant configurations playing the most important role in the hydrogen bond mechanism, i.e.a neutral and a charge transfer configuration, are interchanged by modifying the pressure on the system. A different behaviour is predicted for the less common β form, despite an apparently similar packing motif. Implications for the characterization of hydrogen bonds in the solid state are discussed as well as the possibility of estimating internal energy variations from experimental measurements. [ABSTRACT FROM AUTHOR]

Polymorphs of L-serine have been studied using ab initio density functional theory for pressures up to 8.1 GPa. The SIESTA code was used to perform geometry optimisations starting from the coordinates derived from high-pressure neutron powder diffraction. Between 0 and 8.1 GPa two phase transitions occur, the first of which takes place between 4.5 and 5.2 GPa and the second between 7.3 and 8.1 GPa. A change in molecular conformation occurs during the I-to-II transition, resulting in a stabilisation in intramolecular energy of 40 kJ mol−1. There is good agreement between the theoretical and experimental coordinates, and the largest root-mean-square deviation between experimental and optimised structures is 0.121 Å. Analysis of the effect of pressure on the intermolecular interactions using the PIXEL method showed that none becomes significantly destabilising as the phase-I structure is compressed. It is proposed that the phase transition is driven by attainment of a more stable conformation and from the reduction in the molecular volume. The second phase transition occurs with only a small change in the hydrogen bonding pattern and no substantial difference in molecular conformation. The effect on the energies of attraction between molecules suggests that this transition is driven by the bifurcation of a short OH⋯O interaction. [ABSTRACT FROM AUTHOR]

The crystal structure of the high-pressure ζ-form of the high explosive CL-20 has been determined using a combination of X-ray single crystal and powder diffraction techniques. [ABSTRACT FROM AUTHOR]

l-Alanine crystallises as a zwitterion in space group P212121at ambient pressure. The strongest intermolecular interactions are three N–HO hydrogen bonds. The H-bonds link the molecules into puckered layers in the acplane, which are then stacked along the baxis. PIXEL calculations indicate that the H-bond mediated intermolecular energies within the layers are 118 and 145 kJ mol−1, while the stacking interactions are considerably weaker (31 kJ mol−1). Neutron powder diffraction data on l-alanine-d7to 9.87 GPa show that the effects of pressure are most prominent along a, the direction which is least well aligned with the H-bonds. The aaxis decreases in length more rapidly than the caxis, and the two axis lengths are equal at ca.2 GPa. Although the structure is metrically tetragonal at this point, the true symmetry is still orthorhombic. In fact, the structure remains in a compressed form of its starting orthorhombic phase throughout the pressure range studied, in contradiction of previous Raman and energy dispersive power diffraction studies, in which data were interpreted in terms of phase transitions at ca.2 GPa and 9 GPa. It is likely that the spectroscopic signature of the apparent transition at 2 GPa is the result of a conformational change at the ammonium group. The molecular coordination number in l-alanine is 14, and the packing topology is a distorted form of body-centred cubic at ambient pressure. As pressure increases this topology becomes more regular, until at 9.87 GPa it is very similar to the perfect BCC topology of a structure such as tungsten. It is suggested that this behaviour can be traced to the sphericity of the alanine molecule. [ABSTRACT FROM AUTHOR]

The crystal structure of the high-pressure ζ-form of the high explosive CL-20 has been determined using a combination of X-ray single crystal and powder diffraction techniques. [ABSTRACT FROM AUTHOR]

The crystal structure of the high-pressure ζ-form of the high explosive CL-20 has been determined using a combination of X-ray single crystal and powder diffraction techniques. [ABSTRACT FROM AUTHOR]

l-Alanine crystallises as a zwitterion in space group P212121at ambient pressure. The strongest intermolecular interactions are three N–HO hydrogen bonds. The H-bonds link the molecules into puckered layers in the acplane, which are then stacked along the baxis. PIXEL calculations indicate that the H-bond mediated intermolecular energies within the layers are 118 and 145 kJ mol−1, while the stacking interactions are considerably weaker (31 kJ mol−1). Neutron powder diffraction data on l-alanine-d7to 9.87 GPa show that the effects of pressure are most prominent along a, the direction which is least well aligned with the H-bonds. The aaxis decreases in length more rapidly than the caxis, and the two axis lengths are equal at ca.2 GPa. Although the structure is metrically tetragonal at this point, the true symmetry is still orthorhombic. In fact, the structure remains in a compressed form of its starting orthorhombic phase throughout the pressure range studied, in contradiction of previous Raman and energy dispersive power diffraction studies, in which data were interpreted in terms of phase transitions at ca.2 GPa and 9 GPa. It is likely that the spectroscopic signature of the apparent transition at 2 GPa is the result of a conformational change at the ammonium group. The molecular coordination number in l-alanine is 14, and the packing topology is a distorted form of body-centred cubic at ambient pressure. As pressure increases this topology becomes more regular, until at 9.87 GPa it is very similar to the perfect BCC topology of a structure such as tungsten. It is suggested that this behaviour can be traced to the sphericity of the alanine molecule. [ABSTRACT FROM AUTHOR]

l-Alanine crystallises as a zwitterion in space group P212121at ambient pressure. The strongest intermolecular interactions are three N–HO hydrogen bonds. The H-bonds link the molecules into puckered layers in the acplane, which are then stacked along the baxis. PIXEL calculations indicate that the H-bond mediated intermolecular energies within the layers are 118 and 145 kJ mol−1, while the stacking interactions are considerably weaker (31 kJ mol−1). Neutron powder diffraction data on l-alanine-d7to 9.87 GPa show that the effects of pressure are most prominent along a, the direction which is least well aligned with the H-bonds. The aaxis decreases in length more rapidly than the caxis, and the two axis lengths are equal at ca.2 GPa. Although the structure is metrically tetragonal at this point, the true symmetry is still orthorhombic. In fact, the structure remains in a compressed form of its starting orthorhombic phase throughout the pressure range studied, in contradiction of previous Raman and energy dispersive power diffraction studies, in which data were interpreted in terms of phase transitions at ca.2 GPa and 9 GPa. It is likely that the spectroscopic signature of the apparent transition at 2 GPa is the result of a conformational change at the ammonium group. The molecular coordination number in l-alanine is 14, and the packing topology is a distorted form of body-centred cubic at ambient pressure. As pressure increases this topology becomes more regular, until at 9.87 GPa it is very similar to the perfect BCC topology of a structure such as tungsten. It is suggested that this behaviour can be traced to the sphericity of the alanine molecule. [ABSTRACT FROM AUTHOR]

The crystal structure of the high-pressure ζ-form of the high explosive CL-20 has been determined using a combination of X-ray single crystal and powder diffraction techniques. [ABSTRACT FROM AUTHOR]

l-Alanine crystallises as a zwitterion in space group P212121at ambient pressure. The strongest intermolecular interactions are three N–HO hydrogen bonds. The H-bonds link the molecules into puckered layers in the acplane, which are then stacked along the baxis. PIXEL calculations indicate that the H-bond mediated intermolecular energies within the layers are 118 and 145 kJ mol−1, while the stacking interactions are considerably weaker (31 kJ mol−1). Neutron powder diffraction data on l-alanine-d7to 9.87 GPa show that the effects of pressure are most prominent along a, the direction which is least well aligned with the H-bonds. The aaxis decreases in length more rapidly than the caxis, and the two axis lengths are equal at ca.2 GPa. Although the structure is metrically tetragonal at this point, the true symmetry is still orthorhombic. In fact, the structure remains in a compressed form of its starting orthorhombic phase throughout the pressure range studied, in contradiction of previous Raman and energy dispersive power diffraction studies, in which data were interpreted in terms of phase transitions at ca.2 GPa and 9 GPa. It is likely that the spectroscopic signature of the apparent transition at 2 GPa is the result of a conformational change at the ammonium group. The molecular coordination number in l-alanine is 14, and the packing topology is a distorted form of body-centred cubic at ambient pressure. As pressure increases this topology becomes more regular, until at 9.87 GPa it is very similar to the perfect BCC topology of a structure such as tungsten. It is suggested that this behaviour can be traced to the sphericity of the alanine molecule. [ABSTRACT FROM AUTHOR]

The reproducible crystallisation of elusive polymorphs and solvates of molecular compounds at high pressure has been demonstrated through studies on maleic acid, malonamide, and paracetamol. These high-pressure methods can be scaled-up to produce ‘bulk’ quantities of metastable forms that can be recovered to ambient pressure for subsequent seeding experiments. This has been demonstrated for paracetamol form II and paracetamol monohydrate. The studies also show that the particular solid form can be tuned by both pressure and concentration. [ABSTRACT FROM AUTHOR]

At ambient pressure the crystal structure of L-serine monohydrate (L-serine monohydrate-I) contains H-bonded layers of zwitterionic serine molecules linked by H-bonds to water molecules. The waters act as donors to oxygen atoms on carboxylate and alcohol groups in separate layers. This phase remains stable from ambient pressure to 4.5 GPa; the most prominent structural change in this range is a reduction in the interlayer distance. On increasing the pressure to 5.2 GPa the structure transforms to a high-pressure polymorph, termed L-serine monohydrate-II. The structures of both polymorphs have been determined using a combination of X-ray single-crystal and neutron powder diffraction. During the transition the serine interlayer distances reduce further and the water molecules rotate so that both donor interactions are made to the same serine layer. The serine ammonium group adopts an eclipsed conformation, reconfiguring the H-bonding within the serine layers. The disruption of H-bonding as water is pushed into the serine layers suggests that a similar process may occur as a first step in the pressure-denaturation of proteins. Though the water molecules become coordinatively saturated with respect to H-bonding, and interlayer serine-serine Coulombic interactions are strengthened, PIXEL calculations show that overall the intermolecular interactions are weaker in phase-II than phase-I. The lattice enthalpy becomes more negative through the transition as a result of the smaller PVterm applying to the more efficiently packed phase-II structure. [ABSTRACT FROM AUTHOR]

Using a combination of X-ray single crystal and neutron powder diffraction, the crystal structure of the high-pressure γ-form of RDX has been determined at 5.2 GPa and shows that the RDX molecules adopt different conformations compared to the conformation found in the ambient-pressure α-form. [ABSTRACT FROM AUTHOR]

As part of an on-going programme to study the high pressure structural behaviour of hydrated small molecular systems, sodium formate dihydrate has been studied using high pressure single crystal X-ray and neutron powder diffraction methods. A new phase was initially identified at 17 kbar by X-ray diffraction and high level quantum mechanical calculations completed the structure, allowing definitive hydrogen atom positioning. The resulting structure compared favourably with that found subsequently by high pressure neutron diffraction. The neutron diffraction study also revealed that the deuterated form, NaDCO2·2D2O, is stable in a different structural form to that of the non-deuterated material at ambient pressure. The structure of this phase is related to that of the high pressure phase via a simple translation of the molecular layers. [ABSTRACT FROM AUTHOR]

The crystal structure of the highly metastable β-form of RDX shows that the molecules adopt different conformations compared to the α-form and that, contrary to previous reports, the β-form obtained at ambient pressure is not the same form as that obtained at elevated temperatures and pressures. [ABSTRACT FROM AUTHOR]