Understanding the Energetics in Polymorphs

Lead Research Organisation: University of Glasgow
Department Name: School of Chemistry


The factors controlling the ways in which molecules assemble themselves into crystals are still not fully understood. Indeed, despite considerable effort, it is still generally not possible to predict theoretically the crystal structure (i.e. a unique way in which molecules pack themselves together in the repeat units of crystals) of even quite simple molecules with any degree of certainty. Nevertheless there is much research into the factors which control the molecular recognition process, and which allows molecules to pack together in a well controlled regular fashion (i.e build up a crystal). We propose to obtain accurate experimental information about the electrostatic attractions between molecules by studying the electron charge distributions in crystals. This is new experimental information, which is difficult to obtain in other ways. We will obtain this iformation by analysing highly accurate X-ray and neutron diffraction data using the new single crystal beam-line on the Diamond synchrotron source. We propose to study two similar, but different, cases where the same molecule packs in different arrangements in(a) in polymorphs, in which there are two (or more) different packing arrangements for the same molecule which have an independent existence (i.e each form can be isolated separately ) and(b) single-crystal to single-crystal phase transitions, where the two packing forms interchange over a certain temperature range in the same crystal.By studying these two cases where the energy difference between the various packing forms is very small, we hope to gain an insight into the factors controlling how molecules construct themselves into crystals.


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Description We have examined the electron density in a number of chemical compounds showing polymorphism. The method involves analysis of high resolution X-ray diffraction data. From analysis of the resulting multipole models, the detailed electron distribution in the crystals were obtained. From this distribution, the full electrostatic potential can be calculated and hence the energies of the intermolecular interactions in the crystals. This leads to an estimation of the electrostatic contribution of the lattice energies, which in turn allows an estimation of the relative energies of different polymorphs. It was hoped that this method would allow a deeper understanding of why certain molecules possess polymorphic forms, while other molecules do not
Exploitation Route We have obtained neutron diffraction data and high resolution X-ray diffraction data, which is the public domain. With further future refinements of the multipole modelling technique, more detailed information could be extracted form this data by other workers
Sectors Chemicals,Pharmaceuticals and Medical Biotechnology

Description First paper published in March 2014. So far 3 cited by three independent research groups
First Year Of Impact 2014