How do molecular crystals form?

maps of diffuse

Maps of diffuse scattering in the (a) hk0 and (b) h1/2l planes of para-terphenyl. Key features that relate to the molecular ordering have been highlighted.
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How does the ordering in a crystal arise from the interactions present? Solving the structure shows what the structure is, but not how it arises. Such information is crucial in trying to predict crystal structure from the chemistry of the molecule, a long-held aim in pharmaceutical research.

The influence of a molecule on the positions of its neighbours results in broad scattering features in a neutron diffraction pattern known as diffuse scattering.  By modelling this scattering it is possible to determine the key molecular interactions and cooperative molecular motions and how these lead to the ordering of the molecules. 

We used diffuse scattering measured on SXD to examine the intermolecular interactions in deuterated para-terphenyl, C18D14 (D means the hydrogen has been replaced by its heavier isotope, deuterium). It was found that intramolecular and intermolecular interactions, and the molecules themselves, can be thought of as acting as nanoscale mechanical linkages. This work shows that we can use the analysis of diffuse scattering to build up a picture of how molecular interactions lead to molecular ordering.

DJ Goossens (Australian National University), MJ Gutmann (ISIS)

Research date: September 2009

Further Information

Contact: DJ Goossens, goossens@rsc.anu.edu.au

Further information: DJ Goossens and MJ Gutmann, Phys Rev Lett 102 (2009) 015505

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