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 ((HO2CCH)-C-13(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, theta(D/R)/theta*(L/R) = theta(*L/S)/theta(D/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, Delta Delta E-DL approximate to 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.