Equilibrium Adsorption of d- and l-Alanine Mixtures on Naturally Chiral Cu{3,1,17}^R&S Surfaces

Equilibrium adsorption of gas phase mixtures of d - and l -alanine (Ala) onto the naturally chiral Cu{3,1,17} R&S surfaces has been studied by both experiment and DFT-based modeling. Isotopically labeled * l -Ala (HO 2 13 CCH(NH 2 )CH 3 ) and unlabeled d -Ala allow mass spectrometric enantiodifferentiation of the adsorbed species during temperature-programmed decomposition, following equilibrium adsorption. Measurements of the relative equilibrium coverages of d - and * l -Ala on the Cu{3,1,17} R&S surfaces, θ D/R /θ *L/R = θ *L/S /θ D/S, at gas phase partial pressure ratios of P *L / P D = 1/2, 1, and 2 indicate that the d -Ala and * l -Ala conglomerate phases are more energetically stable than a d * l -Ala racemate phase, but that their adsorption energies are not measurably enantiospecific, ΔΔ E DL ≈ 0. Although the DFT simulations provide a self-consistent structure of Ala overlayers on Cu{3,1,17} R&S they overestimate the enantiospecificity of the adsorption energetics.