Small molecule X-ray crystallography (SMX), the study of a tiny single crystal using X-ray diffraction, is often said to fail in its ability to accurately mirror chemical processes and that single crystal analysis allows for only a snapshot of a chemical process at either the beginning stage or after a chemical modification has taken place. These modifications normally come in the form of different crystals formed at different stages of a chemical reaction and often do not give a true representation of the overall system.
It is therefore unusual to monitor the dynamic progress of a reaction or interaction (e.g., the permeation of gases through a zeolite) using small molecule X-ray crystallography. The ability of the synchrotron radiation source to allow small molecule work to be undertaken at crystal sizes which are of an order magnitude smaller than conventional sources can accommodate. The smaller crystal size allows for the study of samples too small for regular laboratory sources and also provides for a preferential surface to bulk ratio, aiding processes such as gas diffusion. Combining this advantage together with both novel and familiar chemical systems alters the traditional and stereotypical views of this long standing technique to be changed.
SMX is one of the most powerful experimental techniques available to the scientist. The results from which have ramifications in all disciplines. The exquisite results produced, in collaboration with DL through work on Station 9.8 at the SRS by Matt Rosseinsky's group, has captured this change in thinking of SMX and has been recognised through a recent Science publication.[Science, 315, 2007, 977-980] The work is based on a relatively simply metal organic framework (MOF) and employed an experiment utilising variable temperature SMX that reveals a remarkable and perhaps unique in situ reaction. The work highlights for the first time the ability to follow a dynamic process, that of a reaction pathway, in a single sample. A complete characterisation of the “reactant†and “product†states along with an idea of the the direct through space route for the reactant moieties was possible by SMX. SMX is unique as a technique in that it can provide all the three dimensional information for the molecules in a single crystal in a single experiment (including their relative spatial positions) and provide this information relatively quickly (quicker than an episode of Star Trek).
Figure 1 : Bipy is aligned toward the Cobalt centre and the water is on the cobalt. The Methanol is also sat in the channel (Figure 2 :The water has left and the bipy has coordinated, you get two waters leaving two bipy binding on every other cobalt on the other cobalt you get two methanol instead)
The reaction was resolved on the atomic scale, crystallographically, but it was also visible at a macroscopic level in the form of a simple colour change from pink to purple. The story, however, does not end there as the same MOFs are also being developed and explored for use as a hydrogen storage medium and have repercussions on novel fuel storage solutions, energy and the climate.[See SRS annual report 2005-2006, Nasty things happen to nice crystals for more information of dynamic gas studies undertaken at the SRS]
Energy conservation and novel fuel sources are not the only socially relevant experiments envisaged with the environmental gas cell (picture of cell). Other developments include the controlled release of very low concentrations of gases, such as CO and NO. The controlled release of a simple gas from a robust storage medium such as a zeolite could be used in medical applications. Russell Morris's group at St. Andrews is currently exploring this area of science in collaboration with Daresbury laboratory and the environmental gas cell.
SMX is clearly being revitalised thorough the development of new combined techniques. This is exemplified by the recent award of a facility development grant to develop new equipment, techniques and to support a PDRA (Dr. Alexandra Griffin) in non-ambient SMX at DL. The grant led by Dr. J E Warren (CCLRC DL) and Prof. Paul Raithby (Bath University), also boasts an impressive line-up of academic co-investigators, has the key aim of providing a knowledge transfer of these state of the art techniques from the SRS to DLS. This would bring DLS up to the established cutting edge level at day one and allow the SMX station there (ID19) to follow in the footsteps of the giant leaps made by SMX at DL.
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