Bonding and assembly of the chiral amino acid S-proline on Cu( [formula omitted]): the influence of structural rigidity

Abstract

The adsorption of S-proline, vacuum deposited on a clean Cu(1 1 0) surface held at room temperature, has been investigated using reflection absorption infrared spectroscopy and low energy electron diffraction. Throughout the adsorption regime at 300 K, a (4×2) phase is formed in which the molecule bonds to the copper surface in an anionic form, via the oxygen atoms of the carboxylate (COO −) functionality and the nitrogen of the imino (NH) group which forms part of a pyrrolidine ring. Both carboxylate oxygen atoms are found to be equidistant from the surface while the pyrrolidine ring is held at a small angle to the surface plane. Unlike other amino acids, the molecule does not show a range of different phases with varying coverage or temperature conditions and does not significantly reorient at high coverage or on annealing. This difference in behaviour is attributed to the structural rigidity of the molecular structure which severely restricts the degrees of freedom generally associated with amino acid end and side groups. As a result, the proline is forced to adopt the same footprint at the copper surface at all coverages, which also requires slightly more physical space than other amino acids. The bonding of the proline layer to Cu(1 1 0) is strong, creating a robust adlayer which is stable up to 450 K, after which the molecule dehydrogenates and, subsequently, decomposes. Finally, this strong and defined mode of interaction with the metal surface must play an important role in the use of this molecule as a chiral modifier in heterogeneous diastereoselective catalysis, where its attachment to an organic molecule determines the adsorption and orientation adopted by the latter at a surface, thus creating a strong inequality in the probability of hydrogen addition at the two prochiral faces of the reactant.