Theoretical Investigation of Formamide Adsorption on Ag(111

May 19, 2009 - The surface−adsorbate interaction is rather weak, as demonstrated by small interaction energies, large vertical distances, and small ...
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J. Phys. Chem. C 2009, 113, 10541–10547

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Theoretical Investigation of Formamide Adsorption on Ag(111) Surfaces Werner Reckien,† Barbara Kirchner,‡ Florian Janetzko,† and Thomas Bredow*,† Theoretische Chemie, Institut fu¨r Physikalische und Theoretische Chemie, UniVersita¨t Bonn, Wegelerstr. 12, D-53115 Bonn, Germany, and Wilhelm-Ostwald Institut fu¨r Physikalische und Theoretische Chemie, UniVersita¨t Leipzig, Linnestr. 2, D-04103 Leipzig, Germany ReceiVed: December 17, 2008; ReVised Manuscript ReceiVed: April 9, 2009

The cooperativity of intermolecular hydrogen bonds and adsorbate-surface interaction was studied theoretically for the prototype system formamide-silver. A dispersion-corrected density-functional method was employed for the study of the adsorption of hydrogen-bonded formamide networks on the Ag(111) surface, in order to account for weak van der Waals interaction. The dispersion correction considerably improved the calculated adsorption energy of formaldehyde with respect to experiment. We observed anticooperativity of formamide hydrogen bonds and molecule-surface interaction: at low coverage the formamide monomers adsorb vertically at the Ag(111) surface forming a weak O-Ag chemical bond. In contrast, formamide dimers, and also higher polymers formed by linear dimer chains, preferentially adsorb parallel to the surface. The surface-adsorbate interaction is rather weak, as demonstrated by small interaction energies, large vertical distances, and small perturbation of the molecular electronic structure. Consequently the adsorbate structures are very close to the corresponding gas-phase polymers. In this way, the reduction of adsorption energy per molecule in the parallel arrangement compared to the vertical adsorption is overcompensated. 1. Introduction Hydrogen bonds widely exist in many chemical and biological systems such as proteins, nucleic acids, and supramolecular complexes. They play a pivotal role for molecular and supramolecular structures, properties, and reactivity.1-4 One example is the hydrogen bond mediated self-assembly of organic compounds which is frequently exploited for the functionalization of surfaces.5-13 In this process it is often desired that the surface does not strongly influence the interactions between the adsorbed molecules, and therefore inert substrates (e.g., gold) are chosen. At surfaces intermolecular interactions compete with surfaceadsorbate interactions. This has been studied experimentally and theoretically (e.g., for ammonia adsorption on Si(001) surfaces,14 glycine on Cu(110),15 guanine networks on the Au(111) surface,16 anthraquinone and 9,10-dithioanthracene on Cu(111),17 the adsorption of adenine monolayers on a Ag-terminated Si(111) surface,18 the interactions between self-assembled adenine molecules and the Au(111) surface,19 and melamine on the Au(111) surface).20 However, to our knowledge, there are no systematic studies of the mutual influence of hydrogen bonding and adsorbate-surface interaction. The present study contributes to the theoretical analysis of the cooperativity between lateral hydrogen bonds and chemical interaction between peptides and metal surfaces. As a prototype system for peptide hydrogen bonds we chose formamide FA due to its structural simplicity. The FA crystal structure contains a twodimensional network of strong intermolecular hydrogen bonds which is interconnected by weak hydrogen bonds and van-derWaals interactions.21-23 The interaction of FA with silver nanoparticles and small gold and silver clusters has been * To whom correspondence should be addressed. † Universita¨t Bonn. ‡ Universita¨t Leipzig.

investigated experimentally and theoretically.24,25 Interestingly, also the formation of N-H · · · Ag hydrogen bonds has been discussed. The substrate Ag(111) is selected because it is frequently used in surface science studies for adsorption of molecules in ultrahigh vacuum.26 Furthermore it provides structural simplicity because the (111) surface does not reconstruct in contrast to the isostructural Au(111) surface that is usually considered for self-assembled monolayers.27 We employed density functional theory (DFT) methods for our theoretical investigation of the adsorption of formamide molecules and aggregates at the Ag(111) surface. Whereas energetic and structural properties of hydrogen-bonded systems are well described with standard DFT methods,28 these fail to account for dispersion interactions.29 In the last few years, much effort has been spent to improve this situation. Specially parametrized hybrid meta exchange functionals have been developed and tested for noncovalent interactions.30 Approximate nonlocal functionals have been designed particularly for dispersion interactions.31 Up to the present, however, these methods do not allow the calculation of energy gradients needed for the optimization of complex structures. Very recently a combination of exact Hartree-Fock-like exchange and approximate correlation energy derived from the adiabatic connection fluctuation-dissipation theorem has been suggested.32 A similar idea was proposed independently by Rohlfing and Bredow.33 These methods are based on first principles and computationally demanding. An empirical, but much more efficient approach is an a posteriori correction of the total Kohn-Sham energy by effective interatomic potentials. Such simple schemes have been proposed some time ago.34 In the present study we used an empirical DFT energy correction scheme developed by Grimme.35 For the periodic slab calculations presented in this study, we implemented the Grimme correction in the plane wave program VASP.36 A similar approach where the DFT energies and energy gradients of VASP

10.1021/jp811146m CCC: $40.75  2009 American Chemical Society Published on Web 05/19/2009

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Reckien et al. 4 supercell with three Ag layers was used for the adsorption of higher polymers. We find that the size of this supercell is sufficient to guarantee that the adsorbates of neighboring cells do not interact: An increase to a 10 × 4 or 9 × 5 supercell resulted in no significant change of interaction energies. In addition we examined the adsorption of an infinite polymer chain on the Ag(111) surface. This system was modeled by a formamide octamer adsorbed on a three-layer

( ) 7 0 2 4

Figure 1. Structure of the formamide dimer (1), tetramer (2), hexamer (3), and octamer (4) obtained with the PBE functional (plane-wave calculations). The hydrogen bond distances RON are given in pm. Red, oxygen; blue, nitrogen; orange, carbon; light gray, hydrogen.

are corrected externally has been proposed recently by Sauer et al.37 Our approach offers the special feature of selective interfragment interaction energy correction which will be discussed in the following. We focused our study on linear chains composed of FA dimers with C2h symmetry, because the cyclic dimer structure motif leads to a hydrogen-bonded network that is stabilized by the maximum number of hydrogen bonds. Furthermore, it can be systematically extended in one dimension. The corresponding gas-phase dimer structure is highly stable and has been studied before.38-42 The proposed one-dimensional extension, however, leads to novel structures that have not yet been studied to our knowledge. Since the cyclic FA dimer is prochiral, we have two enantiomers on the surface. Supramolecular surface chirality of two-dimensional supramolecular assemblies of prochiral molecules has been observed in previous experiments.43,44 This topic is, however, beyond the scope of our present investigation. We took into account only complexes that are built from the same enantiomers.

supercell. We find a nearly perfect commensurability for this system. The ratio between the ideal lattice vector for the infinite octamer chain and the corresponding lattice parameter for the silver surface is 1.005. The optimization of the surface was limited to only one layer for the larger supercells. Since no experimental data are available for the formamide-Ag(111) system, we performed calculations for the formaldehyde-Ag(111) system in order to check the reliability of our DFT calculations. For this system the adsorption enthalpy has been measured.45,46 3. Methodology In preliminary calculations we tested the accuracy of standard DFT methods for the prediction of energetic and structural properties of gas-phase FA aggregates. Geometry optimization of selected gas-phase formamide polymers was carried out with the TURBOMOLE 5.947 suite. We applied the MP2 method within the resolution of identity approximation (RIMP2) and employed TZVPP basis sets.48 The basis set superposition error (BSSE) of the atomic orbital calculations was corrected via the counterpoise correction method.49 Gas-phase FA aggregates and the adsorption of formamide on the most stable Ag(111) surface were investigated by means of periodic plane wave DFT calculations with the VASP program package.50-52 We applied a recent implementation36 of Grimme’s dispersion correction to density functional theory (DFT-D).35,53 The dispersion corrected DFT-D energy

2. Systems Investigated The systems investigated are substrate-adsorbate complexes which are formed by the clean, unreconstructed Ag(111) surface as substrate and formamide as adsorbate. We chose a single formamide molecule as well as hydrogen-bonded formamide polymers as adsorbates. We restricted the study to linear formamide chains with C2h symmetry which consist of the most stable cyclic dimers, see Figure 1. This was done because the extension of the cyclic dimer structure motif leads to a onedimensional hydrogen-bonded network which is stabilized by the maximum number of amide hydrogen bonds. The formamide molecules at each end of the polymer chain build up three hydrogen bonds, while all other molecules form four hydrogen bonds to their neighbors, see Figure 1. Accordingly, the total number n of hydrogen bonds for a linear chain which consists of N formamide molecules is n ) 2N - 2. Depending on the size of the adsorbed systems, we chose three different Ag(111) supercells as models of the surface. A systematic study was performed in which different arrangements for the adsorption of both a single formamide molecule and a simultaneous adsorption of two molecules were considered. In this case the Ag(111) surface was modeled by a 4 × 4 supercell containing four Ag layers. The first three atomic layers were relaxed in the adsorption calculations while the atoms of the fourth layer were kept at their bulk-like positions. A larger 9 ×

EDFT-D ) EKS-DFT + EDisp

(1)

is calculated by adding an empirical correction EDisp N-1

EDisp ) -s6

N

∑ ∑

i)1 j)i+1

Cij6 Rij6

fdmp(Rij)

(2)

to the Kohn-Sham energy. C6ij is a dispersion coefficient for atom pair ij which is calculated from C6 parameters for atoms i and j (C6ij ) [C6i C6j ]1/2), Rij is the distance between the atoms, and s6 ) 0.75 is the global scaling factor for the PBE functional. fdmp(Rij) which is given as

fdmp(Rij) )

1 -dRij/(Rr-1)

1+e

(3)

is a damping function which was introduced in order to avoid near-singularities for small distances. Rr is the sum of modified atomic van der Waals radii, and d is a global parameter. For the present study, we used the original values of all atomic and global parameters as suggested by Grimme.35 The summation

Formamide Adsorption on Ag(111) Surfaces is performed over all N atoms in the reference cell and shells of neighboring cells until convergence is obtained. A special feature of our implementation in the VASP code is the possibility to switch on or off the dispersion correction between predefined fragments. Because the PBE-D approach is known to overestimate the strength of amide hydrogen bonds,54 and because standard PBE calculations of the FA gas-phase polymers yielded an excellent agreement to RIMP2/TZVPP calculations, as will be shown below, we decided to apply the dispersion correction only between the adsorbate and the Ag(111) surface. We also checked that the DFT-D and DFT results for bulk silver and the Ag(111) surface are very similar, so that dispersion corrections among the Ag atoms are unnecessary. In the VASP calculations we used the projector augmentedwave method to account for the core electrons.55 The PBE as well as the Perdew-Wang PW91 exchange-correlation functional were employed.56-58 A cutoff energy of 400 eV was used for the plane-wave valence basis. The vacuum distance between the Ag surfaces was set to 18 Å. In preliminary calculations convergence of calculated adsorption properties was examined with respect to the vacuum distance, the cutoff energy, the number of k points, and the size of the supercell. With the abovementioned parameters, the calculated adsorption energies and geometries are converged within a few kJ/mol and pm, respectively. Unlike for LCAO-based methods, interaction energies obtained with plane wave methods are not biased by BSSE, therefore no correction is necessary. The stabilization energy Estab ) E0(system) - E0(surface) NE0(FA) is the energy gain due to adsorption and lateral interaction of N FA molecules on the Ag(111) surface. The adsorption energy Eads ) E0(system) - E0(surface) - E0(FAN) is the energy gain due to adsorption of a FA polymer consisting of N molecules. In all cases E0 indicates that the corresponding structure is optimized. Therefore both Estab and Eads are thermodynamic quantities which refer to different reference systems. A nonadiabatic interaction energy between the different parts of the system is defined as EI ) E0(system) - E1(surface) - ∑E1(FA). The total energies E1 of the isolated systems are derived for fixed geometries taken from the optimized surface-adsorbate system. The difference between the interaction energy and the stabilization energy is denoted as deformation energy. Edef ) E0(surface)E1(surface) + NE0(FA) ∑E1(FA) ) EI - Estab is positive and shows how much energy each part loose due to complex formation. 3.1. Validation of Methods: Formaldehyde on the Ag(111) Surface. Before we started the investigation of formamide on the Ag(111) surface, we tested the reliability of our DFT-D approach for a similar system for which experimental data are available. For this purpose we chose the adsorption of formaldehyde on the Ag(111) surface. An experimental study has shown that the formaldehyde is weakly bound to the silver surface with an adsorption energy of about -26 kJ/mol. The formaldehyde molecule is tilted by ∼60° away from the surface normal.45,46 We performed geometry optimizations with (PBED) and without (PBE) the additional dispersion correction between the surface and the adsorbate. The results of these calculations are summarized in Table 1. The adsorption energy of - 32 kJ/mol obtained with PBE-D is in good agreement with experiment, whereas standard PBE gives essentially no binding. Even though both the PBE-D as well as the PBE approach result in an on-top adsorption, in which the oxygen is directly placed above one Ag atom, we notice pronounced differences between the PBE-D and the PBE structures, see Figure 2. In

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10543 TABLE 1: Formaldehyde Adsorption on the Ag(111) Surfacea PBE PBE-D experiment

Eads

RO-surf

RH-surf

-7.5 -32.3 -26

286.3 268.0

368.3 257.8

a Adsorption energy is Eads (kJ/mol), and molecule-surface distances are RO-Surf and RH-Surf (pm).

Figure 2. Calculated structure for the adsorption of formaldehyde on the Ag(111) surface with (5a, left) and without dispersion correction (5b, right).

TABLE 2: Stabilization Energy per Hydrogen Bond (1/n)Estab (kJ/mol) for Formamide Polymer Chains Obtained with RIMP2 and Plane-Wave (PW) PW91 and PBE (1/n)Estab dimer tetramer hexamer octamer ∞ chain

LCAO-RIMP2

PW-PW91

PW-PBE

- 27.8 - 25.4 - 25.3 - 25.3

- 31.0 - 27.5 - 27.1 - 27.0

- 29.6 - 26.2 - 25.9 - 25.7 - 25.4

particular the surface-hydrogen distance RH-Surf is much shorter with PBE-D, 258 pm compared to 368 pm (PBE). This indicates that the PBE-D calculation covers also an attractive interaction between the CH group and the surface. These results encouraged us to use the PBE-D approach for the following study of the formamide adsorption. 4. Results and Discussion 4.1. Gas-Phase Formamide Polymers. We begin with an investigation of isolated formamide polymers with different methods and basis sets. Detailed computational studies of dimer structures38-42 and of other, singly hydrogen-bonded formamide chains can be found in the literature.40,59-63 We performed planewave calculations with the PBE and PW91 functionals. In order to test the quality of the DFT calculations, we carried out RIMP2 optimizations in a TZVPP basis set as reference. In all cases we get planar structures with hydrogen bond parameters typical for an amide hydrogen bond. The stabilization energies per hydrogen bond (1/n)Estab are given in Table 2. This result reveals an unusual behavior: The studied formamide polymer chains show, contrary to other hydrogen-bonded networks,60-64,16 no cooperativity, the interaction energy per hydrogen bond is almost constant (-25 kJ/mol per hydrogen bond for RIMP2). We even find a small anticooperative behavior by comparing the dimer (-28 kJ/mol per hydrogen bond for RIMP2) with the higher ordered polymers. This is surprising since a chain of singly hydrogen-bonded formamide molecules shows a pronounced cooperativity.62,23 We ascribe the observed noncooperativity to the fact, that the polymer chain consists of C2h symmetric cyclic dimers which have no dipole moment due to symmetry reasons. The small difference of the stabilization energy per hydrogen bond ((1/n)Estab) for the cyclic dimer compared to the higher ordered polymers can be explained by the fact, that in the latter different kinds of hydrogen bonds are present.

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TABLE 3: Adsorption of a Formamide Monomer on the Ag(111) Surfacea Estab

RO-surf

RH-surf

PBE-D PBE

on-top position - 54.6 241.3 -20.2 258.4

216.3 272.4

PBE-D

fcc 3-fold position - 49.2 243.1

240.3

a Estab is the stabilization energy for the adsorption of one formamide molecule on the Ag(111) surface in kJ/mol. RO-surf is the vertical distance between the formamide oxygen and the surface, and RH-surf is the distance between the hydrogen atom pointing towards the surface and the surface. All distances in pm.

The DFT calculations show the same trend as the RIMP2 calculation: The calculated stabilization energy per hydrogen bond is -27 kJ/mol for the PW91 and -25 kJ/mol for the PBE functional, the latter value being in better agreement with MP2. Therefore we chose the PBE functional for the further studies on the Ag(111) surface. Furthermore, these results confirm that the addition of dispersion corrections to the PBE functional is not necessary for an accurate description of amide hydrogen bonds. Accordingly we restrict the additional dispersion correction to the surface-adsorbate interaction in the following studies. 4.2. Formamide Adsorption on the Ag(111) Surface. The formamide adsorption study starts with the investigation of the monomer and different dimers on the Ag(111) surface which was modeled with the 4 × 4 × 4 supercell. We examined the preferred adsorption geometry for these systems by considering both different adsorption positions (oxygen on-top of an Agatom, oxygen bridging two Ag-atoms, and oxygen above the center of three Ag atoms (fcc 3-fold)) and different orientations of the molecules relative to the surface as starting configurations. For the monomer adsorption, we find that only the on-top (with both PBE-D and PBE) and the fcc 3-fold positions (only with PBE-D) are local minima on the potential energy surface. All optimizations with different starting structures lead to a transition to an on-top position. Furthermore we notice that the formamide CO groups point toward the surface in all cases. We call this situation a vertical adsorption. A flat adsorption where the molecule is oriented parallel to the surface could not be observed for the monomer adsorption. The results are summarized in Table 3. The on-top adsorption (system 6 in Figure 3) is most stable, with an adsorption energy of -55 kJ/mol compared to -49 kJ/mol for the fcc 3-fold position (PBE-D). The dispersion correction leads again to a pronounced increase of surfaceadsorbate interaction energy, by -25 kJ/mol, and to shorter surface-adsorbate distances. In principle the attractive interaction can be attributed to a chemical bond between the oxygen and the nearest surface Ag atom, a hydrogen bond between the NH group and the surface electron density, and van der Waals interaction. The existence of the chemical bond between the formamide oxygen and the underlying Ag(1) atom, was evidenced by a density of state (DOS) analysis; see Figure 4. The states between -2.7 and -2.8 eV have significant contributions only from these two atoms, indicating a strong hybridization of their orbitals. In addition, the short distance between the NH hydrogen and the surface (RH-Surf ) 216 pm) indicates that some kind of hydrogen bond interaction is also present. This observation is in line with previous studies in which weak hydrogen bonds between the N-H group of formamide and formic acid and small gold and silver clusters were discussed.25 It has to be noted that the

Figure 3. Adsorption of formamide on Ag(111). Calculated structures for the adsorption of one FA molecule (system 6), the vertical adsorption of two FA molecules on adjacent Ag atoms (system 7), the vertical adsorption with a single hydrogen bond between two FA molecules (system 8), and the flat adsorption of a cyclic FA dimer parallel to the surface (system 9).

Figure 4. Projected density of states showing the existence of a bond between the FA oxygen and the surface atom Ag below O.

potential curve for the adsorption is quite flat. A rotation of the adsorbed formamide along the Ag-O axis leads only to minor changes of the calculated adsorption energy (less than 1 kJ/ mol). In the next step we studied the adsorption of two formamide molecules in two different ways. The first possibility is closely related to the adsorption of a single formamide molecule: both molecules are adsorbed perpendicularly to the Ag surface, see system 7 in Figure 3. The calculated stabilization energy Estab of about -104 kJ/mol is almost twice the energy for the monomer. Therefore there is no significant interaction between the two formamide molecules within the accuracy of our DFT calculations. A slightly different arrangement of the perpendicular adsorption of two formamide molecules is system 8 in Figure 3. This system is additionally stabilized by -19 kJ/mol via a single amide hydrogen bond between the two molecules. However, the distance between the surface and both the corresponding oxygen acceptor and the NH donor is increased due to the formation of this hydrogen bond. This causes a smaller adsorption energy of Eads ) -88 kJ/mol compared to -110 kJ/mol for two independently adsorbed molecules. As a result of these effects, we obtain a total stabilization energy of -107 kJ/mol. The second possibility is the adsorption of a flat-lying dimer which resembles the most stable configuration in the gas phase

Formamide Adsorption on Ag(111) Surfaces

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TABLE 4: Adsorption of Two Formamide Molecules at the Ag(111) Surface (4 × 4 Supercell, Four Ag Layers)a structure

Estab

7 8 9

-103.4 -106.6 -135.3

7 8 9

-37.0 -55.8 -64.0

Eads

RO-surf

-87.7 -76.9

252.2, 254.0 237.5, 269.3 286.4, 289.5

-37.5 -6.0

259.1, 261.7 240.8, 327.2 323.7, 348.0

TABLE 5: Adsorption of Formamide Polymers on the Ag(111) Surface (9 × 4 Supercell, Three Ag Layers)a (1/N)Estab

PBE-D

PBE

a

Estab is the stabilization energy of the complex, and Eads is the adsorption energy of the hydrogen-bonded polymer (kJ/mol). See Figure 3 and Table 3 for the definition of RO-surf.

(system 9 in Figure 3). For this system we get significantly larger O-surface distances (∼290 pm) compared to the vertical adsorption. Furthermore no chemical bond between the oxygens and any surface atom could be detected by means of a partial DOS analysis. Accordingly we calculate for the whole complex a smaller adsorption energy Eads of -77 kJ/mol. However, this arrangement is energetically favored compared to both, two independently adsorbed monomers, and the vertical singly hydrogen-bonded structure due to the formation of two amide hydrogen bonds. The total stabilization energy Estab is calculated to -135 kJ/mol. For comparison the results of uncorrected PBE calculations are also given in Table 4. As expected we find significantly lower interaction energies and considerably larger adsorbate-surface distances as for PBE-D. Based on our test calculations for formaldehyde and also benzene at Ag(111),36 we conclude that the PBE-D results are much more reasonable than the PBE results. PBE without dispersion correction leads to a monomer adsorption energy which is smaller than the experimental value for formaldehyde. However, we expect a larger adsorption energy for the FA molecule due to the additional NH-Ag interaction. The interaction between the flatlying dimer 9 and the surface is mainly caused by dispersion which is not covered by the PBE functional. 4.3. Adsorption of Higher Polymers. Next we studied the adsorption of higher formamide polymers on the Ag(111) surface. Only flat-lying polymers were considered since the previous study has shown that these are the most stable arrangements. This assumption was confirmed by an investigation of a vertical hydrogen-bonded trimer structure for which we find that one of the hydrogen bonds is broken during the optimization. Therefore an expansion of the vertical hydrogen bond motif on the surface is unlikely. The PBE-D results are summarized in Table 5 and Figure 5. As expected, the total stabilization and adsorption energies, Estab and Eads, are linearly correlated with the number of formamide molecules. An extension of the chain by one cyclic dimer increases Estab by ∼-150 kJ/mol and Eads by ∼-65 kJ/mol, respectively. Accordingly, the adsorption of these polymer chains exhibits no cooperativity. From the large values of (1/N)Edisp, it can be seen that the adsorbate-surface interaction is dominated by van der Waals interaction. We can therefore distinguish two different mechanisms of the surface-adsorbate bond for the FA-Ag(111) system: chemisorption for the monomer and physisorption for higher flat-lying polymers. This finding is corroborated by the large average surface distance of ∼300 pm of the flat-lying polymers and the infinite chain, compared to the much shorter distance of 240 pm for the monomer oxygen. With uncorrected PBE we obtained much smaller stabilization energies of ∼-50 kJ/mol per FA molecule, negligible adsorption

(1/N)Eads

monomer dimer tetramer hexamer octamer infinite chain

-58.3 -69.3 -74.3 -77.9 -77.5 -85.8

PBE-D -58.3 -39.7 -35.1 -34.8 -32.5 -35.1

monomer dimer tetramer hexamer octamer

-22.0 -32.5 -42.0 -45.5 -47.2

PBE -22.0 -3.0 -2.8 -2.3 -2.2

(1/N)Edisp -38.9 -45.4 -42.3 -42.7 -41.0 -43.7

Radsorb-surf 239.61 291.9 300.9 299.1 305.0 297.1

352.9 376.9 377.8 389.5

a (1/N)Estab is the stabilization energy per FA molecule, (1/N)Eads the adsorption energy per FA molecule, (1/N)Edisp is the dispersion contribution per FA molecule, and Radsorb-surf is the average distance between the surface and the adsorbate atoms. 1 RO_Surf; in Table 3 a different surface model was used. All energies are in kJ/mol, and distances, in pm.

Figure 5. Optimized structure of the octamer (4) adsorbed on the Ag(111) surface.

energies, -2 kJ/mol per FA molecule, and larger adsorbatesurface distances of ∼390 pm. These findings are in line with a recent study, in which the dispersion interaction between melamine and the Au(111) surface was studied.20 The adsorption of formamide on the Ag(111) surface is connected with stabilization due to hydrogen bonding and adsorbate-surface interaction. In order to analyze the mutual influence of these two kinds of intermolecular interactions, we decomposed the total interaction energy EI in two different ways, similar as in a previous study of the interaction between adenine molecules and the Au(111) surface.65 EI can be divided into the polymer hydrogen bond interaction energy EHB ) E1(polymer) - ∑E1(monomer) and the polymer adsorption poly energy Eads ) E0(surface + polymer) - E1(surface) E1(polymer). As an alternative EI can be divided into the mono ) E0(surface + adsorption energy of the FA monomers Eads 1 1 polymer) - E (surface) - ∑E (monomer) and the hydrogen surf ) E0(surface + polymer) +(N bond interaction energy EHB 1)E1(surface) - ∑E1(surface + monomer) of the FA monomers on the surface. surf mono EI ) EHB + Epoly ads ) EHB + Eads

Again the superscript 0 denotes energies of the optimized species and the superscript 1 denotes energies of the species in the geometry on the surface. The corresponding data are listed in table 6. The two energy decomposition schemes lead to different definitions of the contributions of adsorption- and poly hydrogen bond interaction to the total interaction energy. Eads mono does not match Eads . This can be interpreted (i) as a reduction

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TABLE 6: Decompositions of the Total Interaction Energy EI into Hydrogen Bond Interaction EHB and Esurf HB and poly mono a Surface-Adsorbate Interaction Eads and Eads dimer tetramer hexamer octamer infinite chain

EHB

poly Eads

surf EHB

-68.9 -185.5 -301.5 -423.8 -468.4

-83.1 -144.7 -214.7 -264.9 -283.3

-52.1 -140.5 -231.1 -316.3 -352.7

mono Eads

poly ∆Esurf ∆Edef

-99.9 16.8 -189.7 45.0 -285.1 70.4 -372.4 107.5 -399.0 115.7

3.2 3.5 4.4 3.5 1.5

a The surface influence on interaction energies is ∆Esurf, and the poly . All energies are in kJ/mol. polymer deformation energy is ∆Edef

Figure 6. ∆ESurf plotted against the number of hydrogen bonds.

of the adsorption energy due to the formation of hydrogen bonds. However, likewise we notice that Esurf HB is smaller than EHB. which can be interpreted (ii) as a reduction of hydrogen bonding interaction due to adsorption on the Ag(111) surface. To quantify these effects we introduce ∆Esurf as a measure of the mutual influence of hydrogen bonds and adsorbate-surface interactions. poly mono surf ∆Esurf ) Eads - Eads ) EHB - EHB

∆Esurf is a measure for cooperative effects between the adsorption interaction and the hydrogen bonding interaction. Since it is positive one must speak from anticooperativity in the present case. The very small values of the polymer deformation energy poly , ∼4 kJ/mol, indicate that the presence of the Ag(111) ∆Edef surface has no significant influence on the structure and hydrogen bonding situation of the adsorbate. Therefore we assume that interpretation (i) is correct, the formation of peptide hydrogen bonds parallel to the Ag(111) surface leads to a weakening of the adsorbate-surface interaction. A closer look at ∆ESurf enables us to quantify this effect. It is an almost perfect linear function of the number n of hydrogen bonds (Figure 6). The slope is a measurement for the adsorptionhydrogen bond anticooperativity. Each peptide hydrogen bond parallel to the surface contributes about 7 kJ/mol to this effect. ∆ESurf should vanish if no hydrogen bonds are present. Accordingly we get an intercept which is almost zero (0.4 kJ/mol). 5. Summary and Conclusions In this work we theoretically investigated the mutual influence of hydrogen bonding and adsorbate-surface interaction of FA polymers on the Ag(111) surface. The structural motif was the most stable cyclic FA dimer. In the gas phase the interaction

energy of this dimer is -56 kJ/mol at the RIMP2/TZVPP level of theory. Similar results are obtained with standard DFT functionals PBE and PW91. No cooperativity of the hydrogen bonds is found in extended linear dimer chains, the binding energy per hydrogen bond is constant, -25 kJ/mol. We ascribe this unusual noncooperativity to the fact that the FA polymers are build by symmetric dimers which have no dipole moment. Two bonding mechanisms can be differentiated for the formamide/Ag(111) system depending on the polymer size, chemisorption for the monomer, and physisorption for larger polymers. A vertical on-top adsorption in which the FA oxygen forms a chemical bond to the underlying silver atom is preferred for the monomer. Since the calculated adsorption energy for an adsorption on a fcc 3-fold position is only about 5 kJ/mol higher, we expect a high mobility of FA monomers on the Ag(111) surface. For the dimer and the higher polymers a flat adsorption mode that resembles the most stable isolated polymer configurations is most stable. Here we find no hint for a chemical bond between the surface and the adsorbate, the polymer-adsorption energy is mainly caused by dispersion interaction. A lateral displacement of the adsorbate systems leads to no significant changes, we find no preferred adsorption place for the flat lying polymers. Accordingly we also expect a high mobility of the FA polymer on the surface which is important for the formation of FA monolayer on the Ag(111) surface. We find an anticooperativity of intermolecular hydrogen bonding and molecule-surface bonding of ∼7 kJ/mol per monomer, which is caused by the redistribution of electrons in the hydrogen bonds according to our analysis. In future studies we plan the investigation of FA monolayers on the Ag(111) surface. We expect that the combination of two monomers to a flat lying dimer is the first step of this process. The next step would be the combination of two dimers to a tetramer. At this point one must consider that the C2h symmetric cyclic FA dimer is prochiral and that two enantiomers are present on surfaces. Only the combination of two identical enantiomers leads to the formation of the most stable tetramer with the maximum number of hydrogen bonds. Therefore one can speculate that same kind of chiral recognition leads to linear FA chains which are build by the same enantiomers. An alternative is the formation of a FA monolayer which is related to the molecular sheets of the formamide crystal structure. Such a monolayer can only be constructed if each cyclic dimer is hydrogen-bonded to four opposite enantiomers. In both cases we get an infinite network with 2N hydrogen bonds. Acknowledgment. We acknowledge the financial support from the collaborative research center SFB 624 “Templates” at the University of Bonn. We are grateful for computing time at the NIC in Ju¨lich. References and Notes (1) Mingos, D. M. P. , Ed.; Supramolecular Assembly Via Hydrogen Bonds; Springer: Berlin, 2004. (2) Balzani, V.; Venturi, M.; Credi, A. Molecular DeVices and Machines; Wiley-VCH: Weinheim, Germany, 2003. (3) Steiner, T. Angew. Chem. 2002, 114, 50–80. (4) Schalley, C. A.; Reckien, W.; Peyerimhoff, S. D.; Baytekin, B.; Vo¨gtle, F. Chem.sEur. J. 2004, 10, 4777. (5) Cicoira, F.; Santato, C.; Rosei, F. Top. Curr. Chem. 2008, 285, 203–267. (6) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. Single. Mol 2002, 3, 25. (7) Dmitriev, A.; Lin, N.; Weckesser, L.; Barth, J.; Kern, K. J. Phys. Chem. B 2002, 106, 6907.

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