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ABSTRACT: Enantioselective hydrogenation by heterogeneous catalysts has gained great importance in organic synthesis, chemical engineering and pharmaceutical industry. However, the mechanism of enantioselectivity is not well understood due to the absence of evidence at the molecular level. Recently, the great advance on this topic has been made by applying scanning tunneling microscopy (STM) and other surface characterization techniques to get direct evidence about the enantioselective mechanism of catalysts. Here, the progress on the adsorption mode of chiral modifiers such as cinchona and tartaric acid and the related enantioselective mechanism is reviewed from the viewpoint of surface science. Particularly, some STM works on this topic are summarized. Finally, several key issues for further investigation are outlined.
Topics in Catalysis 05/2005; 35(1):131-139. · 2.62 Impact Factor
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ABSTRACT: Scanning tunneling microscopy (STM) combined with cyclic voltammetry has been employed to investigate the adsorption of cinchonine on Cu(111). Similar to cinchonidine, cinchonine forms a long-range ordered adlayer with (4 x 4) symmetry on the substrate. The structural details on molecular adsorption were obtained by high-resolution STM images. On the basis of the previous results and obtained STM images, the quinoline ring is proposed to lie parallel to Cu(111) and serve as an anchoring group. The chiral quinuclidine moiety extends out of the surface to facilitate the interaction with the prochiral reactants.
Langmuir 05/2004; 20(8):3006-10. · 4.19 Impact Factor
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ABSTRACT: Triptycene molecular orientation has been tuned with a STM tip at a Cu(111) surface in solution from flat, to tilt, to vertical. The tuning is completely bias dependent and reversible. The study is important in the fields of nanoscience and technology.
Chemical Communications 01/2004; · 6.17 Impact Factor
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ABSTRACT: The adsorption of enantiomers and a racemic mixture of tyrosine (tyr) on Cu(111) has been investigated by cyclic voltammetry and electrochemical scanning tunneling microscopy. d-tyr and l-tyr molecules adsorb on Cu(111) and show a (4 × 4) structure. The mirror symmetry between two adlayers is demonstrated by high-resolution scanning tunneling microscope images. The spontaneous separation of racemate into homochiral domains is observed. The result presented here is hoped to supply direct evidence for the chiral adlayer ordering at the solid/liquid interface.
02/2003;
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ABSTRACT: The adsorption mode of cinchonidine on Cu(111) was directly obtained by in situ STM. The molecules were found to adsorb on the substrate surface and form a long-range ordered adlayer with (4 x 4) symmetry. While the quinoline rings lie parallel to Cu(111), the chiral quinuclidine moiety extends out of the surface. The enantioselectivity of catalysts may relate to this special adsorption conformation of cinchonidine on the surface.
Journal of the American Chemical Society 01/2003; 124(48):14300-1. · 9.91 Impact Factor
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ABSTRACT: In situ STM and cyclic voltammetry are employed to investigate the adsorption of organic molecules with different functional groups on Cu(111) in a 0.1 M HClO4 solution. The molecules of cyanuric acid, cyanuric chloride, and 1,3,5-tribromo-benzene adsorb on the Cu(111) electrode surface and form well-defined adlayers with (3 × 3) structure in the double layer potential region. The STM image feature for each molecule is a set of three bright spots, suggesting a flat-lying orientation for the molecules. Structural models are proposed for the three adlayers. The results are compared with those obtained on aromatic and heterocyclic molecular adlayers.
10/2002;
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Angewandte Chemie International Edition 10/2002; 41(18):3408-11. · 13.45 Impact Factor
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Angewandte Chemie International Edition 09/2002; 41(18):3408 - 3411. · 13.45 Impact Factor
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ABSTRACT: Adsorption of pyridine, pyrazine, and triazine molecules on a Cu(111) electrode surface was studied in aqueous solution by using cyclic voltammetry and electrochemical scanning tunneling microscopy (STM). High-resolution STM imaging reveals the details of the adlayer structures and internal structure of individual molecules. The molecules are found to form long-range well-ordered adlayers. The same (3 × 3) structures with a coverage of 0.11 are observed on the three adlayers. All three molecules adopt flat-lying orientation in the double-layer potential region, although the adsorption geometries are different. π electrons are believed to be important in stabilizing the molecules in flat-lying orientation. Three structural models are proposed for the three adlayers.
05/2002;
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ABSTRACT: Scanning tunneling microscopy (STM) has been employed to study the organization of photosynthetic bacteria. A high-resolution spatial image is acquired on a bacteriochlorophyllide c (BChlide c) monolayer prepared by a simple method. BChlide c molecules are found to self-organize a well-defined adlayer on a highly oriented pyrolytic graphite (HOPG) surface. A dimeric structure different from those observed in Chls a, and c and BChlide d is first revealed in the adlayer. A hydrogen bond is assumed to exist in the dimer between the OH groups at C31 positions.
03/2002;
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ABSTRACT: The effect of chemically modified tips on STM imaging was investigated. An electrochemical technique was used to prepare a monolayer of sulfur and iodine on the apex of STM tips. The chemically modified STM tips were first employed to image a 1-octadecanethiol (1-C18H37SH) adlayer on a highly oriented pyrolytic graphite (HOPG) surface. The images acquired with S-modified tips could ascertain the structural detail of thiol groups of the adsorbed molecules, while that acquired with I-modified tips appeared in a low resolution. The contrast variations of STM images are attributed to the changes of electronic states of the tips.
10/2001;
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ABSTRACT: A self-assembled monolayer of l-cysteine was prepared on Au(111) under potential control. Cyclic voltammetry and in situ electrochemical scanning tunneling microscopy were employed to investigate the molecular adsorption and adlayer structure in perchloric acid solution. The molecules chemically adsorb on Au(111) and form a well-defined adlayer. A new structure of (4 × √7)R19° was observed in the double-layer region. A phase transition at the positive potential region resulted in the appearance of a disordered layer. A structural model is proposed to interpret the molecular registry with Au(111) substrate.
08/2001;
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ABSTRACT: Adsorption and adlayer structures of ferrite magnetic anion [Fe(C2O4)3]3−, organic π-electron donor bis(ethylenedithio) tetrathiafulvalene (bedt-ttf) and charge-transfer complex (bedt-ttf)5(H3O)Fe(C2O4)3·CH2Cl2 have been investigated by using electrochemical scanning tunneling microscopy on Cu(1 1 1) electrode surfaces in aqueous HClO4 solution. It was found that [Fe(C2O4)3]3− formed a (4×4) structure with a characteristic propeller shape for each anion and that bedt-ttf adlayer had a (4×6) symmetry. For the complex, [Fe(C2O4)3]3− and bedt-ttf co-adsorbed on Cu(1 1 1) surface and yielded a well-defined two-dimensional network, in which one [Fe(C2O4)3]3− interacted with three bedt-ttf molecules.
Surface Science. 517:52-58.
