J. Phys. Chem. C 2007, 111, 8331-8336
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Enantiospecific Adsorption of Alanine on the Chiral Cu{531} Surface Michael J. Gladys,† Amy V. Stevens,† Nicola R. Scott,† Glenn Jones,† David Batchelor,‡ and Georg Held*,†,§ Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K., Experimentelle Physik II, UniVersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany, and Department of Chemistry, UniVersity of Reading, Whiteknights, Reading RG6 6AD, U.K. ReceiVed: January 24, 2007; In Final Form: March 27, 2007
We have studied enantiospecific differences in the adsorption of (S)- and (R)-alanine on Cu{531}R using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. At saturation coverage, alanine adsorbs as alaninate forming a p(1 × 4) superstructure. LEED shows a significantly higher degree of long-range order for the S than for the R enantiomer. Also carbon K-edge NEXAFS spectra show differences between (S)- and (R)-alanine in the variations of the π resonance when the linear polarization vector is rotated within the surface plane. This indicates differences in the local adsorption geometries of the molecules, most likely caused by the interaction between the methyl group and the metal surface and/or intermolecular hydrogen bonds. Comparison with model calculations and additional information from LEED and photoelectron spectroscopy suggest that both enantiomers of alaninate adsorb in two different orientations associated with triangular adsorption sites on {110} and {311} microfacets of the Cu{531} surface. The experimental data are ambiguous as to the exact difference between the local geometries of the two enantiomers. In one of two models that fit the data equally well, significantly more (R)-alaninate molecules are adsorbed on {110} sites than on {311} sites whereas for (S)-alaninate the numbers are equal. The enantiospecific differences found in these experiments are much more pronounced than those reported from other ultrahigh vacuum techniques applied to similar systems.
1. Introduction The past decade has seen a dramatic increase in research into chiral surface systems, driven by the growing demand for optically pure chemicals in drug manufacturing and, hence, a desire for enantioselective heterogeneous catalysts. These avoid the problem of phase separation inherent in homogeneous enantioselective processes which are predominantly used today. So far, significant success has been achieved by modifying achiral Pt, Pd, or Ni catalyst surfaces with chiral molecules thus creating a stereoselective reaction environment for R- and β-ketoesters.1-3 Alternatively, intrinsically chiral metal and mineral surfaces show enantioselective behavior without such modifiers,4-6 but mechanisms are much less well understood. Both chirally modified and intrinsically chiral surfaces have been studied with surface science methods in some detail, mainly by scanning tunneling microscopy, photoemission spectroscopies (XPS, NEXAFS), IR spectroscopy, and density functional theory (DFT) (see refs 2 and 7-9 and references therein). The main emphasis in recent studies of intrinsically chiral substrates has been on high-Miller-index kinked surfaces of metal single crystals with cubic bulk lattice symmetry, such as Pt or Cu. These have no mirror symmetry and can therefore not be superimposed onto their mirror images, which makes them chiral substrates. It has been found that such surfaces show enantioselectivity with respect to the adsorption and reactions of chiral * To whom correspondence may be addressed at the University of Reading. E-mail:
[email protected]. † University of Cambridge. ‡ Experimentelle Physik II, Universita ¨ t Wu¨rzburg. § University of Reading.
molecules,10,11 but theoretical and experimental studies also showed that they are thermally relatively unstable.12,13 Due to the choice of experimental methods, very little is known to date about the exact geometries of chiral adsorption complexes, on such surfaces, although this information is crucial for the understanding of stereoselectivity in any system. Of particular interest is the question to which extent the actual shape of the adsorption site and the lateral interaction between neighboring molecules, mutually acting as chiral modifiers, determine adsorption geometries and reaction pathways. Our recent combined quantitative low-energy electron diffraction (LEED-IV) and DFT studies of Pt{531} and Cu{531}14,15 show that these surfaces do not reconstruct and have atomic arrangements very similar to that of bulk termination, except for very strong inward relaxation of the kink atoms in the topmost layer. No detailed LEED-IV structure analysis of adsorbates on chiral substrates has been carried out to date because of the extreme electron-beam sensitivity of most chiral molecules of interest. Certain aspects of the adsorption complex, such as the molecular orientation and adsorption site, can, however, be determined by spectroscopic methods, which usually cause much less beam damage than LEED. In a recent study of alanine (CH3-CH(NH2)-COOH) on Cu{110}, we were able to determine experimentally the orientation of the molecules and identify the atoms through which the surface bond is formed using X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure spectroscopy (NEXAFS).16 Here we apply a similar approach for the first time to study the adsorption of a chiral molecule, alanine, on a chiral substrate, Cu{531}. On the basis of a large body of experimental and theoretical work studying alanine on achiral copper surfaces, it is well-established that
10.1021/jp070621f CCC: $37.00 © 2007 American Chemical Society Published on Web 05/22/2007
8332 J. Phys. Chem. C, Vol. 111, No. 23, 2007
Figure 1. Arrangement of atoms in the bulk-terminated Cu{531}R or (5h3h1h) surface. The surface unit cell and the most relevant crystallographic directions are indicated at the top of the figure; the {311} and {110} microfacets are indicated at the bottom.
this molecule usually chemisorbs as alaninate (CH3-CH(NH2)COO) forming µ3 adsorption complexes with bonds through the two O atoms and the N atom.8,16-18 Cu{531} has the smallest surface unit cell of all chiral Cu surfaces; it exposes {110} and {311} microfacets, inclined by 17.0° and 14.5° with respect to the {531} plane, which both contain triangular adsorption sites that could accommodate the footprint of alanine/alaninate (see Figure 1). The present study shows that both enantiomers form ordered overlayers, but there are pronounced differences in the quality of long-range order and in the angle dependence of the NEXAFS signal between the R and the S enantiomer of alanine indicating differences in the local adsorption geometries. The degree of enantiospecificity in these experimental data is significantly higher than that in temperature-programmed desorption (TPD)11 or XPS circular dichroism19 data reported earlier for similar chiral systems. 2. Experiment and Model Calculations The Cu single crystal used for these experiments was terminated by a (5h3h1h) surface (equivalent to {531}R 4 or {531}D,20 shown in Figure 1), as determined by a LEED-IV analysis.15 It was cleaned using standard procedures including electropolishing, Ar+-ion sputtering (typically 20 min at 1 µA/ 600 V), and a final annealing step to at least 1000 K in ultrahigh vacuum (UHV). Surface cleanliness and beam-induced damage were regularly monitored by XPS; likely contaminants were found to be below the detection limit (
