Research Scholar
Christopher Williams, Department of Chemistry
John M. Herbert, Faculty advisor
Biography
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Christopher Williams was born in England and did his undergraduate degree at Durham University. As part of his final year research project with Dr. Stuart Althorpe he used time-dependent scattering techniques to investigate the reactive differential cross-sections of the reaction D + H2 ® H + HD. For his Ph.D. he moved to Oxford University and worked on the application of these techniques to atmospheric reactions under the supervision of Professor David Clary. He is currently working with Professor John Herbert on dipole bound anions and developing the major electronic structure package QChem.
About the Research
When bulk water is subjected to ionizing radiation electrons can be ejected from core and valence orbitals of water molecules. These free electrons can be stabilized by the surrounding water molecules solvating them. The resultant solvated electron is important in many chemical reactions. For example, solvated electrons produced by gamma radiation in the water coolant of nuclear reactors can catalyze the corrosion of the stainless steel container. They are also continuously generated in the earth's upper atmosphere where their broad and intense absorption from the upper visible to the lower UV region of the electromagnetic spectrum has a considerable influence on the type and intensity of radiation received by the Earth's surface.
The structure of the solvation shell about the electron is hard to examine with conventional spectroscopy as a result of its transient nature and the considerable statistical averaging that takes place in bulk systems. However, new experimental techniques have become available that allow the probing of water cluster anions with a selected number of water molecules in a low temperature molecular beam, thus allowing a picture of the solvation shell to be built up one water molecule at a time. These experiments have identified at least two separate types of water clusters present in the molecular beam for n=11-200, one of which binds an electron more strongly than the other. It has been proposed that the more strongly binding type are cavity states where the water molecules cluster around the electron and stabilize it with their partially positively charged hydrogen atoms whereas the less strongly bound type bind the electron on the surface of the water cluster. However, calculations where the electron is treated quantum mechanically but the water cluster is treated according to the laws of classical mechanics (QM/MM) have suggested that unless an electron is formed deep within a large cluster it will migrate to the surface and cavity states will not be formed for any appreciable time.
In this study an analysis of the potential used in the QM/MM simulations to represent the interaction between the excess electron and the water cluster finds that it neglects the mutual correlation between the fluctuations in the charge distribution of the excess electron and the water cluster. This dispersive electron correlation produces an additional attraction between the two species and hence an increase in the Vertical Electron Binding Energies (VEBEs). For water cluster anions with n=6-25 VEBEs have been calculated for fixed geometries using perturbation theory to the second order (MP2) to take electron correlation into account. It has been found that in cavity type isomers the electron correlation effect is greater resulting in a greater proportion of the binding energy being due to dispersion. This would result in a preferential formation of surface states in QM/MM simulations. It is demonstrated how an alternative model can be used to take into account dispersion.
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