Competing forces in chiral surface chemistry: enantiospecificity versus enantiomer aggregation

Abstract

The enantiospecific adsorption of enantiomer mixtures on surfaces is dictated by two competing forces: the enantiospecificity of adsorption energetics and the propensity of enantiomers to aggregate into homochiral (conglomerate) or heterochiral (racemate) clusters. These phenomena have been studied by measuring the surface enantiomeric excess, ee(s), of chiral amino acid mixtures adsorbed on Cu single-crystal surfaces and in equilibrium with gas-phase mixtures of varying enantiomeric excess, ee(g). Alanine adsorption on Cu{3,1,17}(R&S) surfaces is non- enantiospecific, ee(s) = ee(g), because alanine enantiomers do not interact with either the surface or with one another enantiospecifically. Aspartic acid adsorbs enantiospecifically on the Cu{3,1,17}(R&S) surfaces; ee(s) not equal ee(g), even during exposure to a racemic mixture in the gas phase, ee(g) = 0. Exposure of the achiral Cu{111}} surface to nonracemic aspartic acid, ee(g) not equal 0, results in local amplification of enantiomeric excess on the surface, vertical bar ee(s)vertical bar > vertical bar ee(g)vertical bar, as a result of homochiral aggregation. Finally, despite the fact that the Cu{653}(R&s) surfaces are chiral, the adsorption of aspartic acid mixtures yields vertical bar ee(s)vertical bar > vertical bar ee(g)vertical bar, indicating that homochiral aggregation dominates enantiospecific adsorbate-surface interactions. All of these types of behavior are captured by a Langmuir-like adsorption isotherm that includes competition between enantiospecific adsorption and both homochiral (conglomerate) and heterochiral (racemate) aggregation of chiral adsorbates.