Understanding the Energetics in Polymorphs

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.