Chiral Metal Surfaces Research Articles

Chiral surfaces and metal/ceramic heteroepitaxy in the Pt/SrTiO 3(621) system

Low-energy electron diffraction (LEED), atomic force microscopy (AFM), and X-ray diffraction (XRD) have been used to investigate the structural and morphological character of a naturally chiral ceramic SrTiO 3(621) substrate and of Pt and Cu thin films deposited on its surface. AFM experiments showed that as-received chirally-oriented SrTiO 3(621) substrates display atomically smooth surface morphologies, while LEED patterns revealed that the surface structure has a net chirality. Pt(621) and Cu(621) thin films were grown heteroepitaxially on SrTiO 3(621) substrates, as confirmed by XRD. AFM showed that the film surfaces were atomically smooth and LEED illustrated that the Pt films exhibit surface chirality, and by implication that the atomically-flat chirally-oriented Cu films also have chiral surfaces. The characteristics of the observed LEED patterns, where splitting of diffraction spots is considered to arise from the kinked step features of naturally chiral fcc metal surfaces, are discussed with respect to existing models. These results indicate that the chiral SrTiO 3(621) ceramic surface drives the growth of single-enantiomer, chiral, metal (621) thin films.

Probing Chiral Nanoparticles and Surfaces by Infrared Spectroscopy

Chiral metal surfaces and nanoparticles have the potential to be used for the selective production, the resolution and the detection of enantiomers of a chiral compound, which renders them highly attractive in view of the tremendous consequences of homochirality on earth. Their capability to distinguish between enantiomers of a chemical compound relies on their structure and the ability to form intermolecular interactions. However, molecular-level understanding of the interactions that are at the origin of enantiodiscrimination is lagging behind due to the lack of powerful experimental techniques that are able to spot these interactions selectively with high sensitivity. In this article two techniques based on infrared spectroscopy are presented that are able to selectively target the chiral properties of nanoparticles and interfaces. These are the combination of attenuated total reflection infrared (ATR-IR) with modulation excitation spectroscopy (MES) to probe enantiodiscriminating interactions at chiral solid–liquid interfaces and vibrational circular dichroism (VCD), which is used to probe the structure of chirally modified metal nanoparticles.

Enantioselective Surface Chemistry of R-2-bromobutane on Cu(643)R&S and Cu(531)R&S

The enantioselective surface chemistry of chiral R-2-bromobutane was studied on the naturally chiral Cu(643)R&S and Cu(531)R&S surfaces by comparing relative product yields during temperature-programmed reaction spectroscopy. Molecularly adsorbed R-2-bromobutane can desorb molecularly or debrominate to form R-2-butyl groups on the surfaces. The R-2-butyl groups react further by beta-hydride elimination to form 1- or 2-butene or by hydrogenation to form butane. Temperature-programmed reaction spectroscopy was used to quantify the relative yields of the various reaction products. At low coverages of R-2-bromobutane on Cu(643)R&S and Cu(531)R&S, the surface chemistry is not enantioselective. At monolayer coverage, however, the product yields indicate that the R-2-bromobutane decomposition reaction rates are sensitive to the handedness of the two chiral surfaces. The impact of surface structure on enantioselectivity was examined by studying the chemistry of R-2-bromobutane on both Cu(643)R&S and Cu(531)R&S. The selectivity of R-2-bromobutane desorption versus debromination is enantiospecific and differs significantly from Cu(643) to Cu(531). The selectivity of the R-2-butyl reaction by beta-hydride elimination versus hydrogenation is only weakly enantiospecific and is similar on both the Cu(643) and Cu(531) surfaces. These results represent the first quantitative observations of enantioselectivity in reactions with well-known mechanisms probed using a simple adsorbate on naturally chiral metal surfaces.

Reflections on Chiral Metal Surfaces and Their Potential for Catalysis

Abstract Principal possibilities for imparting chirality to a metal surface are surveyed and an attempt is made to assess their potential for heterogeneous asymmetric catalysis. Most possible strategies for creating catalytically active enantiodifferentiating sites on metal surfaces possess inherent limitations concerning potential application in practical asymmetric catalysis. Among the various possibilities known today for bestowing chirality to a metal surface only chiral modification by adsorbed chiral organic molecules (chiral modifiers) has gained practical relevance in catalysis, though its application range is still relatively narrow, confined to asymmetric hydrogenation of C O and C C bonds in specific substrates. Opportunities and limitations of this approach are discussed using the enantioselective hydrogenation of ketopantolactone over cinchona-modified platinum as an example.

Competition at Chiral Metal Surfaces: Fundamental Aspects of the Inversion of Enantioselectivity in Hydrogenations on Platinum

O-Phenylcinchonidine (PhOCD) is known to efficiently induce inversion of enantioselectivity with respect to cinchonidine (CD) in the enantioselective hydrogenation of various activated ketones on Pt/Al(2)O(3). To understand the origin of the switch of enantioselective properties of the catalyst, the adsorption of PhOCD has been studied by in situ ATR-IR spectroscopy, in the presence of organic solvent and dissolved hydrogen, i.e., under conditions used for catalytic hydrogenation. The adsorption structures and energies of the anchoring group of CD and PhOCD were calculated on a Pt 38 cluster, using relativistically corrected density functional theory (DFT). Both approaches indicate that both modifiers are adsorbed via the quinoline ring and that the spatial arrangement of the quinuclidine skeleton is critical for the chiral recognition. New molecular level information on the conformation of CD relative to PhOCD adsorbed on a surface is extracted from the ATR spectra and supported by DFT calculations. The result is a clearer picture of the role played by the phenyl group in defining the chiral space created by the modifiers on Pt. Moreover, when CD was added to a pre-equilibrated adsorbed layer of PhOCD, a chiral adsorbed layer was formed with CD as the dominant modifier, indicating that CD adsorbs more strongly than PhOCD. Conversely, when PhOCD was added to preadsorbed CD, no significant substitution occurred. The process leading to nonlinear effects in heterogeneous asymmetric catalysis has been characterized by in situ spectroscopy, and new insight into a heterogeneous catalytic R-S switch system is provided.

Reflections on chiral metal surfaces and their potential for catalysis

Principal possibilities for imparting chirality to a metal surface are surveyed and an attempt is made to assess their potential for heterogeneous asymmetric catalysis. Most possible strategies for creating catalytically active enantiodifferentiating sites on metal surfaces possess inherent limitations concerning potential application in practical asymmetric catalysis. Among the various possibilities known today for bestowing chirality to a metal surface only chiral modification by adsorbed chiral organic molecules (chiral modifiers) has gained practical relevance in catalysis, though its application range is still relatively narrow, confined to asymmetric hydrogenation of C O and C C bonds in specific substrates. Opportunities and limitations of this approach are discussed using the enantioselective hydrogenation of ketopantolactone over cinchona-modified platinum as an example.

Chiral metal surfaces from the adsorption of chiral and achiral molecules

Chiral surfaces, capable of existing in two distinguishable mirror forms that cannot be superimposed, are attracting worldwide attention. The adsorption of complex organic molecules provides a means of introducing the ultimate discrimination function of chirality to a metal surface. Here, a comparison of the chiral tartaric acid (HOOC–CHOH–CHOH–COOH) molecule and the achiral succinic acid (HOOC–CH 2–CH 2–COOH) molecule on a Cu(110) surface is presented. For both molecules, two-dimensional assembly is found to depend strongly on molecule–metal bonding interactions, whereas the presence/absence of the OH groups causes subtler, second-order effects on the self-assembled structure. The driving force for creating chiral organisations is shown to arise from adsorption-induced asymmetrisation, via molecular distortion and/or metal reconstruction of the local adsorption unit. The macroscopic chirality of the surface is then determined by whether nucleation points of both chirality can be equally created, or whether non-degeneracy can be introduced to favour one chirality.

Pt thin films on stepped SrTiO3 surfaces: SrTiO3(620) and SrTiO3(622)

Abstract As a first step in understanding Pt growth on chiral SrTiO 3 surfaces we have examined Pt adsorption on the stepped SrTiO 3 (6 2 0) and SrTiO 3 (6 2 2) surfaces using plane-wave density functional theory. We find that for both stepped SrTiO 3 surfaces Pt adsorbs more strongly along the step versus the terrace. We also report on the interface strength of Pt slabs on both terminations of SrTiO 3 (6 2 0) and (6 2 2). We find that the interface strength is considerably larger than found for Pt slabs on the low-Miller index SrTiO 3 surfaces. Our results indicate that step-flow growth of Pt that retains the underlying morphology of the stepped SrTiO 3 surface is likely to be favored over island nucleation on terraces. This preliminary work indicates that the growth of thin metal films on chiral metal oxides may be a viable approach to producing cheap chiral metal surfaces.

From local adsorption stresses to chiral surfaces: (R,R)-tartaric acid on Ni(110).

The chiral molecule (R,R)-tartaric acid adsorbed on nickel surfaces creates highly enantioselective heterogeneous catalysts, but the nature of chiral modification remains unknown. Here, we report on the behavior of this chiral molecule with a defined Ni(110) surface. A combination of reflection absorption infrared spectroscopy, scanning tunneling microscopy, and periodic density functional theory calculations reveals a new mode of chiral induction. At room temperatures and low coverages, (R,R)-tartaric acid is adsorbed in its bitartrate form with two-point bonding to the surface via both carboxylate groups. The molecule is preferentially located above the 4-fold hollow site with each carboxylate functionality adsorbed at the short bridge site via O atoms placed above adjacent Ni atoms. However, repulsive interactions between the chiral OH groups of the molecule and the metal atoms lead to severely strained adsorption on the bulk-truncation Ni(110) surface. As a result, the most stable adsorption structure is one in which this adsorption-induced stress is alleviated by significant relaxation of surface metal atoms so that a long distance of 7.47 A between pairs of Ni atoms can be accommodated at the surface. Interestingly, this leads the bonding Ni atoms to describe a chiral footprint at the surface for which all local mirror symmetry planes are destroyed. Calculations show only one chiral footprint to be favored by the (R,R)-tartaric acid, with the mirror adsorption site being unstable by 6 kJ mol(-1). This energy difference is sufficient to enable the same local chiral reconstruction and motif to be sustained over 90% of the system, leading to an overall highly chiral metal surface.

Photoemission studies of chiral metal surfaces using circularly polarized synchrotron radiation

In an attempt to measure classical circular dichroism (CD) in the photoemission of electrons from a solid surface, valence-band photoemission from clean and adsorbate-covered chiral metal surfaces has been investigated using circularly polarized synchrotron radiation. An experimental geometry was used whereby q, the polarization of the radiation, k, the wave vector of the photoemitted electrons, and n, the surface normal, were coplanar. It was anticipated that by doing this, so-called CDAD (circular dichroism in the angular distribution) effects associated with chiral photoemission geometries would be decoupled from classical CD arising from the difference in absorption cross-section for left and right circularly polarized light of electrons in energy states associated with R and S kinks. However, from the magnitude of the effects observed, it is evident that any classical CD features in the photoelectron spectrum arising from kinks, including photoemission from surface states or resonances, are swamped by other dichroic effects. We conclude that final-state effects in the diffraction of photoelectrons dominate the intensity of all peaks observed in angle-resolved photoemission from chiral, single crystal, metal surfaces using circularly polarized photons.

ChemInform Abstract: Naturally Chiral Metal Surfaces as Enantiospecific Adsorbents

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The electro-oxidation of glucose using platinum–palladium bulk alloy single crystals

A series of chiral and achiral PtPd bulk single crystal electrodes have been prepared using the method of Clavilier. Subsequent characterisation of the surface composition and structure of the electrodes using CV, LEED and AES indicate that all surfaces are well-defined and give rise to good (1×1) surface long range order. The CV profile of the PtPd bulk alloy surfaces are compared with those associated with irreversibly adsorbed palladium adlayers on single crystal Pt electrodes. Using the electro-oxidation of d- and l-glucose as chiral probe reactions, the inherent surface chirality of the PtPd bulk single crystal surfaces is highlighted. As such, these results represent the first report of a bi-metallic chiral metal surface.

Naturally Chiral Metal Surfaces as Enantiospecific Adsorbents

The creation of solid surfaces with intrinsic chirality is one promising route for developing novel enantiospecific processes. This paper focuses on high Miller index surfaces of pure metals such a...

Electrochemical Studies of Enantioselectivity at Chiral Metal Surfaces

The combination of electrochemical methods with the use of well-defined kinked metal surfaces allows the experimentalist to examine many fundamental aspects of asymmetric reactions at solid surfaces in a new and detailed manner. For example, by systematically changing both the stereogenic center on a particular chemisorbing molecule and also the molecular architecture of the metal surface, relationships between adsorption geometry and reactive sites on the metal surface may be deduced. In addition, by measuring the differential rate of chemisorption of chiral auxiliaries on R and S metal surfaces insights in to the mechanism of the so-called “Orito” reaction (whereby enantioselective hydrogenation of α-ketoesters is observed) may be deduced. In the present article, a brief resume of the relevant electrochemical and heterogeneous catalysis literature is expounded together with an analysis of the problem of chirality in two dimensions in order to show that, by utilizing an electrochemical surface science ap...

Induced chiral metal surfaces in asymmetric hydrogenations

On the basis of reviewed works and our experiences we propose that during the reduction of the surface the adsorbing substrates and the chiralcompounds induce the formation of chiral surface sites in dia- or enantioselective hydrogenation reactions catalyzed by precious metal

Scanning Tunneling Microscopy Investigation of l-Lysine Adsorbed on Cu(001)

Adsorption of l-lysine (or, in absolute nomenclature, S-lysine) on Cu(001) has been studied with a UHV−STM. The binding energy is estimated to be not lower than 1.6 eV, and thus lysine is expected to be in its anionic form on the surface. The adsorbates have two phases, that is, the two-dimensional (2D) gas and solid phases. We speculate that in the 2D gas phase the molecules stand upright on the surface with their two oxygen atoms at the atop sites and can diffuse frequently, and that in the 2D solid phase the molecules lie down on the surface and connect to their neighboring molecules by hydrogen bonds so to form two slightly different superstructures, Cu(001)( )- and Cu(001)( )-l-lysine, for which models have been proposed for further investigation. Surprisingly, adsorption of l-lysine can make all steps restructuring into {3 1 17} facets with the same chirality. This phenomenon shows that chiral metal surfaces have potential in discrimination of molecular chirality.