In this award, funded by the Experimental Physical Chemistry Program of the Chemistry Division, Prof. Benjamin J. Schwartz of the University of California, Los Angeles and his postdoctoral and graduate student colleagues will undertake a combined experimental and theoretical program of research to develop a better understanding of electron transfer and photodissociation reactions in solution through the study of simple, prototypical model systems. The theoretical work will center on molecular dynamics simulations incorporating a rigorous, mixed quantum/classical method (including configuration interaction) that Schwartz and his colleagues have developed, and will focus both on on atomic electron transfer reactions and preliminarily on the photodissociation of simple diatomic molecules such as alkali metal dimers or hydrides. The simulations will be complemented by ultrafast spectroscopy measurements of electron transfer reactions on iodide and alkali metal anion charge transfer to solvent systems. These results should be broadly applicable to a wide variety of solution-phase systems, wherever electron transfer and related chemical reactions take place.
Besides the broader scientific impacts of the research being supported, the young researchers working on this project will gain simultaneous experience in both cutting edge theoretical and experimental methods.
Chemical reactions involve the transfer of atoms or electrons between molecules. Most chemical reactions take place in liquids, but the role of the liquid molecules in chemistry is still poorly understood. For reactions involving the transfer of electrons from one molecule to another , the role of solvent molecules is more important than the identity of the molecules directly involved in the chemical reaction. The purpose of this award is to combine both experimental and theoretical studies to investigate how molecules in liquids control the motion of electrons in chemical reactions. The experimental work focuses on techniques that allow observation of electron transfer events in real time. Since these reactions are controlled by solvent motions, the experiment must be able to 'look' faster than these motions, on time scales of less than one billionth of one billionth (10-12) seconds. The theoretical work is aimed at using computer models to describe these same electron transfer reactions; the models are used to calculate what is measured in the experiments, so that if agreement is reached, the model can be used to obtain a detailed molecular understanding of the role of the solvent. Most of the work in this proposal has focused on studying the properties of a single excess electron in liquids. Using computer modeling, we found that excess electrons in liquid water surround the water molecules and cause them to pack together more tightly. This observation is contrary to previous work, which suggested that excess electrons expel water molecules and thus lie in a tiny bubble or cavity in the liquid. Numerous new calculations and experiments are ongoing in an attempt to resovle this controversy, which is important because excess electrons in liquids are the most important intermediates in electron transfer reactions, and also play a role in the damage that ionization radiation causes to living organisms. We also have performed a series of experiments on excess electrons in liquids. We found that excess electrons in the organic liquid acetonitrile due occupy small cavities, but that they also can attach themselves to two of the acetonitrile molecules. Using laser techniques, we were able to convert electrons between the two forms, revealing a great deal about their molecular nature. We also examined the behavior of excess electrons in solvent mixtures, and found that many solvent mixtures contain 'pools' of one type of molecule embedded in the other (for example, water forms small pools when mixed with the organic solvent tetrahydrofuran). Computer modeling verifies this picture of liquid mixtures as not being smooth; the fact that different liquid moleclues in mixtures clump together has extremely important implications for all sorts of chemical reactions in liquids. The students and postdoctoral fellows funded by this award are already going on to careers that make excellent use of the science they have learned through this research. Dr. Jennifer Casey, who recently graduated, has accepted an assistant professor position at Sonoma State University (part of the California State University system). Argyris Kahros, who is currently writing his thesis, will become a management consultant for the European Central Bank. Dr. William Glover will be on the academic job market this fall, seeking positions at Research-I universities. Devon Widmer and Erik Farr are new students who are both learning their way around the laboratory, developing skills in spectroscopy and the safe handling of air-sensitive chemicals. Both Jennifer and Devon are female, and thus come from underrepresented groups in the scientific fields involved in this research.
