Adsorption of chiral molecules on naturally chiral surfaces and chirally templated surfaces.

摘要

Naturally chiral surfaces can be prepared by exposing high Miller index surfaces from a bulk achiral fcc crystal, such as Cu. These high Miller index surfaces have terrace-step-kink structures where the kink sites are chiral. On most of the naturally chiral surfaces the terraces, steps and kinks are formed by the (100), (110) and (111) microfacets. There are six types of chiral surfaces depending on which of the three microfacets forms the terraces, steps and kinks. In this project, one pair of chiral surfaces was chosen from each of the six types of chiral surfaces to be studied. A special pair of chiral surfaces Cu(531)R&S where the length scale of (100), (110) and (111) microfacets are equal was also studied. Low energy electron diffraction (LEED) and X-ray diffraction (XRD) technique were used to identify the surface structure and chirality. Temperature-programmed desorption (TPD) was used to investigate the enantiospecific adsorption of chiral (R)-3-methylcyclohexaone molecule on naturally chiral surfaces. The desorption energy of (R)-3-methylcyclohexanone from the chiral kink sites on the seven pairs of chiral surfaces were observed to be different in all cases. The difference varies from DeltaDeltaE des = 0.5 to 1.0 kJ/mole. This serves as direct evidences to the enantioselectivity of (R)-3-methylcyclohexanone on naturally chiral Cu surfaces.;Chirally templated surfaces were achiral surfaces templated by chiral molecules. The chirality of these surfaces was imparted by the adsorption of chiral molecules. In this work, Cu(100) and Cu(111) single crystal surfaces templated with a few different chiral molecules were studied by co-adsorption of chiral probes. These systems include propylene oxide on 2-butoxide templated Cu(100), (R)-3-methylcyclohexanone on 2-butoxide templated Cu(100), propylene oxide on 2-butoxide templated Cu(111), propylene oxide on 2-butanol templated Cu(111), (R)-3-methylcyclohexanone on 2-butoxide templated Cu(111) and propylene oxide on alaninol templated Cu(111). None of these systems displayed any enantioselectivity despite the fact that enantioselective adsorption was found on Pd and Pt surfaces templated with similar chiral molecules. This suggests that the nature of the substrate plays a very important role in the enantioselectivity on chirally templated surfaces.