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

Abstract

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 (HO213CCH(NH2)CH3) 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, θD/R/θ*L/R = θ*L/S/θD/S, at gas phase partial pressure ratios of P*L/PD = 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, ΔΔEDL ≈ 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.