The Effect of High Pressure on Crystal Structure Topology
Wood, Peter Andrew
This thesis describes the effects of the application of high pressure to single crystals of small organic compounds. A range of different structural analysis techniques have been used with the emphasis on whole molecule interactions rather than atom-atom contacts. A study of the effect of pressure on the crystal structure of salicylaldoxime showed that the size of a pseudo-macrocyclic cavity within the structure is tuneable by compression. This cavity determines the reactivity of salicylaldoxime as a ligand, when deprotonated it is known to preferentially bind Cu^2+ ions over other cations in a bis(salicylaldoximato) complex due to the compatibility between the cavity size and the ionic radius of Cu^2+. Further compression studies on a range of substituted salicylaldoximes with different ambient cavity sizes showed that the application of pressure consistently decreases the cavity size across the whole series. Variation of substituent and the pressure yields cavities which span the covalent radii of many of the 1st transition series metal dications. This should allow the selectivity of metal extraction to be tuned using pressure. Computational studies of lattice energies and conformational energies in the compression studies of L-serine and 3-aza-bicyclo(3.3.1)nonane-2,4-dione have shown that significant molecular distortions can occur during compression of a crystal structure below 10 GPa. L-serine shows different conformations between phases with an energy difference of 40 kJ mol-1, whereas the conformation of 3-aza-bicyclo(3.3.1)nonane-2,4-dione is seen to distort within the same phase. Analysis of a database of compression studies using Hirshfeld surfaces has highlighted the fact that all different types of intermolecular interaction have a lower limit for compression, at least in the pressure regime below 10 GPa. These studies, along with theoretical calculations, have suggested a lower distance limit for H…H contacts of 1.7 A. This is potentially very useful for prediction of the effects of compression as H…H contacts are almost universal across small organic crystal structures.