Pairing of (R,R)-2,3-butanediol molecules on a Si(001) surface

We used a combination of scanning tunneling microscopy and DFT calculations to determine the reaction characteristics of (R,R)-2,3-butanediol (BD) molecules on a Si(001)-2 × 1 surface. The BD molecules were adsorbed onto the surface as either single, isolated molecules or paired configurations. As single adsorbed molecules, the BD molecules preferred to bridge adjacent Si dimers through oxygen atoms while preserving their chirality. The adsorption probability depended on the molecular conformation; the CH₃-gauche conformer is more stable than the CH₃-anti conformer. When BD molecules were adsorbed as paired configurations, BD pairs formed between the CH₃-gauche conformers with several types of configurations. Total energy calculations for the pairs revealed that they are stabilized by a direct molecular interaction and/or a Si-mediated interaction, which can be analyzed as the strain energy involved with BD adsorption and the dangling bond reactivity near the adsorbed BD molecule. These results suggest that alcohol-type chiral molecules are useful agents for homochiral modification of Si surfaces and that the surface-mediated, as well as the direct intermolecular, interactions on the surface may play an important role in enantioselective separation on a Si surface.