Dependence of the adsorption of chiral compounds on their enantiomeric composition

Abstract

The adsorption of two different chiral molecules on platinum surfaces has been explored as a function of enantiomeric composition. In the first case, the saturation monolayers obtained by the adsorption of 1-(naphthyl)ethylamine (NEA) from CCl4 solutions were characterized in situ by reflection–absorption infrared spectroscopy (RAIRS). It was found that racemic mixtures yield different IR spectra than those obtained from enantiopure monolayers, a behavior that was interpreted as the result of the formation of racemate pairs via hydrogen bonding at the amine moiety also responsible for bonding to the surface. NEA adsorption under these conditions is reversible and can be modified by subsequent exposures to solutions of different chiral compositions, but that appears to take place only via changes in the relative fractions of enantiopure versus racemic domains on the surface; no other enantiomeric ratios are apparent in the IR data. The second study focused on the uptake of propylene oxide (PO) on Pt(111) under ultrahigh vacuum (UHV) conditions. In that case, racemic monolayers show densities up to ~20% lower than those obtained with one single enantiomer. This can be explained by kinetic arguments, since data from isothermal molecular-beam experiments indicated that the PO sticking coefficient depends on the chirality of the incoming PO molecules relative to that of the neighboring adsorbed PO species. Monte Carlo simulations could reproduce the experimental data by assuming adsorbate-assisted adsorption and enantiospecific adsorption geometries for molecules impinging on sites adjacent to previously adsorbed surface species.