ARTICLE pubs.acs.org/JPCC
Structure and Distribution of S-R-(1-Naphthyl)-ethylamine on Pd(111) Jorge A. Boscoboinik, Yun Bai, Luke Burkholder, and Wilfred T. Tysoe* Department of Chemistry and Biochemistry, and Laboratory for Surface Studies, University of Wisconsin—Milwaukee, Milwaukee, Wisconsin 53211, United States
bS Supporting Information ABSTRACT: The adsorption geometry of S-R-(1-naphthyl)ethylamine (NEA) on Pd(111) was studied by scanning tunneling microscopy (STM), STM image simulations, and Monte Carlo (MC) simulations. The comparison of the experimental images with STM image simulations based on DFT calculations indicated a good correspondence with a dibridge[7] adsorption geometry. Furthermore, two different exo- and endo-conformers of the molecule were found on the surface that were related to different orientations of the ethylamine group with respect to the naphthyl group, due to the steric constraint imposed by the surface. It was found that the exo-NEA conformer occurs slightly more often than the endo-conformer, accounting for ∼57 ( 3% of the adsorbed NEA. The distribution and coverage of NEA molecules on the surface was compared to MC simulations of random deposition, where it was found that the ratio of exo- to endo-conformer was related to the different packing densities.
’ INTRODUCTION The study of the adsorption of S-R-(1-naphthyl)-ethylamine (NEA) on metal surfaces is relevant to a fundamental understanding of chiral modification in heterogeneous catalysis since NEA has a structure similar to, yet simpler than, cinchonidine, which is the chiral modifier used in the well-known Orito reaction.1 However, NEA has the important experimental advantage that it can be introduced into ultrahigh vacuum without undergoing thermal decomposition. The NEA molecule is anchored to the surface by the strong interaction between the conjugated π-electron system of the naphthyl group (or the quinoline group in the case of cinchonidine) and the metal substrate. The side group contains the chiral center and is thought to induce a certain degree of tilting of the naphthyl group with respect to the surface plane, as has been found for adsorption on Pt(111) surfaces.2 4 In fact, in addition to the proven activity in enantioselective reactions and adsorption, both molecules appear quite similar in scanning tunneling microscopy (STM) images. To the best of our knowledge, the only STM images currently available in the literature for cinchonidine on Pt or Pd are from the work by the Baiker group.5,6 They found that, in the absence of hydrogen and under UHV conditions, two configurations of cinchonidine were identified on the surface, which are attributed to a flat-lying and a tilted species. The flatlying species appears as two different shapes depending on the conditions of the tip used for scanning. Experimental evidence for the adsorption site or a comparative study of the orientation of the molecule with respect to the substrate, for different adsorption geometries, has not been reported. The structure and adsorption site of naphthalene has been recently measured experimentally by STM on Cu(111), and it was found to adsorb with the center of the naphthalene ring located over a bridge site r 2011 American Chemical Society
in which the bond between the two metal atoms that form the bridge site lay perpendicular to the long axis of the molecule.7 The adsorption geometries of naphthalene and quinoline on Pt(111), Pd(111), and Rh(111) surfaces were calculated by Santarossa et al. by means of density functional theory (DFT) calculations, and it was found that the so-called dibridge[7] adsorption site was the most stable one for both molecules on all three different substrates.8 This is in agreement with the geometry previously found for naphthalene on Pt(111) by Morin et al., also by means of DFT.9 On the basis of these predictions, Burkholder et al. have more recently shown that the most stable structure of NEA on Pd(111) is with the naphthyl group on the dibridge[7] site (ΔHads ∼ 222 kJ/mol).10 Two different conformations, the endo conformation, in which the amino group is oriented so that it is adjacent to the naphthyl ring, and the exo conformation, where it is oriented away from it, were explored. The calculations also took into account the van der Waals’ interactions between the adsorbate and the substrate and within the molecule. The NEA conformations are depicted in Figure 4 adsorbed on the most stable dibridge[7] adsorption site and the relative orientations of the ethylamine group in the endo and exo conformations. It was found that the exo conformation was slightly more stable than the endo one by about 2.2 kJ/mol. The DFT calculations by Santarossa et al. also showed that, on Pt(111) and Rh(111), a tilted adsorption mode is stable for quinoline molecules, where the plane of the aromatic system is not parallel to the metal surface.8 Bonello et al. reported tilt angles for quinoline adsorbed both on Pd(111) and on Received: April 28, 2011 Revised: July 15, 2011 Published: July 19, 2011 16488
dx.doi.org/10.1021/jp203960g | J. Phys. Chem. C 2011, 115, 16488–16494
The Journal of Physical Chemistry C
ARTICLE
Figure 1. STM image (20.1 nm 20.1 nm) of NEA on Pd(111) at low coverage (relative coverage θ = 0.057 ML) taken at 100 K. The line profiles along the white lines are shown on the right and correspond to a molecule along its long axis (top) and perpendicular to it (bottom). Imaging conditions: Vb = 101 mV; current set point It = 19.3 pA.
Pt(111) at 295 K. It was found that the molecules lay approximately flat on the surface, bonding via the aromatic π-system, with a maximum tilt angle of 18 ( 5 with respect to the surface plane.11 It has been found that NEA-modified Pd(111) surfaces induce enantioselective adsorption of 2-butanol over a narrow NEA coverage range centered around 0.55 ML.12 Chiral modifiers can operate as “templates”, in which several modifiers form a chiral adsorption pocket.13 Alternatively, they can form a docking complex via a one-to-one interaction between the chiral modifier and prochiral reactant.14 In the former case, the chiral modifier must necessarily form ordered structures, while this is not required for a one-to-one interaction. The disorder found for NEA adsorbed on Pt(111)11 was taken as evidence that they did not operate as templates. Thus, a full understanding of the chiral modification mechanism requires information on both the local structure and the distribution of the chiral modifiers on the surface. Accordingly, in this work, the distribution and conformation of NEA molecules on Pd(111) are studied by STM and analyzed using Monte Carlo simulations and STM image simulations based on previously reported DFT calculations.10
’ EXPERIMENTAL METHODS All images were collected for the S-enantiomer of NEA (Acros, 99% purity), which is a liquid at room temperature, and it was dosed onto the sample using the home-built Knudsen source described elsewhere.12 Because of its high vapor pressure, the NEA source was cooled to
