(44d) Enantiospecific Desorption of Propylene Oxide From Chirally Modified Surfaces

One approach to fabricating chiral surfaces for the production of pure enantiomers is through the adsorption of enantiomerically pure chiral modifiers onto an achiral surface. Our objective is to probe the enantiospecific behavior of copper surfaces chirally modified by amino acids, using temperature programmed desorption by measuring the difference in desorption temperatures and hence desorption energies between the R- and S-enantiomers of a chiral probe molecule from these modified surfaces.

S- or R-propylene oxide was adsorbed, as the chiral probe, onto the L-alanine templated Cu(110) and L-lysine restructured Cu(100) surfaces to detect possible enantiospecific desorption. No enantiospecificity was detected on the L-alanine templated surface at either low or high alanine coverages. On the L-lysine/Cu(100) surface, the system was found to exhibit enantiospecific behavior over a narrow range of lysine coverages in which S-propylene oxide desorbed at a higher temperature than R-propylene oxide from the same L-lysine modified Cu(100) surface. In other words, S-propylene oxide adsorbs more strongly on the L-lysine/Cu(100) surface than R-propylene oxide. This observed enantiospecificity, however, was found to be critically dependent on the lysine modifier coverage. The enantiospecific temperature difference between S- and R-propylene oxide from the L-lysine/Cu(100) surface becomes larger with increasing L-lysine coverage before reaching a maximum of ~ 5.5 K. The enantiospecificity then decreases with increasing lysine coverage until no more available sites on the surface can be occupied by incoming propylene oxide.