One of the most successful ways of inducing enantioselectivity in a heterogeneous catalytic system is by the adsorption of chiral "modifier" molecules on the reactive metal surface. However, little is known about the nature of the active sites present on the modified metal surface and how such modifiers bestow chirality to an achiral metal surface. In this paper we report the behavior of R,R-tartaric acid adsorption on a Cu(110) surface using high-resolution surface analytical techniques. R,R-Tartaric acid is known to be an extremely successful modifier molecule for the enantioselective hydrogenation of methyl acetoacetate, the simplest β-keto ester, to the R-enantiomer of the product molecule methyl 3-hydroxybutyrate. A combination of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and Fourier transform reflectionabsorption infrared spectroscopy (FT-RAIRS) techniques has allowed us to demonstrate that a complicated adsorption phase diagram exists for this system. A rich variety of ordered overlayer structures are produced, in which preferred molecular forms, bonding and orientations of the chiral molecules are adopted, dependent on coverage, temperature and time. These different adlayers will clearly play a different role in the enantioselective reaction. Of particular interest is the fact that under certain conditions, the 2-dimensional order of the R,R-tartaric acid adlayer destroys all symmetry elements at the surface, leading to the creation of extended chiral surfaces! Such chiral surfaces may be an important factor in defining the active site in heterogeneous enantioselective reactions.
We have determined the azimuthal orientation of an adsorbate on a metal surface from an intramolecular-transition-derived feature in reflectance anisotropy spectroscopy (RAS). Adsorption of 9-anthracene carboxylic acid onto p͑2 3 1͒O͞Cu͑110͒ led to an ordered structure with a strong (2%), derivativelike feature at 4.5 eV. Fresnel theory predicts the measured intensity, functional behavior, and sense of the RAS signal for the molecule aligned along [110]. IR measurements confirm that the molecular plane is perpendicular to the surface and STM measurements support the azimuthal orientation. We reassign the sense of the clean Cu (110) surface RA spectrum. [S0031-9007(98)06135-3] PACS numbers: 68.45. 33.20.Kf, 73.20.At, Reflectance anisotropy spectroscopy (RAS), also known as reflectance difference spectroscopy (RDS) using linearly polarized, visible light to detect azimuthal dependence in surface structure, has evolved from fundamental semiconductor surface studies to the control of semiconductor processing in situ under higher pressures and with real-time feedback [1]. Early applications of the technique to metal surfaces showed strong anisotropy of fcc (110) surfaces associated with transitions between surface states as well as interband transitions in the near surface region [2,3]. Recently, there have been several attempts to follow the effects of molecular adsorption on metal surfaces [4][5][6][7]. Although the molecule-surface bond clearly modifies the inherent anisotropy of the substrate metal electronic states, the information is not directly interpretable and may contain little information regarding the orientation of nonbonding moities in multifunctional species. To date, none of the adsorbates studied [4-6,8] have shown an intramolecular electronic transition within the 1.5-5.5 eV energy range available. To demonstrate the potential of RAS, we chose a larger conjugated molecular species, so as to shift the intramolecular electronic transitions into the visible region. We report here the first observation of reflectance anisotropy originating from an intramolecular transition, following the adsorption of 9-anthracene carboxylic acid (9-AA) onto the p͑2 3 1͒O͞Cu͑110͒ surface, with a structure approximately described as p͑8 3 1͒g. No previous studies of this adsorption system exist.In principle, the azimuthal orientation of an adsorbate can be determined from the RA spectrum if a well-defined transition dipole, relative to the molecular coordinates, exists within the available energy range and the metal does not screen the E field of the light parallel to the metal surface. In favorable cases, such as the one chosen here, the application of RAS to molecular adsorbates requires understanding (i) the intensity; (ii) whether an adsorbate transition causes an increase or decrease in reflectivity, since the sense corresponds to a rotation of the adsorbate by 90 ± , and (iii) whether or not Fresnel equations, which are valid in the continuum limit, apply at microscopic dimensions and describe the substrate respon...
The interplay between the surface band structure and possible surface reconstructions of Mo (112)" (2000). Peter Dowben Publications. 118.
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