Development of Facilities for Single-Crystal Diffraction at High Pressure on KOALA

The way in which a solid responds to changes in pressure and temperature reveals a wealth of detail about the nature of intermolecular interactions, and their influence on thermodynamic properties. When pressure is applied to a crystalline solid it may persist in a compressed form of its ambient pressure phase or it may undergo a transition to a new phase, providing detailed experimental data on the deformability of different classes of intermolecular interactions (e.g. H-bonds, halogen bonds, stacking interactions) and on 'energy landscapes' in solids. In no area is this information of more importance than in molecular organic solids: the pharmaceutical industry spends billions of pounds per year on discovery of new solid forms such as polymorphs and hydrates of drugs and on modelling and understanding their properties. The design of new functional molecular materials via crystal engineering also depends critically on control of intermolecular interactions. The results are even applicable to planetary sciences, where the thermal and compressibility behaviour of 'molecular ices' yield important data for modelling deposits in astrophysical settings: carbon is the fourth most abundant element in our galaxy, and it is thought that the largest repository for this element is within polycyclic aromatic hydrocarbons. Many classes of intermolecular interaction involve hydrogen atoms: hydrogen bonds are an obvious example, but hydrogen atoms dominate the outer surfaces of most organic molecules and they play an important role in dispersion and electrostatic interactions. In this project we will explore the effect of high pressure on interactions involving hydrogen in organic crystals at pressures between 1 000 and 100 000 atm. We will focus on classes of weak contacts that are increasingly being invoked as structure directing interactions. Examples are interactions such as CH...X 'hydrogen bonds' where X = a halogen, oxygen or nitrogen, stacking and CH...pi interactions which occur in polyaromatic hydrocarbons, and new classes of interaction such as halogen, chalogen, pnictogen and tetrel bonds, which have also been invoked as structure directing interactions. The response of crystalline materials containing these interactions will be studied at high pressure using single crystal and powder neutron diffraction, and the results interpreted with the aid of semi-empirical and ab initio computational data. A key element of our approach will be to determine the changes in energy, enthalpy, free energy and entropy not only computationally but also experimentally, using a new method that we have developed that transforms variable-pressure and temperature crystallographic data into experimental thermodynamic information. Neutron diffraction is necessary for this work because it accurately locates H-atoms and because the penetrating nature of neutron radiation means that complete, high-quality data can be obtained for samples in elaborate extreme conditions environments. Our work, which will also help develop new techniques and experimental methodologies at central facilities, is relevant to EPSRC priority areas such as functional materials and materials characterisation, research areas including analytical science, energy storage and condensed matter, the Physical Sciences and Manufacturing the future themes and to the Directed Assembly of Extended Materials with Targeted Properties grand challenge.