Rasmita Raval

Complex organic molecules at metal surfaces: bonding, organisation and chirality

Abstract Surface science techniques have now reached a stage of maturity that has enabled their successful deployment in the study of complex adsorption systems. A particular example of this success has been the understanding that has been gained regarding the behaviour of multi-functional organic molecules at metal surfaces. These organic–metal systems show enormous diversity, starting from their local description which can vary in terms of chemical structure, orientation and bonding. Additionally, in many cases, these complex organic molecules self-organise into beautiful, ordered superstructures held together by networks of intermolecular bonds. Both these aspects enable a single organic molecule–metal system to exhibit a wide-ranging and flexible approach to its environment, leading to a variety of adsorption phases, according to the prevailing temperature and coverage conditions. In this review we have attempted to capture this complexity by constructing adsorption phase diagrams from the available literature for complex carboxylic acids, amino acids, anhydrides and ring systems, all deposited under controlled conditions onto defined metal surfaces. These provide an accessible, pictorial basis of the adsorption phases which are then discussed further in the text of the review. Finally, interest has recently focused on the property of chirality that can be bestowed at an achiral metal surface by the adsorption of these complex organic molecules. The creation of such architectures offers the opportunity for ultimate stereocontrol of reactions and responses at surfaces. We have, therefore, specifically examined the various ways in which chirality can be expressed at a surface and provide a framework for classifying chiral hierarchies that are manifested at surfaces, with particular attention being paid to the progression of chirality from a local to a global level.

Extended surface chirality from supramolecular assemblies of adsorbed chiral molecules

The increasing demand of the chemical and pharmaceutical industries for enantiomerically pure compounds has spurred the development of a range of so-called ‘chiral technologies’ (ref. 1), which aim to exert the ultimate control over a chemical reaction by directing its enantioselectivity. Heterogeneous enantioselective catalysis is particularly attractive because it allows the production and ready separation of large quantities of chiral product while using only small quantities of catalyst. Heterogeneous enantioselectivity is usually induced by adsorbing chiral molecules onto catalytically active surfaces. A mimic of one such catalyst is formed by adsorbing (R,R)-tartaric acid molecules on Cu(110) surfaces: this generates a variety of surface phases, of which only one is potentially catalytically active, and leaves the question of how adsorbed chiral molecules give rise to enantioselectivity. Here we show that the active phase consists of extended supramolecular assemblies of adsorbed (R,R)-tartaric acid, which destroy existing symmetry elements of the underlying metal and directly bestow chirality to the modified surface. The adsorbed assemblies create chiral ‘channels’ exposing bare metal atoms, and it is these chiral spaces that we believe to be responsible for imparting enantioselectivity, by forcing the orientation of reactant molecules docking onto catalytically active metal sites. Our findings demonstrate that it is possible to sustain a single chiral domain across an extended surface—provided that reflection domains of opposite handedness are removed by a rigid and chiral local adsorption geometry, and that inequivalent rotation domains are removed by successful matching of the rotational symmetry of the adsorbed molecule with that of the underlying metal surface.

A one-dimensional ice structure built from pentagons

Heterogeneous ice nucleation has a key role in fields as diverse as atmospheric chemistry and biology. Ice nucleation on metal surfaces affords an opportunity to watch this process unfold at the molecular scale on a well-defined, planar interface. A common feature of structural models for such films is that they are built from hexagonal arrangements of molecules. Here we show, through a combination of scanning tunnelling microscopy, infrared spectroscopy and density-functional theory, that about 1-nm-wide ice chains that nucleate on Cu(110) are not built from hexagons, but instead are built from a face-sharing arrangement of water pentagons. The pentagon structure is favoured over others because it maximizes the water-metal bonding while maintaining a strong hydrogen-bonding network. It reveals an unanticipated structural adaptability of water-ice films, demonstrating that the presence of the substrate can be sufficient to favour non-hexagonal structural units.

FT-rairs, eels and leed studies of the adsorption of carbon monoxide on Cu(111)

Abstract The adsorption of CO on Cu(111) has been re-investigated using Fourier transform reflection absorption infrared spectroscopy (FT-RAIRS), electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED). The combination of these techniques has revealed important new information about this system. At 95 K three adsorption stages (1 to 3) can be distinguished, leading to successive ordered overlayer structures (I to III). Stage 1 exists for coverages up to 0.33 and involves linearly bound species only. The sharpest band observed during the course of these experiments, with an intrinsic FWHM of 4.5 cm −1 , is associated with a coverage of 0.33 and an ordered (√3 × √3) R 30 ° structure (I). Stage 2 is associated with the coverage regime 0.33−0.44. In this stage EELS data show tilted, linear CO species are present. No bridged species exist at this point. On the basis of this, we propose a new overlayer structure (II), for θ = 0.44. Increasing the coverage gives stage 3. During this process, the high resolution FT-RAIRS spectra resolved two bands associated with linear CO species. The behaviour of these bands suggests that the final structure (III) is formed in a step-wise manner, with islands of (II) and (III) coexisting in the adlayer. Both linear and bridged species exist in stage 3. The bandshapes suggest significant inhomogeneities in the adlayer are present at the saturation coverage. Finally, the concentration of bridged species is found to be very sensitive to the adsorption temperature.

The bonding and orientation of the amino acid l-alanine on Cu{110} determined by RAIRS

Reflection absorption infrared spectra have been obtained for the amino-acid l-alanine on Cu{110} as a function of exposure at 300 K. The RAIR spectra reveal a complicated set of vibrational bands which can be interpreted with reference to Cu-(ala)2 complexes. A detailed analysis in terms of the individual functional groups present in the molecule yields a detailed geometry of the adsorbed species. Two chemisorbed phases are identified: a thermodynamically stable phase which forms at low exposures, in which both the carboxylate oxygen atoms are equidistant from the surface, and a second phase, created by diffusion-limited adsorption processes, in which a differently oriented alanine species is accommodated at the surface, possessing titled carboxylate groups where the two oxygen atoms are no longer equidistant from the surface.

The catalytic role of water in CO oxidation

Water, one of the most popular species in our planet, can play a catalytic role in many reactions, including reactions in heterogeneous catalysis. In a recent experimental work, Bergeld, Kasemo, and Chakarov demonstrated that water is able to promote CO oxidation under low temperatures (∼200 K). In this study, we choose CO oxidation on Pt(111) in the presence of water as a model system to address the catalytic role of water for surface reactions in general using density functional theory. Many elementary steps possibly involved in the CO oxidation on Pt(111) at low temperatures have been investigated. We find the following. First, in the presence of water, the CO oxidation barrier is reduced to 0.33 eV (without water the barrier is 0.80 eV). This barrier reduction is mainly due to the H-bonding between the H in the H2O and the O at the transition state (TS), which stabilizes the TS. Second, CO can readily react with OH with a barrier of 0.44 eV, while COOH dissociation to produce CO2 is not easy (the barr...

CHx hydrogenation on Co(0001): A density functional theory study

Hydrogenation is an important process in the Fischer-Tropsch synthesis. In this work, all the elementary steps of the hydrogenation from C to CH4 are studied on both flat and stepped Co(0001) using density functional theory (DFT). We found that (i) CH3 hydrogenation (CH3+H-->CH4) is the most difficult one among all the elementary reactions on both surfaces, possessing barriers of around 1.0 eV; (ii) the other elementary reactions have the barriers below 0.9 eV on the flat and stepped surfaces; (iii) CH2 is the least stable species among all the CHx(x=1-3) species on both surfaces; and (iv) surface restructuring may have little effect on the CHx(x=0-3) hydrogenation. The barriers of each elementary step on both flat and stepped surfaces are similar and energy profiles are also similar. The reason as to why CHx hydrogenation is not structure-sensitive is also discussed.

Heat-to-Connect: Surface Commensurability Directs Organometallic One-Dimensional Self-Assembly

Recent experiments demonstrated the assembly of unfunctionalized porphyrin molecules into organometallic wires on the Cu(110) surface through the formation of stable C-Cu-C bonds involving Cu adatoms. The remarkable property of the observed structures is that they adopt a clear direction, despite the lack of functional ligands to direct the assembly. Here we use density functional theory calculations and scanning tunneling microscopy to clarify the mechanism for the highly one-dimensional assembly of the observed nanostructures. An energetic preference for the formation of C-Cu-C bonds is found in several lattice directions, but self-assembly critically relies on the commensurability of appropriate adsorption sites for the Cu atoms involved in the coupling. The experimentally observed structures arise from a geometric self-limitation of the assembly process, which proceeds in the energetically and geometrically most preferred direction. A further extension of the structure in the orthogonal dimension to form 2D assemblies is prevented by the lattice mismatch between the repeat lengths in the 001 and 110 directions of the underlying (110) lattice and the apparent rigidity of the molecules involved. However, the fusing of two parallel chains is geometrically allowed and leads to some of the energetically most favorable configurations. Finally, the role of van der Waals forces is investigated for the covalent couplings and chemisorbed interactions found in this system.

Probing Conformers and Adsorption Footprints at the Single-Molecule Level in a Highly Organized Amino Acid Assembly of (S)-Proline on Cu(110)

Establishing the nanoscale details of organized amino acid assemblies at surfaces is a major challenge in the field of organic-inorganic interfaces. Here, we show that the dense (4 x 2) overlayer of the amino acid, (S)-proline on a Cu(110) surface can be explored at the single-molecule level by scanning tunneling microscopy (STM), reflection absorption infrared spectroscopy (RAIRS), and periodic density functional theory (DFT) calculations. The combination of experiment and theory, allied with the unique structural rigidity of proline, enables the individual conformers and adsorption footprints adopted within the organized assembly to be determined. Periodic DFT calculations find two energetically favorable molecular conformations, projecting mirror-image chiral adsorption footprints at the surface. These two forms can be experimentally distinguished since the positioning of the amino group within the pyrrolidine ring leads each chiral footprint and associated conformer to adopt very different ring orientations, producing distinct contrasts in the STM images. DFT modeling shows that the two conformers can generate eight possible (4 x 2) overlayers with a variety of adsorption footprint arrangements. STM images simulated for each structural model enables a direct comparison to be made with the experiment and narrows the (4 x 2) overlayer to one specific structural model in which the juxtaposition of molecules leads to the formation of one-dimensional hydrogen bonded prolate chains directed along the [110] direction.

Understanding the Interaction of the Porphyrin Macrocycle to Reactive Metal Substrates: Structure, Bonding, and Adatom Capture

We investigate the adsorption and conformation of free-base porphines on Cu(110) using STM, reflection absorption infrared spectroscopy, and periodic DFT calculations in order to understand how the central polypyrrole macrocycle, common to all porphyrins, interacts with a reactive metal surface. We find that the macrocycle forms a chemisorption bond with the surface, arising from electron donation into down-shifted and nearly degenerate unoccupied porphine π-orbitals accompanied with electron back-donation from molecular π-orbitals. Our calculations show that van der Waals interactions give rise to an overall increase in the adsorption energy but only minor changes in the adsorption geometry and electronic structure. In addition, we observe copper adatoms being weakly attracted to adsorbed porphines at specific molecular sites. These results provide important insights into porphyrin-surface interactions that, ultimately, will govern the design of robust surface-mounted molecular devices based on this important class of molecules.