Interplay between Supramolecularity and Substrate Symmetry in the

Apr 18, 2013 - ... delle Ricerche (CNR), Via Fosso del Cavaliere 100, 00133 Roma, Italy ... on Cu(100) or inhibits hydroxyl group dehydrogenation on C...
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Interplay between Supramolecularity and Substrate Symmetry in the Dehydrogenation of D‑Alaninol on Cu(100) and Cu(110) Surfaces G. Contini,*,†,‡ P. Gori,†,‡ M. Di Giovannantonio,† N. Zema,† S. Turchini,† D. Catone,† T. Prosperi,† and A. Palma§ †

Istituto di Struttura della Materia, Consiglio Nazionale delle Ricerche (CNR), Via Fosso del Cavaliere 100, 00133 Roma, Italy Centro Interdipartimentale Nanoscienze & Nanotecnologie & Strumentazione (NAST), University of Rome “Tor Vergata”, 00133 Roma, Italy § Istituto per lo Studio dei Materiali Nanostrutturati, CNR, Via Salaria Km 29.3, 00015 Monterotondo S. (RM), Italy ‡

S Supporting Information *

ABSTRACT: The adsorption of organic chiral molecules on metallic substrates is widely studied as a tool to obtain chiral surfaces. The chirality transfer from the molecules to the surface is strongly driven by the availability of hydrogen atoms, which guides a specific chiral self-assembled structure. In this paper we report, by combination of photoelectron spectroscopy, low-energy electron diffraction, and density functional theory calculations, on the adsorption of the D-enantiomer of alaninol on Cu(100) and on Cu(110) with the aim of revealing dehydrogenation in the formation of the molecular chiral superstructure. We show that, on both surfaces, at low coverage alaninol is dehydrogenated at the hydroxyl group, whereas at saturated coverage the substrate symmetry, in combination with intermolecular interactions, induces partial amino group dehydrogenation on Cu(100) or inhibits hydroxyl group dehydrogenation on Cu(110).

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INTRODUCTION Two-dimensional (2D) chiral surfaces obtained functionalizing metal surfaces with chiral molecules are active in biological processes and biomedicine and are of fundamental importance in enantiomeric compound separation, catalysis, and sensors.1−4 Chirality is transferred from molecules to the surface via the formation of self-assembled monolayers, driven by supramolecular effects such as long-range interactions and hydrogen bond network formation.5−9 Several molecules have been used to produce chiral surfaces, containing one or more functional groups, carboxylic, alcoholic, or amino, that may be involved in bonding to the metal surface; some of them are chiral and others prochiral, leading to a form of 2D chirality.10,11 H-bondings involving alcoholic and/or amino groups play a central role, through formation of intermediate clusters and chains, in guiding a specific chiral self-assembled structure; in this respect, the number and positions of the hydrogen atoms are relevant for the degree of surface enantioselectivity,12 and possible dehydrogenation due to the adsorption should be considered. Dehydrogenation processes of the hydroxyl group on surfaces have been mostly studied in the case of alcohols, both experimentally and theoretically; as an example, oxidation of methanol on Cu(110) starts with its decomposition into methoxy (CH3O) and hydrogen,13,14 in contrast to platinumbased catalysts, on which the O−H and the C−H bond scission pathways have comparable barriers.15 Preadsorbed oxygen on © 2013 American Chemical Society

copper plays an important role in any, partial or total, oxidation of methanol. Dehydrogenation of alcohols has been observed on metal catalysts, and hydrogen-bonded neighbors assist it: hydrogen bonds modulate the alcohol dehydrogenation process, either an intermolecular hydrogen bond with a neighboring molecule or an intramolecular hydrogen bond in a polyol.16 Primary amines, on the other side, usually adsorb molecularly on metal surfaces at room temperature, as has been observed for, e.g., methylamine on Ni3Al(111) and NiAl(110)17 or on Cu(110).18 In this paper, we focus our attention on the chemical behavior of D-alaninol, the simplest chiral amino alcohol presenting two reactive groups, when it is adsorbed on Cu(100) and Cu(110) surfaces. D-Alaninol produces a chiral surface when adsorbed on Cu(100) since it gives rise to a longrange-ordered self-assembled monolayer, with a 14° clockwise rotation of the molecular superstructure with respect to the high-symmetry directions of the substrate.5,19−21 We show here, using a combination of core level and valence band photoelectron spectroscopy, low-energy electron diffraction (LEED) experiments, and density functional theory (DFT) calculations, how alcohol dehydrogenation occurs on the two surfaces with different extents and that, in some cases, Received: February 21, 2013 Revised: April 18, 2013 Published: April 18, 2013 10545

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far from the surface. The surface unit cell is sufficiently large that an hydrogen atom located on the surface does not influence the relaxation of dehydrogenated alaninol.

dehydrogenation of the amino group also takes place. On Cu(100), the hydroxyl group is always dehydrogenated, while the amino group is partly dehydrogenated only at saturated coverage where intermolecular forces are more effective. On the contrary, on Cu(110) we never observe dehydrogenated nitrogen, whereas the hydroxyl group partly retains the hydrogen atom at saturated coverage. These results have been confirmed by theory, modeling the adsorption of isolated D-alaninol molecules with different degrees of dehydrogenation on the two copper surfaces.

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RESULTS AND DISCUSSION D-Alaninol was found to adsorb on the Cu(110) surface at RT. The molecular amount, as determined by the XPS peak area, grows to saturation, indicating that only a single layer of Dalaninol is obtained at RT (the Langmuir isotherm curve is reported in Figure S1 of the Supporting Information). At saturated coverage, the self-assembled globally chiral phase produces the LEED pattern reported in Figure 1a, witnessing only one domain rotated with respect to the high-symmetry directions of the substrate, described by the epitaxy matrix (2,1|−3,2) (Figure 1b).

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EXPERIMENTAL AND THEORETICAL METHODS The experiments have been performed at room temperature (RT) in ultra-high-vacuum (UHV) conditions with base pressure in the low 10−8 Pa range, using laboratory X-ray and synchrotron radiation (SR) sources. SR data obtained on Cu(110) have been recorded on the circular polarization beamline at the Elettra facility (Trieste, Italy) using a hemispherical electron energy analyzer combined with a grazing or normal incident monochromator for the core level and valence band investigations, respectively. C 1s, O 1s, and N 1s photoelectron spectra have been obtained at 0.04 and 3 langmuirs for low coverage (LC) and 0.3 and 15 langmuirs for saturated coverage (SC) conditions for Cu(110) and Cu(100), respectively (1 langmuir is the exposition at 1.33 × 10−4 Pa for 1 s). The D-alaninol multilayer phase has been obtained on the Cu(110) surface at 100 K and measured using a laboratory Al Kα source. Copper surfaces were cleaned with repeated cycles of 600 eV Ar+ sputtering followed by annealing at 800 K and checked by X-ray photoelectron spectroscopy (XPS) and LEED. D-Alaninol (2-amino-1-propanol) was purchased from Sigma-Aldrich and purged by repeated freeze and dry cycles before it was introduced into the chamber through a leak valve. The purity of alaninol during evaporation was checked by a mass spectrometer. DFT calculations of isolated adsorbed D-alaninol molecule on Cu(100) and on Cu(110) have been performed using a planewave pseudopotential code.22 The employed pseudopotentials make use of a semilocal Perdew−Burke−Ernzerhof treatment of exchange and correlation energy.23 A kinetic energy cutoff of 25 Ry has been used in the plane-wave expansion. Both systems have been represented in a repeated slab approach, in a 4 × 4 geometry of the copper surface, to decouple the molecule from its periodic replicas. The Cu(100) surface has been represented by three layers separated by 16.8 Å of vacuum, while the Cu(110) surface has been modeled by a seven-layer slab separated by 21.2 Å of vacuum. The surface Brillouin zone has been sampled by a 4 × 4 Monkhorst−Pack mesh of k-points24 for D-alaninol/Cu(100) and by a 4 × 3 mesh for D-alaninol/ Cu(110). The systems have been relaxed using the Broyden− Fletcher−Goldfarb−Shanno algorithm25−28 until the forces on each ion were reduced below 10−4 au. Semiempirical dispersion corrections to the forces have been applied to better describe weak physical interactions, such as van der Waals interactions or noncovalent bonds.29,30 Core level binding energy (BE) shifts for N 1s and O 1s have been calculated, in the framework of pseudopotential DFT, as total energy differences between ionized and neutral states in the same geometry; relative BEs are obtained as differences with respect to a reference configuration.31 The detached H atom in the dehydrogenated structures has been located on the copper surface, for the two substrates, since this configuration is energetically preferred at 0 K compared with the one in which H belongs to a H2 molecule

Figure 1. (a) Experimental LEED pattern after a 0.3 langmuir exposure (SC) of D-alaninol on Cu(110) (Ep = 52 eV). Circled spots are due to Cu(110). (b) Corresponding direct lattice. Black points and the red lattice represent molecular diffractive units and the Cu(110) surface, respectively. Arrows indicate unit cell vectors.

The evolution of the interaction of D-alaninol with Cu(110) as a function of the molecular amount on the surface and of surface temperatures (RT for LC and SC and 100 K for a multilayer) has been studied. Experimental C 1s, O 1s, and N 1s core level photoelectron spectra were measured, and the results are reported in Figure 2a,b. The multilayer phase provides reference spectra for noninteracting D-alaninol since at 100 K multiple layers of physisorbed molecules cover the substrate. The corresponding spectra of N 1s and O 1s can be well fitted by using one Voigt function, in agreement with the chemical formula of D-alaninol (at 400.4 and 533.3 eV, respectively). The C 1s spectrum shows that the tree carbon atoms of alaninol have similar BE values and cannot be energetically resolved; the same shape of the C 1s spectrum is reported for alaninol in the gas phase.32 For low coverage at RT, the C 1s spectrum in Figure 2b can be fitted by three Voigt functions with the same Gaussian and Lorentzian widths (two Voigt functions at lower BE are superimposed). No significant changes in the BE of the three components, within the experimental uncertainty, have been found, increasing the coverage to saturation. In the N 1s core level spectrum only one component is detected at LC, located at a BE of 399.8 eV, indicating a single contribution from the NH2 group; this peak moves to 399.6 eV at SC, showing that slight changes take place in the nitrogen surroundings upon self-assembly. The O 1s core level spectrum, reported in Figure 2b, shows, at LC, only one component at a BE of 530.6 eV. At SC this peak shifts to a BE of 530.7 eV, while a second feature appears at a BE of 532.4 eV. The presence of two components, shifted by 1.7 eV, shows that 10546

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The comparison between core level photoelectron spectra for D-alaninol adsorbed on Cu(110) and Cu(100) indicates that different interactions with the surface are present as a function of the substrate symmetry and molecular coverage. At LC, the BEs of O 1s and N 1s present the same values, showing that, when intermolecular interactions are negligible, D-alaninol interacts with the substrate in a similar manner. On the contrary, at SC, where the self-assembled structure is present at the surfaces, differences stem from molecule−molecule interactions mediated by the surface. Cu(110) and Cu(100), indeed, present different electrostatic potentials and surface unit cells that impose different space constraints to selfassembly processes. We observe that the additional peaks detected at SC, besides occurring for the two different heteroatoms on the two surfaces, shift in two opposite BE directions with respect to the LC peak positions, i.e., at lower BE for N 1s on Cu(100) and at higher BE for O 1s on Cu(110), indicating different self-assembly behavior on the two surfaces. Figure 3b,c reports the valence bands of D-alaninol adsorbed on Cu(110) and Cu(100) surfaces at LC and SC, together with

Figure 2. (a) Experimental C 1s, O 1s, and N 1s core level photoelectron spectra for D-alaninol on Cu(110) obtained at 100 K for a multilayer phase using a laboratory Al Kα source. (b) Experimental C 1s (hν = 366 eV), O 1s (hν = 610 eV), and N 1s (hν = 450 eV) core level photoelectron spectra for D-alaninol on Cu(110) obtained at RT for LC and SC using SR. (c) Experimental C 1s (hν = 320 eV), O 1s (hν = 620 eV), and N 1s (hν = 508 eV) core level photoelectron spectra for D-alaninol on Cu(100) obtained at RT for LC and SC using SR.33 In all plots, experimental results are reported as points together with the results of the fit as solid lines (the components and background are reported in all panels).

the surface unit cell includes alaninol molecules in two chemically different forms. Figure 2c reports the experimental C 1s, O 1s, and N 1s core level photoelectron spectra for LC and SC situations for Dalaninol adsorbed on Cu(100) at RT.33 As for Cu(110), the C 1s spectra can be fitted by three Voigt functions with the same Gaussian and Lorentzian widths (two components overlapping at lower BE). Their BE shows no significant changes as a function of the coverage within the experimental uncertainty; a small component, which increases with the exposure to radiation, is observed at lower BE due to molecular photoinduced damage. The O 1s and N 1s spectra obtained at LC present one component at BEs of 530.8 and 399.6 eV, respectively, similarly to the case of adsorption on Cu(110). At SC, the O 1s spectra can be well fitted by only one component (a shift of 0.3 eV toward lower BE is observed in comparison with the LC case). The N 1s spectrum, instead, presents two components with a relative shift of 1.9 eV; one corresponds to the N 1s LC component (a shift of 0.2 eV toward lower BE is observed, similar to that of the O 1s spectrum), and the other at a BE of 397.5 eV indicates the copresence of two chemically different nitrogen atoms on the surface.

Figure 3. Experimental photoelectron valence band spectra obtained at room temperautre (hν = 50 eV, Γ point) for D-alaninol in the gas phase (a) and D-alaninol adsorbed on Cu(110) (b) and on Cu(100) (c) for LC and SC. The higher BE parts of the spectra in (b) and (c) (on the left of the dotted line) have been multiplied by the reported factor.

the valence spectrum obtained for D-alaninol in the gas phase (Figure 3a).32 The valence photoelectron spectra at SC for both Cu(110) and Cu(100) surfaces show several structures, labeled from A to F, not present in the valence band of clean copper. The higher BE features (from C to F), which can be 1:1 correlated with the corresponding ones in the gas-phase spectrum, are attributed, on the basis of calculations, to electronic molecular orbitals delocalized on the whole 10547

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molecule.19,32 Two additional structures, A and B, appear in the valence band at 1.7 eV (A) and 4.3 eV (B) for both Cu(110) and Cu(100). These structures, attributed to bonding (B) and antibonding (A) states, are the fingerprints of alaninol chemisorption on the copper surfaces. Localized electronic states close to transition-metal 3d orbitals are formed in presence of a strong interaction between adsorbate and nearestneighbor substrate metal atoms. If the interaction is strong enough or the apparent d bandwidth is narrow enough, then localized states appear outside the d band; the chemisorption energy of the bond is related to the energy shift of the adsorbate electronic level and to the width of the formed surface state.34 These valence states have been studied using circular dichroism in the angular distribution of photoelectrons to have direct information on the chirality behavior of a chiral surface obtained by self-assembled molecules.6 A comparison between the valence bands obtained for LC and SC situations shows an increase in the intensity of the structures attributable to D-alaninol. Changes in peak shape will be interpreted with the help of theoretical results. Ab initio DFT calculations, performed for an isolated molecule on a surface, allow determination of the building blocks composing the self-assembled structures on Cu(100) and Cu(110). Several geometries have been tested to find the most stable adsorption configurations. In the following, to identify the different structures, we refer to dehydrogenation of the hydroxyl and of the amino groups as O-dehydro and Ndehydro, respectively. The most stable geometries, sketched in the Supporting Information, give the relative total energies reported in Table 1.

stability. The adsorption of an isolated D-alaninol molecule on Cu(100) has already been addressed theoretically,19,35,36 where, however, only intact molecules were considered to model the low-coverage adsorption. We extend the analysis considering also the adsorption of D-alaninol molecules dehydrogenated at the alcoholic and/or at the amino group, showing that the Odehydro geometry is energetically favored compared with the adsorption of an intact D-alaninol molecule. The Cu−O and Cu−N bonds lengths, reported in Table 1, provide direct evidence of the electronic structure change when a dehydrogenated D-alaninol interacts with the copper surface. Core level shifts (CLSs) between different adsorption configurations have been calculated to understand the chemical origin of the peak splitting of O 1s and N 1s spectra experimentally observed when passing from the LC to the SC regime. The BE shift cannot be attributed to different adsorption geometries of intact D-alaninol molecules on the surfaces, which would only give rise to shifts up to 0.4 eV when comparing different configurations. Instead, a more profound chemical origin, such as a dehydrogenation process, has to be taken into account to interpret the experimental data, in agreement with the energetics discussed previously. For the Cu(110) surface, the second component in the O 1s spectrum has been found experimentally on the higher BE side (Figure 2b), with a CLS of 1.7 eV. Its BE resembles that obtained in the multilayer case, where noninteracting molecules with the copper surface can be assumed. In the comparison between spectra of the multilayer with those of the submonolayer or monolayer phases, it should be taken into account that surface screening produces a BE decrease of about 0.5 eV. The high-BE component at SC can therefore be related to the presence of intact alcoholic groups on the surface, whereas the single component obtained at LC has to be connected with O-dehydro-D-alaninol molecules. Calculations indeed support this interpretation, as CLSs obtained for O 1s spectra always indicate an increase of BE, in the range of 2.3− 2.7 eV (see Table 2), of intact molecules compared to

Table 1. Calculated Relative Total Energies and Cu−O and Cu−N Bond Lengths of the Most Stable Structures Obtained for the Adsorption of an Isolated D-Alaninol Molecule on Cu(110) and on Cu(100) D-alaninol/Cu(110)

relative total energy (eV)

Cu−O distance (Å)

Cu−N distance (Å)

G1, O-dehydro

0.00

2.06

G2, intact molecule G3, intact molecule G4, O-dehydro

0.03 0.10 0.14

G5, O-dehydro

0.21

1.98 1.99 2.13 2.23 1.94 2.05 2.03 2.05 Cu−O distance (Å)

configuration

D-alaninol/Cu(100)

configuration

relative total energy (eV)

g1, O-dehydro

0.00

g2, intact molecule g3, N-dehydro

0.21 0.42

g4, O- and N-dehydro

0.54

1.97 1.98 2.21 2.16 2.07 2.19

Table 2. Calculated O 1s and N 1s CLSs between a Configuration (A) Representative of LC and a Configuration (B) Appearing at SC for D-Alaninol on Cu(110) and Cu(100), Respectivelya

2.05 2.07 2.12

A configuration 2.04

B configuration

BE(B) − BE(A) (eV)

D-Alaninol/Cu(110):

O 1s Core Level Shifts G1, O-dehydro G2, intact molecule 2.35 G1, O-dehydro G3, intact molecule 2.73 G4, O-dehydro G2, intact molecule 2.36 G5, O-dehydro G2, intact molecule 2.45 experimental second peak at SC 1.7 experimental multilayer 2.6 D-Alaninol/Cu(100): N 1s Core Level Shifts g1, O-dehydro g4, O- and N-dehydro −2.39 experimental second peak at SC −1.9

Cu−N distance (Å) 2.07 2.07 1.97 1.97 1.97 1.99

a

For the Cu(110) surface, an O-dehydro geometry (G1) is the lowest energy configuration. A second adsorption configuration (G2), which refers to an intact molecule, is almost degenerate in energy with G1 (0.03 eV less stable). Another configuration, marginally less stable than G2 (by 0.07 eV), has been found for intact alaninol (G3). Two other O-dehydro configurations (G4 and G5) follow in energetics, resulting not too far in relative

Experimental BE shifts for both systems are reported.

dehydrogenated ones. We can therefore, also taking into account the energetics reported in Table 1, consider an Odehydro geometry (G1 or, slightly less probable, G4 or G5) as a candidate for describing the LC situation and an intact molecule (in configuration G2 or G3) coexisting with an Odehydro molecule to describe the surface at the SC condition. 10548

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copper 3d states, corresponding to the experimental feature B in Figure 3b, is also due to oxygen 2p states that are involved in the molecule−substrate bonding. It is present in both G1 and G2 configurations at slightly different energies. For what concerns D-alaninol on Cu(100), Figure 4c,d reports the DOS and PDOS for the g1 geometry, which can be considered representative of the LC situation, and the g4 geometry, considered as the new configuration appearing at SC together with g1. Also for this surface, the peak labeled A″ in Figure 4c can be related to the experimental feature A in Figure 3c and is attributed to the contribution by oxygen 2p orbitals. At SC, a further contribution in the same energy range comes from the nitrogen atom of the imino group, as shown in Figure 4d. Peak B″ is attributed to oxygen in the g1 configuration, further showing the similarity of the two substrates at LC, whereas it is smeared over a larger range and built by both nitrogen and oxygen 2p orbitals in the g4 configuration. The theoretical results obtained for the valence band can be fruitfully compared with the experimental data obtained at SC in the high-BE range (Figure 5a), where states mostly due to the molecular overlayer are present. The shapes and the BE positions of the structures are essentially the same for the two substrates, while their relative amplitudes are slightly different, in particular at 11.5 eV (high-BE part of peak C) and 16.1 eV (peak E). The sum of the calculated PDOS of the G1 and G2

Similar analysis has been performed in the case of adsorption on Cu(100). A calculated N 1s CLS of 2.39 eV toward lower BE is obtained going from the g1 geometry (dehydrogenated at the hydroxyl group) to the g4 geometry (dehydrogenated at both the hydroxyl and the amino groups). The calculated shift, in fair agreement with the experimental value of 1.9 eV, suggests the coexistence, at SC, of D-alaninol molecules in g1 and g4 geometries, binding through an amino or an imino group in the molecular self-assembled structure. Total and partial electronic densities of states (DOS and PDOS) have been calculated at the Γ point for the most stable adsorption configurations of D-alaninol on Cu(110) and Cu(100). For what concerns D-alaninol on Cu(110), we focused on the G1 geometry as the most probable candidate geometry of the LC situation (Figure 4a) and on the G2

Figure 4. Total DOS and PDOS of 2p orbitals for C, N, and O for Dalaninol on Cu(110) ((a) O-dehydro G1 geometry; (b) intact G2 geometry) and Cu(100) ((c) O-dehydro g1 geometry; (d) O- and Ndehydro g4 geometry). A broadening of 0.27 eV has been applied. Peaks are labeled to correspond with the experimental structures.

configuration (Figure 4b), which can be considered as the new configuration appearing at SC. At LC, it is possible to correlate the peak labeled with A′ (at 1.1 eV; see Figure 4a), essentially due to oxygen 2p orbitals, with the increase of intensity experimentally observed in the valence band at 1.7 eV (feature A in Figure 3b). A similar peak is observable in the DOS of the other two configurations dehydrogenated at the hydroxyl group (G4 and G5, not shown). The DOS of an intact alaninol molecule, on the contrary, would not show a significant signal in the energy range between the 3d states of copper and the Fermi edge (Figure 4b). Peak B′, at the higher BE side of

Figure 5. (a) High-BE experimental valence bands obtained for Dalaninol at SC on Cu(110) and Cu(100) (intensities have been normalized for better comparison). (b) Theoretical molecular DOS of G1 + G2 (Cu(110) substrate, solid black line) and g1 + g4 (Cu(100) substrate, solid red line). A broadening of 0.50 eV has been applied to the theoretical DOS. 10549

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ACKNOWLEDGMENTS The experiments with synchrotron radiation have been performed under the approval of the ELETTRA Proposal Review Panel. We thank M. Brolatti for technical assistance during the experiments. CPU time was granted by ENEACRESCO.

configurations and of the g1 and g4 configurations has been used to model SC on Cu(110) and Cu(100), respectively; the results are displayed in Figure 5b, showing a large band centered at 6 eV and three electronic states at higher energy, in agreement with the experimental data. Of particular interest for the comparison are the theoretical peaks around 13 eV, corresponding to peak E, and at 9.2 eV, corresponding to peak C. The analysis of atomic contributions to the DOS shows that the amplitude reduction of peak C can be traced back to amino group dehydrogenation, which occurs on Cu(100) but not on Cu(110). Peak C is indeed largely due to nitrogen in the NH2 configuration (see Figure 4a−c), a contribution that is shifted to higher BE (see Figure 4d) in a molecule that presents an imino rather than an amino group. The amplitude and BE position of peak E are related to the different oxidation states of the oxygen atom on the two substrates because no DOS contributions in this energy range exist for intact D-alaninol molecules on Cu(110) (see Figure 4b).

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CONCLUSIONS Combining photoelectron spectroscopy and LEED experiments with first-principles calculations, we presented some unexpected features of the adsorption of D-alaninol on Cu(100) and on Cu(110). The theoretical results, concerning local minimum configurations and their electronic structures, help in the interpretation of photoelectron spectra, showing that dehydrogenation processes occur to different extents for the two substrates on amino and alcoholic groups of the molecule. We showed that supramolecular effects that occur when D-alaninol forms chiral self-assembled structures, mediated by the two different surfaces of copper, influence molecular dehydrogenation. The comparison of experimental O 1s and N 1s spectra with the results of ab initio calculations allowed the following picture to be drawn. At low coverage OH dehydrogenation manifests both on Cu(100) and on Cu(110) and is therefore ascribed to molecule−surface interaction. At saturated coverage, instead, supramolecular interactions act in opposite ways on the two surfaces: they partly inhibit hydroxyl dehydrogenation on Cu(110), whereas they induce amino dehydrogenation on Cu(100). The reason for the different adsorption mechanisms may stem from the different coordinations of surface atoms. The electronic structure and charge analysis of the two copper surfaces, however, do not show relevant differences between them. The origin of the different behaviors has to be found then in the interplay of surface substrate symmetry, which imposes different geometrical constraints in the two situations, and of supramolecular interactions. Their balance acts in a subtle way, yielding different effects on the two surfaces. ASSOCIATED CONTENT

S Supporting Information *

Langmuir isotherm for D-alaninol on Cu(110) at RT and calculated most stable structures for D-alaninol on Cu(110) and Cu(100). This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail: [email protected]. Phone: +39 0649934139. Notes

The authors declare no competing financial interest. 10550

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