Behaviour of intermolecular interactions at extreme pressures

In organic solids, pressures of only a few gigapascals modify and rearrange intermolecular contacts such as H-bonds and van der Waals contacts leading to extensive phase diversity. Applications in this rich area of research include searches for new phases and solvates of pharmaceutical materials; modelling of detonation mechanisms of energetic materials, and modelling of the driving forces of phase transitions. The overarching theme of this PhD thesis is to obtain new, often difficult to isolate, high-pressure polymorphs of small molecules and elucidate the role of intermolecular interactions in their phase stabilities. The need to obtain precise structural information at atomic resolution demands the use of single crystal diffraction methods but scattering intensities are typically low, and the pressure apparatus used in these studies (the diamond anvil cell) results in incomplete data. This can make direct structure determinations for some materials difficult or even impossible. Third generation synchrotron X-ray sources are therefore used for their brightness, high energies, and small focused beams to extract as much structural information from samples as possible. The amino acid L-threonine, characterised by its hydrogen bond network, has been structurally characterised at 22 GPa which is an unusually high-pressure for a complex organic molecule. L-threonine undergoes two isosymmetric phase transitions at ca. 2 and ca. 9 GPa, and a phase transition at ca. 18 GPa that results in a loss of crystal symmetry. Structures of L-threonine were determined by single-crystal X-ray diffraction to 22 GPa; which is the highest-pressure structure ever reported for an amino acid. High-pressure polymorphism in pyridine was studied extensively by single-crystal X-ray diffraction, Raman spectroscopy and neutron powder diffraction. Pyridine has at least three polymorphs in the narrow pressure range of ca. 1 to ca. 2 GPa but the sluggish nature of the phase transitions has made isolating and characterising one of the phases difficult, until now. Here, we used in situ crystal growth in the diamond anvil cell to obtain a stable, diffraction quality single crystal of the elusive phase III and determined its crystal structure for the first time. A mechanism for the transformation is also proposed. The halogen bonded molecule, 4-iodobenzonitrile was studied experimentally by single-crystal X-ray diffraction and Raman spectroscopy up to 10 GPa. 4-iodobenzonitrile undergoes a reconstructive phase change above 5 GPa that results in crystals breaking apart, making it difficult to obtain meaningful diffraction data. Nevertheless, the structure of the new high-pressure phase was determined for the first time by rapidly pressurising a crystal grown in situ to 8 GPa. Crystal lattice and intermolecular PIXEL energy calculations have been validated for use with small organics to 22 GPa, as well as for halogen containing molecules at very high pressures; allowing the roles of stabilising, or destabilising, molecular interactions to be probed in high-pressure polymorphs for a range of organic molecules. Finally, a neon co-crystal was obtained on compression of a Cu2 Pacman complex. This single-crystal structure represents one of only a few published neon containing organometallic structures. Neon resides within the interstitial voids as a result of the Pacman complex reconfiguring to allow neon-uptake. The study shows the interplay between the pressure transmitting medium and crystal structure and we discuss the potential applications of pressure mediated guest-uptake in the Pacman complexes