(73a) Chiral Surfaces and Enantioselectivity

High Miller index metal surfaces with structures lacking mirror symmetry are naturally chiral and have enantiospecific interactions with chiral adsorbates. This set of surfaces or structures spans the interior of the stereographic projection of surface structures. In the case of face-centered-cubic lattices the chiral surfaces are those that have kink-step-terrace structures. The kinks occur at the intersections of the three low Miller index microfacets that project out of the bulk and the handedness of the structure is dictated by the rotational orientation of the microfacets.

Past work on the Cu(643)^R&S surfaces has shown that the adsorption of R-3-methylcyclohexanone is enantiospecific in the sense that its desorption energies from the chiral kinks are dependent on the handedness of the kink. In addition, we have shown that the orientation of the R-3-methylcyclohexanone adsorbed at the kinks is dependent on the chirality of the kinks.

The kinks, steps and terraces on the high Miller index surfaces are formed by the (100), (110) and (111) microfacets. Thus there are, in principle, only six types of kinked surfaces dictated by which of the three microfacets forms the kink, step and terrace. The different surfaces within the stereographic projection differ in the lengths of the steps separating the kinks and in the widths of the terraces separating the steps. Things may in fact be even simpler, if the enantiospecificity of the adsorbate-surface interaction is dictated only by the relative orientations of the three microfacets. This problem has been addressed by studying the adsorption of R-3-methylcyclohexanone on a set of surfaces spanning the entire stereographic projection.

R-3-methylcyclohexnanone adsorption and temperature programmed desorption has been studied on a set of 9 achiral Cu surfaces and on both the R- and S-enantiomers of a set of 7 chiral Cu surfaces. Six of the chiral surfaces have been chosen to fall within the six zones of the stereographic projection having surface structures formed of the six combinations of kink, step and terrace formed by (100), (110) and (111) microfacets. The seventh surface pair of chiral surface is the Cu(531)^R&S pair which are formed of single unit cells of the (100), (110) and (111) microfacets.

The desorption spectra of R-3-methylcyclohexanone can be resolved into components arising from desorption from the kinks, the step edges and form the terraces of each of the surfaces. The desorption from the kink sites is enantiospecific in the sense that the R-3-methylcyclohexanone has different desorption energies from the two enantiomers of the surfaces. The enantiospecificity of the desorption energies varies from ΔΔE_des = 0.2 to 1.0 kJ/mole.