Research Guides

Department of Chemistry University of Oxford

Dr Kirsten Christensen

With higher intensity sources available today more and more complex diffraction patterns are observed, showing us that we need to extract this additional information to be able to tell the full story about a material’s properties. This is enhanced by the chemist synthesising more and more advanced materials. Our research is focused on several areas that will establish a fundamental understanding of modulation in molecular materials e.g. their formation and properties. This knowledge can be put to immediate use in Crystal Engineering, and will also affect research in pharmaceuticals, developments in molecular modelling and crystal packing predictions.
 

Trimesic Acid (TMA) and related molecules.

Trimesic Acid (TMA) is a fairly simple small molecule that can form a variety of structures. The simplicity of the molecule itself makes it a good model system as there is only one form of molecular interaction e.g. hydrogen bonding, and the structural complexity comes only from the molecular arrangement. At the same time TMA lends itself to substitution that allows exploring the effect of adding other interactions and investigate how these affect the modulation.
Reconstructed precession image of the h7l-layer of trimesic acid (left) and schematic of TMA (right) (R = nothing; R' =H). Modifying R and R' gives access to a range of interesting materials.

Crystal structures with Z’ > 1.

It is often found that organic molecules pack with more than one unique molecule in the unit cell. We do know a few examples of higher Z’ structures that are better described using a modulation approach rather than a super cell approach. We need to search the structure databases to establish how many of the reported high Z’ structures would benefit from this alternative description.

The boundary between diffuse scattering and modulation.

During crystallization a variety of different molecular interactions are at play, and they determine the final structure. These interactions are often competing, and depending on their mutual strengths, this can result in various degrees of order, that in turn can give rise to diffuse scattering or well-defined reflections in the diffraction pattern. It is important to investigate how order – part order –disorder affects the diffraction pattern and how this information can give us vital information concerning packing of molecules. The balance between the interactions is very fine, and can often be changed by external parameters such as temperature or pressure. This can result in a solid state order – disorder transformation, with a transition between diffuse scattering and modulation. It is also possible to disrupt the balance by weakening specific interactions; this can be done by increasing the distance between the interacting parts of the structure. In this way it is possible to determine whether it is long distance interactions or local environment that determines how the molecules pack in the crystal.
               
Part of a recontructed precession image of the hhl-layer of a ferrocene anion receptor illustrating structured diffuse scattering.
 

Phase transitions and polymorphs in higher dimensions.

It has been suggested that some phase transitions and certain polymorphic systems can best be understood in higher dimensions. Verifying these proposals could revolutionize our understanding of phase transitions and polymorphism - areas critical to the pharmaceutical industry. If we can understand what happens during a phase transition and link polymorphs together in higher dimensions we will gain a better understanding for solid-state reactions.
 

Modulation by design.

The above gained information raises the question if it is possible to design solid state molecular structures that are modulated by design. Can we choose a combination of metal and organic molecules, so modulation will occur? If we can start to design the solid state we can also adjust properties to suit our needs. 
 

I19, the small-molecule single-crystal diffraction beamline at Diamond Light Source (2012) H. Nowell, S. A. Barnett, K. E. Christensen, S. J. Teat & D. R. Allan, J. Synchrotron Rad. 19, 435-441.