Polymorphism and disorder: subtle effects controlling solid state molecular form
Studying molecular materials on SXD at ISIS
There is increasing interest in the development of molecular materials with optimised physical properties, using directed molecular design and the control of intermolecular interactions, notably hydrogen bonds, to self-assemble molecular units into favourable configurations. A complication that often arises when developing solid state molecular materials is that of polymorphism, where a compound can crystallise in more than one form. This can be of key importance, for example in the pharmaceutical industry. Co-crystallisation is often used in preparing solid state molecular materials. In some cases this yields a molecular complex of an active material with an inert co-molecule, often improving solubility and of importance in drug delivery, while in others such crystallisation attempts can yield unexpected new polymorphs. Recent work using Sxd at ISIS generated such an accidental new polymorph of 3-fluorobenzoic acid when co-crystallised with 4-acetylpyridine. This polymorphism results in patterns of molecular order and disorder that produce characteristic neutron scattering patterns and which underpin the subtle energetics of the formation of the two crystal forms.
LH Thomas, G Craig (University of Glasgow), MJ Gutmann (ISIS), CC Wilson (University of Glasgow)