Professors David Sholl and Andrew Gellman seek to understand the characteristics of chiral surfaces that determine enantioselectivity in heterogeneous catalysis. They will pursue this goal using surface science, quantum chemical and Monte Carlo techniques. Their first objective is to study chemisorption and surface reactions on naturally chiral surfaces, such as Ag(643)S, Cu(643)S, Pt(643)S and Ag(643)R, Cu(643)R, Pt(643)R, which have S and R configurations. The differences in desorption energies, adsorbate orientation angles and reaction rate constants will be calculated and measured, in order to isolate and quantify the role of each type of interaction in adsorption-desorption enantiodifferentiation. Enantiomerically pure adsorbates will consist of alkyl chlorides, secondary or tertiary alcohols, alkenes, nitriles, and substituted aromatics. LEED will be used to characterize the surfaces, IRAS to determine adsorbate orientation, and TPD to measure desorption energy. Utilizing semiempirical potentials, molecule-surface and intermolecular interactions will be parametrized and adjusted to experimental measurements of alkyl chloride adsorption. Monte-Carlo simulations will afford the energy and orientation of adsorbed species in order to study surface coverage and adsorbate size effects on enantiodifferentiation. Extended Huckel Molecular Orbital (EHMO) theory will be used in the case of alcohol, ketone and alkene adsorption on naturally chiral Cu and Pt surfaces. The second objective is to modify achiral surfaces (such as Pt(111) and Cu(111)) by adsorption of a chiral template, to model and quantify the types of interaction producing enantiospecific behavior. Templates will consist of R or S configurations of thermally stable molecules bound to the metal through oxygen or methylene groups, and containing side groups such as alkyl, vinyl, phenyl, etc.. Co-adsorbates will consist of reversible bound monofunctional species such as primary alcohols, ketones and nitriles. Interaction energies will be measured using TPD. The interaction between functional groups will be studied theoretically utilizing semiempirical potentials for group-surface and function-function interactions, as well as EHMO theory and Monte-Carlo simulation to obtain adsorbate orientation and desorption energy. The third objective is to investigate the surface chemistry of enantiospecific surfaces. Series of alcohols on well-characterized kinked and stepped surfaces will be used to study structural factors such as relative size of adsorbate with respect to spacing between selective sites. Theoretical prediction of energies and molecular orientation for various combinations of surfaces, templates and co-adsorbents will be used to guide the experimentation. The final goal is to generate fundamental knowledge of the surface and intermolecular interactions that produce enantiodifferentiation. Such knowledge could be used to guide the design of heterogeneous enantioselective catalysts. This theorico-experimental research will involve the training of students.
