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We report a pressure-induced transition to a new crystalline phase of biurea (C2D6N4O2). Neutron-powder diffraction and Raman spectra, measured up to 3.89 GPa reveal this transition to be isosymmetric, where substantial intramolecular reorientation leads to an abrupt decrease in unit cell volume at ∼0.6 GPa and an increase in some intermolecular contact distances. DFT and vibrational energy calculations suggest that the ambient pressure phase is stabilised by zero point energy and entropy, until the pressure–volume–driven transition at 0.6 GPa.

The crystal structure of the complex formed between acetone and bromine has been determined between 14 K and 200 K. The structure exhibits negative 2D thermal expansion at low temperature and colossal thermal expansion in the perpendicular dimension. At low temperature the C[double bond{,} length as m-dash]O bond is significantly longer than that observed in free acetone.

The structure of the complex formed between trimethylamine and iodine monochloride had been redetermined by a combination of neutron powder diffraction and X-ray single crystal diffraction techniques. The structure determined using NPD shows that the geometry of the trimethylamine moiety resembles that in trimethylammonium cations rather than in the free trimethylamine molecule. The structure shows the presence of weak C–D⋯Cl hydrogen bonds, The I–Cl bond length is in agreement with that derived from an analysis of the pKb values of a series of nitrogen bases and previously determined structures containing N⋯ICl fragments.

The crystal structures of the simple monoclinic halogen-bonded complexes pyridine–ICl and pyridine–IBr have been redetermined using modern X-ray and neutron diffraction techniques. The representation quadric surfaces for the thermal expansion tensor have been found to consist of a hyperboloid of one sheet. Negative thermal expansion occurs parallel to the crystallographic b axis, whilst there is colossal thermal expansion in a direction approximately parallel to the a* direction. These effects arise because of a decrease in the strength of an already weak C–H⋯X (X = Br, Cl) hydrogen bond with increasing temperature. The decrease in strength of the hydrogen bonding originates because of the reduction in the strength of the halogen bond formed between the nitrogen atom and the IX moiety with increasing temperature. Thus since at 298 K the I–X bonds are shorter than at 110 K, there will be a smaller partial negative charge on the X halogen at 298 K, leading to a weaker C–H⋯X hydrogen bond.

Two crystalline phases of aniline have been investigated by a combination of single crystal X-ray diffraction data on aniline-h7 and neutron powder diffraction data on aniline-d7. Phase-I, which is formed on cooling the liquid at ambient pressure, is monoclinic (P21Ic). Orthorhombic (Pna21) phase-II was crystallised at 0.84 GPa at room temperature and structurally characterised at pressures up to 7.3 GPa. The strongest intermolecular interactions in both structures are NH···π contacts and NH···N H-bonds. These interactions occur within layers in both phases, and the phases differ in the way the layers are stacked. The structures of both phases have been obtained under two sets of identical conditions, at 0.84 GPa and 0.35 GPa and studied at room temperature by neutron powder and X-ray single-crystal diffraction. At 0.84 GPa phase-II is the thermodynamically stable form because it has a lower molar volume than phase-I, but as the pressure is reduced the volume of phase-I becomes less than that of phase-II, and at 0.35 GPa phase-II partially transformed into phase-I. PIXEL calculations indicate that the intermolecular interaction energy for pairs of molecules connected by H-bonds is -9 to -16 kJ mol-1 in phase-I and II at 0.84 GPa, but one of these becomes destabilising in phase-II at 7.3 GPa, with an energy of +1 kJ mol-1, making it similar to several compressed CH center···π contacts. The results demonstrate how the hierarchy of intermolecular interaction energies can be manipulated with pressure, driving a H-bond beyond its ambient-pressure distance limit into repulsive region of its potential, and trapping it within a compressed crystal structure.

A combination of single-crystal X-ray (at 110 and 200 K) and high-resolution neutron powder diffraction (110 K only) using perdeuterated samples has been used to re-determine the crystal structure of the well-known acetone–Br2 complex. The results indicate a significant interaction between the carbonyl oxygen and the nearest Br atom leading to a marked elongation of the molecular Br–Br bond. In the earlier widely cited crystal structure, no such elongation was reported. Furthermore, the new structure indicates the presence of additional weaker C–H⋯O and C–H⋯Br interactions. In this simple complex, in which the halogen bonding is strong compared to the hydrogen bonding (C–H⋯O), the former interaction is found to occur close (~12°) to the plane of the carbonyl group, with the C–H⋯Br complementing the stronger halogen bond.

L-Alanine is the smallest chiral amino acid, crystallising in the space groupP212121. 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.

L-Alanine crystallises as a zwitterion in space group P212121 at ambient pressure. The strongest intermolecular interactions are three N–H⋯O hydrogen bonds. The H-bonds link the molecules into puckered layers in the ac plane, which are then stacked along the b axis. 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-d7 to 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 a axis decreases in length more rapidly than the c axis, 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.

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.

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

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 PV term applying to the more efficiently packed phase-II structure.