Gas electron diffraction
Electrons have both wave and particle properties, so they are diffracted by atoms and molecules, but can also be observed as distinct particles. Both properties are used in the electron diffraction experiment, which gives information about distances between atoms in gas-phase molecules. In Edinburgh we use this method to study volatile compounds, both inorganic and organic.
Electron diffraction is the most widely applicable technique for determining structures of gaseous molecules. However, locating light atoms in molecules which contain heavy atoms can be difficult, and it is also limited in resolving interatomic distances which are similar to one another. In both these respects it is complementary to liquid crystal NMR spectroscopy. We have therefore developed ways of performing combined analyses of the data from both these experiments, and from rotational spectroscopy. We can thus derive structures which are much more accurate and complete than can be obtained using one technique by itself.
Combining all these experimental data with flexible restraints derived from ab initio or DFT calculations is a further technique developed here, known as the SARACEN method. This allows us to determine accurate structures of much larger and more complex molecules.
We were recently faced with a problem where the very large-amplitude modes of vibrations of some molecules made it difficult to determine their structures accurately. In those cases we used state-of-the-art molecular-dynamics calculations to approximate experimental equilibrium structures.
GED can be problematic when performed on compounds that have low volatility or a low vaporisation rate. Work has recently been done to develop new nozzles and reservoir systems to overcome these problems.
State-specific electron diffraction. New apparatus which features electronic recording of scattering data, a supersonic molecular beam, a high-intensity electron beam and facilities for laser excitation of the molecular beam has been constructed and is being tested.
Structures of molecules in different phases are not necessarily the same, and sometimes they are strikingly different. Determination of crystal structures of compounds which are liquids or gases at room temperature is another Edinburgh speciality, and wherever possible we compare gaseous and crystalline phase structures.
For information about structures determined world-wide by electron diffraction, see the Gas Electron Diffraction Information Service.