Department of Chemistry and Biochemistry

Research Synopsis:
Inorganic Chemistry/X-ray Crystallography:

X-ray crystallography - The use of X-ray crystallography to solve chemical problems is well developed. Using extensive low temperature X-ray diffraction data it is possible to go far beyond the simple determination of molecular structures. We are able to map the entire electron density distribution in molecules and thus obtain information on chemical bonding previously only available from quantum mechanics. Any one-electron property is available given the atomic arrangement and the electron density distribution. In the simplest cases we may obtain atomic charges, or the orientation of lone pairs. This information is also used to map the molecular electrostatic potential, which tells us the direction of electrophilic or nucleophilic attack, or how a molecule would interact with a biological receptor (molecular recognition). From the complete topological analysis of the electron density, we are able to determine energy density distributions, and the energy of intermolecular interactions, not only the dissociation energy of hydrogen bonds, but also that of weaker interactions, e.g. O¡­O, O¡­N, H¡­H. Current studies examine the deformation of the pi-electron density due to pyramidalized sp2 carbon in a series of organic molecules, the charge density distribution in high energy (explosive) molecules to understand shock sensitivity, and the electrostatic potential of estrogens in order to understand binding affinities to the estrogen receptor, and the initiation step for developing breast cancer. Variable temperature diffraction experiments allow us to determine the thermal expansion tensors of materials which, coupled with compressibility studies, lead us to equations of state.

 

In addition, we are developing new techniques to exploit the cutting edge diffraction hardware available to us, e.g. we have recently described a new cooling device using liquid helium and have benchmarked a new image plate detector for the rapid measurement of electron density quality data.

 

Much of our research is carried out in collaboration with other groups, either in national laboratories or at other universities around the globe.