So much of our knowledge and understanding of the world around us comes from a consideration of the structures of molecules. But how do we know what is happening at a molecular or atomic level? Diffraction techniques can give us directly information such as the geometry that a molecule adopts, whether that geometry changes depending on the physical state of the substance, and what products are yielded when two or more molecules react.

My research uses electron diffraction as a probe to study the structures of chemical species undergoing changes that occur on a variety of timescales. Electrons are particularly well suited to studying structures in the gas phase, where the lack of influence from neighbouring molecules (an issue with solid-state techniques) allows model systems to be studied. Electrons are efficient probes of molecular structure, with a high scattering cross section and a low proportion of inelastic scattering (which contains little or no structural information).

Because electrons are charged they repel one another. This has consequences when very short pulses of electrons are required, and the theoretical limit of temporal resolution in a laboratory is 0.5 picoseconds. However, it is possible to break through this barrier using electrons with very high energies. Such electrons are routinely used in accelerator physics, where they are sped up until X-rays are emitted. I will ultimately harness these electrons to give pulses with a length of around 100 femtoseconds; when used in a diffraction experiment these electrons will allow the formation and breaking of chemical bonds to be observed.

SELECTED RECENT PUBLICATIONS

1. Experimental equilibrium structures: application of molecular dynamics simulations to vibrational corrections for gas electron diffraction. D. A. Wann, A. V. Zakharov, A. M. Reilly, P. D. McCaffrey and D. W. H. Rankin, J. Phys. Chem. A, 2009, 113, 9511-9520.
2. Structures and aggregation of the methylamine-borane molecules, Me_nH_3-nN·BH₃ (n = 1-3), studied by X-ray diffraction, gas-phase electron diffraction, and quantum chemical calculations, S. Aldridge, A. J. Downs, C. Y. Tang, S. Parsons, M. C. Clarke, R. D. L. Johnstone, H. E. Robertson, D. W. H. Rankin and D. A. Wann, J. Am. Chem. Soc. , 2009, 131 , 2231-2243.
3. Primary phosphines studied by gas-phase electron diffraction and quantum chemical calculations. Are they different from amines? R. Noble-Eddy, S. L. Masters, D. W. H. Rankin, D. A. Wann, H. E. Robertson, B. Khater and J.-C. Guillemin, Inorg. Chem. , 2009, 48 , 8603-8612.
4. Accurate structures from combined gas electron diffraction and liquid crystal NMR data; the importance of anisotropy of indirect couplings for 1,4-difluorobenzene. E. M. Brown, P. D. McCaffrey, D. A. Wann and D. W. H. Rankin, Phys. Chem. Chem. Phys., 2008, 10, 738-742.
5. Dimethylalkoxygallane incorporating a donor-functionalised alkoxide: the monomeric gas-phase structure. C. E. Knapp, C. J. Carmalt, P. F. McMillan, D. A. Wann, H. E. Robertson and D. W. H. Rankin, Dalton Trans., 2008, 6880- 6882.

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