Substituted Benzene Derivatives on the Cu (111) Surface

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Substituted Benzene Derivatives on the Cu(111) Surface Scott Simpson and Eva Zurek* Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000, United States S Supporting Information *

ABSTRACT: First-principles calculations on substituted benzenes adsorbed to the Cu(111) surface illustrate that quintessential activating/deactivating substituents can be used to control the amount of charge transfer at the adsorbate− surface junction. A frontier orbital perspective is employed to develop an understanding of the factors influencing the magnitude and direction of electron donation upon absorption and the strength of the metal−organic interaction. Classic activating groups generally increase and prototypical deactivating groups decrease the amount of charge transferred from the adsorbate to the metal surface. In the case of trihalogenated benzenes, the donor strength is inversely proportional to the electronegativity of the substituent. An interplay of the Coulomb repulsion (a result of the charge transfer) between the adsorbate molecules along with attractive supramolecular forces (i.e., hydrogen bonding, van der Waals interactions) is important in determining the network architecture.

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INTRODUCTION Understanding how to engineer the metal−organic interface to design devices such as molecular electronics,1 organic lightemitting diodes,2,3 antifouling agents,4 molecular lubricants, molecular traps,5 and sensing technologies6 is one of the major goals of nanotechnology and materials science.7,8 Some ways to control the pattern the molecules self-assemble into include constructing covalently bound networks9 or making use of metal coordination chemistry.10 For example, an artistic array of honeycomb arrangements (such as perylene and melanine molecules on the Au(111) surface11,12), molecular chains,13 and Celtic trinity shapes14 have been constructed. In some cases the patterns into which the molecules arrange is coverage dependent15 or different patterns can coexist.16 The interactions between the adsorbate molecules may be attractive or repulsive. Attractive or organized networks, such as 2H-TPP,17,18 phenylacetylene19 on Ag(111), anthraquinone on Cu(111),20 or xanthine on Au(111)21 organize due to weak van der Waals interactions between the surface and the adsorbate and relatively strong supramolecular forces.22,23 The surface diffusion barrier for these molecules is small compared to that of the intermolecular interactions, which allows for aggregation on the metal surface.24 Repulsive or disorganized networks, such as benzene on Cu(111),25 2H-TPP on Cu(111),17 or HBC on Au(111)26 only form organized networks when the coverage is high. We have previously shown that repulsive architectures arise as a result of significant charge transfer from the adsorbate to the metal surface.27 Organic molecules are attracted to the surface via van der Waals forces. However, because of Pauli repulsion between filled orbitals, charge density gets pushed out from between the adsorbate and metal giving rise to the so-called “pillow effect”.28 © 2012 American Chemical Society

This rearrangement of charge density has a profound impact on the interface dipole and work function.28−33 Of course, another reason for charge redistribution comes from the interaction of the molecule’s orbitals and the surface bands. The conventional description of the adsorption of benzene is essentially based on Blyholder’s model for surface− CO bonding:34 electron donation from the highest occupied molecular orbital (HOMO) of CO (5σ) to the d bands on the metal surface, and back-donation from the d bands to the lowest unoccupied molecular orbital (LUMO) on CO. Modifications of the Blyholder model, such as the strong mixing of the CO 4σ and 5σ orbitals as well as the importance of the surface s and p bands,35 have been put forward. The Blyholder picture adapted to unsaturated hydrocarbons (the Dewar, Chatt, Duncanson model36,37) has been applied to ethylene, benzene,38 acetylene,39 and the free base porphine40 on Cu(110). For some metals the charge redistribution leads to a significant loss of the bond order which results in an increase in the C−C distance and/or a Kekulé distortion, along with a concomitant bending of the hydrogen atoms away from the surface.41 Below, we apply a frontier orbital perspective42 to analyze the bonding between the Cu(111) surface and substituted benzenes. We consider how the perturbation of the electronic structure of the organic molecule as well as the neighboring surface atoms may influence the supramolecular interactions. It is our hope that this type of analysis will ultimately lead to a chemical way of rationally predicting the patterns which may Received: March 20, 2012 Revised: May 15, 2012 Published: May 18, 2012 12636

dx.doi.org/10.1021/jp302682n | J. Phys. Chem. C 2012, 116, 12636−12643

The Journal of Physical Chemistry C

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levels in the middle and right panels will not be discrete; they will disperse. Interactions that were not possible or unimportant in molecules may become important for molecules interacting with a surface as well. In a four-electron event, similar to that illustrated in Figure 1c, the raising of the antibonding combination higher in energy than the lowering of the bonding one is a result of the antisymmetrization of the wave functions (Pauli repulsion). Between two molecules this scenario would lead to a repulsive interaction. However, if the energy of the antibonding combination rises above the Fermi level, electrons will be “dumped” into the surface bands, thereby relieving the Pauli repulsion.52,53 Since the degree of orbital overlap dictates the splitting of the bonding and antibonding combinations, Pauli repulsion may result in a barrier to chemisorption.41,52,54 Similarly, the interplay of two empty orbitals will have no consequence for molecules. However, if the bonding combination falls below the Fermi level of the surface, as illustrated in Figure 1d, it will become occupied, leading to a net bonding interaction. Second-order electron shifts (not shown) may also be of importance in a metallic solid.42 Clearly many factors other than van der Waals attraction influence the surface−adsorbate bonding: the energy of the Fermi level and the frontier orbitals of the adsorbate along with their overlap with the surface bands. In addition, the surface− adsorbate interactions in turn affect the supramolecular ones.

form upon molecular surface deposition. The focus will be on physisorbed or weakly chemisorbed systems: ones where strong chemical bonding between the metal and the adsorbate does not dominate the network architecture. In reality, molecules often aggregate on step edges and defects, which will not be considered here. Because of the plethora of studieswhether they be of the extended Hückel type,41,43 wave function based,28,30,32,44 or performed with density functional theory and plane waves45−48 or atom-centered basis sets49we start by considering benzene. The bonding between this quintessential aromatic and metal surfaces is understood, but there are some variations50 which generally follow the well-established trends of chemical reactivity of the metals (a point we will return to in a follow-up study). Further, we examine the effects that various electrondonating and -withdrawing substituents have on the bonding energies as well as the magnitude and direction of charge transfer of various trisubstituted benzene derivatives interacting with Cu(111). Upon the basis of the results of our dispersioncorrected density functional theory (DFT-D3)51 calculations, interaction diagrams are constructed which are used to explain the observed trends.

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SURFACE−ADSORBATE INTERACTIONS During the interaction between the surface and the adsorbate MOs the Fermi level (EF) does not change: the Fermi sea is both a large reservoir of electrons and an electron sink. As in molecules, two-electron events, which give rise to electron transfer between the surface and the adsorbate, dominate the attraction. In the Blyholder model a picture similar to Figure 1a

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RESULTS AND DISCUSSION Benzene on Cu(111). Standard density functionals yield virtually no bonding between Cu(111) and benzene, −0.5 kcal/ mol,47 as they are not able to describe van der Waals forces. MP2 calculations using a 32-atom cluster as a model for the surface are closer to the experimental estimate of −13.6 kcal/ mol,55 yielding a bonding energy of −8.1 kcal/mol.30 The interaction energies computed with van der Waals DFT and plane wave basis sets are in even better agreement: they range from −11.3 to −12.7 kcal/mol, with a slight dependence upon the site to which benzene adsorbs and upon whether or not the six-layer surface slab was relaxed.48 We optimized this system using DFT-D3, and a 64- atom copper cluster consisting of two layers was used to model the Cu(111) surface. It was assumed that the center of mass of benzene lies on top of the central copper so as to maintain C3v symmetry. The BSSE-corrected bonding energy of −25.5 kcal/ mol is about twice as large as experiment. Using a two-layer deep 166-atom cluster lowers the bonding energy slightly to −25.1 kcal/mol. The reason for the too high value we calculate is likely due to overestimation of dispersion interactions by the functional: the DFT-D3 adsorption energy of benzene on an Ag(111) cluster was ∼1.7 times too large in magnitude.51 The bonding strength we calculate for Cu(111) is greater, and the metal−adsorbate separation of 2.94 Å is shorter than the 3.630 and 3.75 Å48 obtained in previous work. There is little change of the benzene geometry upon adsorption in our study: the C− C bonds lengthen by less than 0.005 Å, and the hydrogen atoms tilt toward the surface by only 1.2°. It appears that there are no barriers to absorption.30,48 In Figure 2a we provide a schematic diagram showing the interactions between benzene and the copper cluster constructed using the results of a fragment orbital (SFO) analysis.56 The original picture (provided in the Supporting Information) has been modified in a number of ways. First, the energy levels of Cu(111) and the molecule/adsorbate system

Figure 1. Possible interactions between the MOs of a molecule and the surface states. (a/b) Molecule donates/accepts electron density to/from the surface. (c) Four-electron event in which electron density is transferred from the antibonding component to the Fermi level. (d) Interaction between two empty orbitals may lead to a bonding component which falls below the Fermi level with concomitant charge transfer.

would be representative of electron donation from the HOMO of CO to the surface and Figure 1b the back-donation from the metal bands to the LUMO of CO. Of course, in reality, there will not only be a single surface level but a number of bands with the right symmetry and energy at some k-point to mix strongly with the frontier orbitals of the molecule. This will result in more than one molecule−surface state. Thus, the 12637



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Figure 2. (a) Schematic interaction diagram between benzene and a 64-atom Cu(111) cluster model. Red/blue bands contain some benzene LUMO/HOMO character as well as character from surfaces MOs. Top (b) and side (c) views of a charge density difference (CDD) isosurface plot (±0.0003 au), where red signifies a build up and blue a depletion of charge upon adsorption. Frontier orbitals of benzene are also shown (isovalue ± 0.03 au). (d) Contour plot of the CDD, where the plane of the contour passes through two hydrogen atoms.

Figure 3. Contour plots of orbitals which are representative of those formed from the overlap of the benzene HOMO and surface states. Even though the interaction between the adsorbate and surface in (a) is in phase whereas in (b) it is out of phase, both surface/benzene MOs fall below the Fermi level. The metal orbitals involved display primarily Cu d character. The plane of the contour bisects the Cu(111) cluster and contains two H and two C atoms.

are represented as a continuum, whereas in our calculations there are a finite number of states but the spacing between them is small. In the middle panel we provide an approximate bandwidth encompassing the states which have metal and benzene HOMO (blue) and LUMO (red) character, as extracted from the SFO analysis. The projected densities of states (PDOS) of the molecule and surface are provided in the Supporting Information. The PDOS of the Cu(111) slab shows a sharp peak whose top lies ∼2 eV below the Fermi level that can be attributed to the Cu d bands. Importantly for the bonding, there is a small amount of Cu d character which mixes with the s and p extending past EF. Moreover, the s and p bands have a large dispersion, interpenetrating the d bands. Only the major contributions to the Cu DOS are labeled schematically in the left panel of Figure 2a. The picture obtained from our finite calculations matches well with the one expected based upon the Dewar, Chatt, Duncanson model. We observe electron transfer from the benzene HOMO to the metal bands and back-donation from copper to the benzene LUMO. The SFO analysis shows a number of surface/molecule MOs that are a result of twoelectron events, similar to those illustrated in Figure 1a and 1b. The contour plots in Figure 3 are representative of occupied MOs which display character from the benzene frontier orbitals and the metal surface. From these and from the SFOs it appears that it is the Cu d and not the sp orbitals which mix the most with the benzene frontier MOs. The redistribution of charge (CDD) which occurs upon adsorption is illustrated Figure 2b and 2c with red/blue symbolizing a gain/loss of charge. The surface−CDD is reminiscent of the local modifications of the surface electron density, the Friedel oscillations. This picture is qualitatively similar to the one computed by Bagus and Wöll,28 who also analyzed the bonding in the benzene−Cu(111) system.32 However, a contour diagram of the CDD in Figure 2d reveals that it is not only the π system which loses electron density during surface adsorptiona result of surface induced σ−π mixing and of the tilting of the hydrogen atoms toward the surface.

The BSSE-uncorrected bonding energy we calculate (ΔE = ECu−C6H6 − ECu − EC6H6 = −29.6 kcal/mol) can be decomposed as56 ΔE = ΔEgeo + ΔEsteric + ΔEoi + ΔEdisp

(1)

where ΔEgeo, the energy required to change the geometry of benzene and the copper cluster to the one in the optimized system, is small, 2.1 kcal/mol. ΔEsteric, the steric repulsion, is a sum of the Pauli repulsion (ΔEPauli = 73.8 kcal/mol) and the classical electrostatic interaction between the interpenetrating charge densities of the fragments (ΔVelstat = −38.5 kcal/mol). The orbital interaction, ΔEoi = ECu−C6H6 − E[Ψ0] (where Ψ0 is the antisymmetrized and normalized product wave function of the isolated copper cluster and benzene molecule in their final geometries), is −27.6 kcal/mol. The dispersion energy, ΔEdisp, is calculated as being −39.2 kcal/mol. The last three terms in eq 1 are of the same order of magnitude, with the first being a destabilizing and the last two stabilizing interactions. Benzene on Cu(111) forms repulsive networks which aggregate on imperfections or step edges on the metal surface, and organization only occurs at high coverages.25,48,55,57,58 The benzene molecules interact via direct intermolecular forces (van der Waals attraction and Coulomb repulsion) as well as by indirect mechanisms mediated by the surface. Assuming that the indirect forces are weaker than the intermolecular ones, one can set up a computational experiment to determine what the charge on benzene must be so that there is no net attraction between two molecules. We optimized the geometries for the three benzene dimers considered in ref 48 as well as a fourth configuration which was even more stable (see the Supporting Information). The geometries of charged species were also optimized, and it was found that above a charge of ∼0.15 per benzene there was no net bonding (ΔEdimer > 0). A Mulliken/ 12638



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Hirschfeld/Voronoi analysis shows that the overall charge transfer to the surface is −0.19/−0.27/−0.28, suggesting that benzene molecules will repel each other on the Cu(111) surface, which is inline with experimental observations. The computational methodology we use overestimates the binding energy, underestimates the molecule−surface distance, and therefore give rise to a larger dispersion of the bands. A Hirschfeld analysis where the vertical distance between the copper slab and benzene was constrained to 3.4 Å showed that −0.14e (where e is the unit of elementary charge) was transferred to the surface instead. It is also therefore likely that we are overestimating the electron-donating strength of the molecules. However, our primary interest lies in the observed trends and in particular on how the amount of charge transferred by substituted benzenes to Cu(111) compares with that computed for the parent molecule. Benzene Derivatives Trisubstituted with Activating Groups. In order to determine how chemical modification of benzene affects the bonding energies, charge transfer, and concomitant network formation on Cu(111), the geometries of a number of trisubstituted benzene derivatives were optimized on a cluster composed of 64 copper atoms. The functional groups were classified according to those that add electron density to the conjugated π system of benzene (activating groups) and those which withdraw it (deactivating groups). Calculated isosurfaces of the electron density colored by the electrostatic potential (see the Supporting Information) aided the classification. Schematic interaction diagrams as determined from the SFO analysis are provided in the Supporting Information. Activating groups can add electron density to the benzene ring via either the σ or the π system (or both). Classic examples of the two are alkyl and methoxyl. Whereas in electrophilic aromatic substitutions activating groups behave as ortho and para directors, in surface absorption they increase the electrondonating capability (Lewis basicity) of the molecule. As shown below, the direction and magnitude of charge transfer is dependent upon the energies of the molecular frontier orbitals, which in many cases have non-negligible contributions from the substituents and their overlap with the surface bands. Table 1 lists the vertical distance between the central copper and the carbon atom in the benzene ring closest to it, the charge accumulated on the surface after molecular adsorption, and the bonding energy of benzenes trisubstituted with common activating groups adsorbed on Cu(111). In most cases the charges obtained from a Mulliken, Hirschfeld, and Voronoi analysis agree quite well with each other. However, for N(CH3)2 and N(OH)2 the values found using the Mulliken scheme differ by about a factor of 2 from the other methods. In both computations at least one of the hydrogen atoms was within 2.4−2.7 Å of the copper cluster. It is likely that because of the large basis sets employed in the calculations and the close proximity to the surface the Mulliken analysis does not do a good job of determining the charges on these hydrogen atoms. Hirschfeld and Voronoi charges are relatively basis set independent and likely to provide a better description, so they will be employed in deducing the trends below. The substituents in Table 1 have been listed roughly in order of decreasing amount of charge transferred to the copper slab. With a few exceptions, the order agrees well with the activating group strength in standard textbooks. The donating capability of these molecules is at least as good as that of benzene, and most are better nucleophiles upon interaction with Cu(111).

Table 1. Distance between the Surface and the Molecule (DCu−molecule in Å)a, Surface Charge As Calculated by a Mulliken (QM), Hirshfeld (QH), and Voronoi (QV) Charge Analysis, and BSSE-Corrected Bonding Energy (ΔEbond in kcal/mol) for Benzenes Trisubstituted with Activating Functional Groupsb functional group

DCu−molecule (Å)

QM (e)

QH (e)

QV (e)

ΔEbond

N(CH3)2 NH2 N(OH)2 CCH CH3 OHc

2.90 2.86 3.26 3.07 3.01 2.82

−1.16 −0.48 −0.18 −0.45 −0.53 −0.30

−0.63 −0.44 −0.39 −0.35 −0.33 −0.31

−0.66 −0.41 −0.40 −0.37 −0.35 −0.27

−68.1 −52.4 −33.0 −36.7 −39.5 −34.1

a

Taken as the vertical distance between the central copper and the carbon in the benzene ring which was closest to the surface. bFor benzene, DCu−molecule = 2.94 Å, QM = −0.19, QH = −0.27, QV = −0.28, ΔEbond = −25.5 kcal/mol. cOptimized with C1 symmetry. The systems had C3v symmetry, except where reoptimization with C1 led to a structural distortion which substantially lowered the energy of the system.

To understand why this occurs, consider the interaction diagram of 1,3,5-trimethylbenzene (mesitylene) in Figure 4.

Figure 4. Same as Figure 2a−c except for mesitylene. Charge density is donated from the doubly degenerate HOMO of mesitylene to Cu(111), but back-donation into the doubly degenerate LUMO does not occur.

Comparison with the diagram computed for benzene in Figure 2 shows that functionalization raises the energy of the HOMO by about 0.6 eV. This does not affect the bonding much: in both cases charge is donated from the HOMO, which lies below EF, to the metal surface (from the SFO analysis it is difficult to determine how much is a result of two-electron and how much of four-electron events). Importantly, for the magnitude of charge transfer, however, is the fact that the inphase mixture of the LUMO with the metal MOs does not fall below EF, in contrast to what was computed for benzene. The absence of back-donation from the metal to the organic is one of the main reasons why mesitylene donates more charge to the copper cluster than does the parent molecule. The CDD in Figure 4b and 4c supports this analysis, showing a build up of charge on the metal surface and depletion in the π system. The magnitude of the BSSE-uncorrected bonding energy of −45.1 kcal/mol for mesitylene is larger than that for benzene. In order to determine the origin of this difference, we compare the various terms in eq 1. As in benzene, the geometrical distortion was small (ΔEgeo = 2.5 kcal/mol): the C−C bonds lengthen by less than 0.005 Å, and the methyl groups pucker 2.5° away from the surface upon adsorption. Despite the 12639



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bulkier methyl group, the distance between mesitylene and the surface is only 0.07 Å longer, which is in line with the stronger dispersion energy of −61.9 kcal/mol and the larger Pauli repulsion of 91.2 kcal/mol. Overall these two effects nearly cancel, so that Δbenzene−mesitylene(ΔEPauli − ΔEdisp) = 5.3 kcal/ mol. The magnitude of the electrostatic and orbital interactions, −45.3 and −31.6 kcal/mol, is somewhat greater for mesitylene than for benzene, also contributing to the stronger bonding. The main features of the interaction diagram computed for mesitylene are representative of all of the derivatives provided in Table 1. One difference is that in the case of CCH, NH2, and N(CH3)2 other occupied MOs such as the HOMO-1 and HOMO-2 are also found to strongly interact with the copper levels (see the Supporting Information). These MOs, as well as the frontier orbitals of OH and N(OH)2, are not only localized in the benzene π system but also contain a substantial amount of character arising from the functional groups. Regardless, all of the interaction diagrams suggest that functionalization of benzene with activating groups prevents back-donation to the molecule, leading to a greater transfer of charge to the surface. Provided that the attractive van der Waals forces and hydrogen-bonding interactions are not stronger in magnitude than the intermolecular Coulomb repulsion, it is likely that the molecules functionalized with strong activating functional groups in Table 1 will repel each other on Cu(111). Like benzene, organization will occur at high coverage or on step− edges or defects. Benzene Derivatives Trisubstituted with Deactivating Groups. Benzenes functionalized with electron-withdrawing substituents deactivate primarily the ortho and para positions (act as meta directors) in electrophilic aromatic substitutions. In the case of surface adsorption, the electron-donating strength of these systems was decreased as compared to that of the parent molecule. The amount of charge donated to the surface upon adsorption correlates well with textbook knowledge on the deactivating group strength, see Table 2. NO2, NF2, CN, and SO3H were found to be the strongest deactivating substituents, whereas BF2, CF3, CCl3, COOH, and COH were only weakly deactivating.

Figure 5. Same as Figure 2a−c except for 1,3,5-trinitrobenzene. Charge density is donated from the metal surface to the doubly degenerate LUMO and the LUMO+1 of 1,3,5-trinitrobenzene.

must occur. Indeed, the CDD we compute shows a significant loss of charge (blue) on the copper atoms and an increase around the nitrogen, oxygen, and carbon atoms (red). Because of the large build up of charge, 1,3,5-trinitrobenzene molecules are likely to repel each other on Cu(111). The magnitude of the BSSE-uncorrected bonding energy of −39.6 kcal/mol is greater than that of benzene. Despite the bulkiness of the NO2 group, the vertical distance between a carbon atom in the ring and the surface is only 0.1 Å larger than in the underivatized system, resulting in an increased Pauli repulsion (105.4 kcal/mol). The attractive dispersion, electrostatic, and orbital interactions are stronger than for benzene, on the other hand. We calculate these as being −58.4, −49.9, and −44.9 kcal/mol, respectively. The geometric distortion upon adsorption is non-negligible with ΔEgeo = 8.2 kcal/mol. In particular, the nitro groups bend toward the surface by 4.3°, so that the molecule is no longer planar with D3h but instead has C3v symmetry. There is an elongation of the N−C bonds by 0.03 Å and the N−O bonds by 0.06 Å. This geometrical distortion was also observed when the optimization was carried out without employing any symmetry constraints. For all of the systems functionalized with deactivating groups, the HOMO and LUMO energies were computed to be lower than those of benzene. The amount of charge transfer occurring in the species derivatized with NF2 and CN was nearly negligible. The approximate interaction diagrams of most of these systems were similar to that obtained for the parent molecule (see Figure 2) with electron donation from the HOMO to the metal surface and back-donation to the LUMO. In the case of CF3 and COH the bottom of the LUMO/metal band did not fall below EF, so back bonding did not occur. Because of the relatively small amount of charge buildup on benzenes trisubstituted with NF2, CN, SO3H, BF2, CF3, CCl3, COOH, and COH they may organize into attractive networks on the Cu(111) surface provided that the hydrogen-bonding and intermolecular van der Waals attraction is sufficiently strong. In order to test this further we carried out computations on trimesic acid, TMA, which self-assembles into hydrogenbonded networks on Cu(100),59 Cu(110),60 along with other surfaces.61,62 In the gas phase the BSSE-corrected dimerization energy is computed as being −16.9 kcal/mol for the neutral species. A geometry optimization where each TMA was assigned a charge corresponding to the one computed for the adsorbed molecule (+0.18, see the second last row in Table 2) also gave rise to a net attractive interaction. In fact, our gasphase model calculations showed that Coulomb repulsion

Table 2. As in Table 1 except for Benzenes Trisubstituted with Deactivating Functional Groups functional group

DCu−molecule (Å)

QM (e)

QH (e)

QV (e)

ΔEbond

NO2 NF2 CN SO3Ha BF2 CF3 CCl3 COOHa COH

3.05 3.27 3.34 3.11 3.09 3.37 3.42 3.10 3.17

0.30 0.09 −0.01 −0.10 −0.15 0.03 −0.19 −0.18 −0.19

0.22 −0.07 −0.09 −0.08 −0.11 −0.12 −0.17 −0.19 −0.22

0.20 −0.09 −0.11 −0.12 −0.14 −0.16 −0.19 −0.20 −0.27

−33.7 −23.5 −31.4 −45.0 −31.7 −18.9 −35.3 −38.1 −34.5

a

Optimized with C1 symmetry.

Remarkably, for the strongest deactivator, 1,3,5-trinitrobenzene, all of the schemes used to compute charge unambiguously show that electron density is injected from the surface into the molecule. Upon adsorption to Cu(111) 1,3,5-trinitrobenzene behaves as an electron acceptor, a Lewis acid. The interaction diagram in Figure 5 illustrates that the energies of the LUMO and LUMO+1 fall below the Fermi level, so a substantial amount of back-donation from the surface to the molecule 12640



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similar to Figure 1c but with both occupied orbitals falling below EF) and how much is attractive (like Figure 1a, resulting in donation of charge to the metal surface). The bandwidth is small, indicating little overlap between the HOMO and the metal MOs. Back-donation to the LUMO does not occur. Because of the high electronegativity of fluorine little charge is donated to the metal surface and the bonding energy is decreased as compared with benzene. The CDD is indicative of a slight build up of charge localized primarily on the copper atoms below the hydrogens and a decrease of electron density in the π system which has contributions both on carbon and on fluorine. Comparison of the CDD with the one computed for benzene shows that there is a smaller redistribution of charge upon adsorption. The magnitude of the BSSE-uncorrected bonding energy of −23.8 kcal/mol is smaller than that for benzene. The larger distance between the metal and the surface is in part due to the bulkier substituent but also a result of the weaker bonding which manifests in less negative dispersion, electrostatic, and orbital interaction energies of −34.9, −21.6, and −15.1 kcal/ mol. The magnitude of the Pauli repulsion (45.9 kcal/mol) is also decreased from that of the parent molecule. As expected, the geometrical distortion (ΔEgeo = 1.9 kcal/mol) was minimal. Going down the periodic table, the electronegativity of the halogens decreases as does their electron-withdrawing ability. As a result, the amount of charge transferred increases along with the bond strength. The HOMO is raised and the LUMO lowered in energy. Because the size of the halogen atoms increases whereas the surface−benzene distance does not change much, the overlap between the frontier MOs and the metal bands increases. This gives rise to a larger bandwidth, with the top of the HOMO/metal band falling above and the bottom of the LUMO/metal band below EF. The latter results in back-donation of charge to the molecule. However, as shown in the Supporting Information, it is not only the frontier orbitals which participate in bonding between the copper slab and the substituted benzene: as a result of the close proximity of the surface and the adsorbate coupled with the large size of the substituents, other orbitals which contain a substantial amount of halogen π character become important as well. It may be that for the more electronegative halogens the van der Waals attraction will be sufficiently large to overcome the Coulomb repulsion between the substituents so that organized, attractive networks form.

overcame the hydrogen-bonding and dispersion forces (ΔEdimer > 0) above a charge of ∼0.67 per TMA molecule. Geometry optimization has also been carried out on the TMA dimer adsorbed to a copper cluster composed of 166 atoms. The BSSE-uncorrected bonding energy was determined by ΔEdimer = Edimer ‐ Cu − E TMA1‐ Cu − E TMA 2 ‐ Cu + ECu

(2)

where Edimer‑Cu, ETMA1‑Cu, and ETMA2‑Cu are the energies of the TMA dimer as well as of the individual monomers adsorbed to the copper cluster. The energy of the surface, ECu, is added in order to avoid double counting. The BSSE-corrected dimerization energy of −19.3 kcal/mol is nearly equivalent to the one computed in the gas phase, suggesting that TMA will form attractive networks on Cu(111). Benzene Derivatives Trisubstituted with Halogens. Despite behaving as deactivating groups, the halogens are ortho and para directors in electrophilic aromatic substitutions. The peculiar behavior of the halogens arises because of two competing factors: due to their high electronegativity the halogens withdraw electron density from the benzene ring through σ bonds while at the same time donating electron density via the π system. The charges we calculate, see Table 3, Table 3. As in Table 1 except for Benzenes Trisubstituted with Halogen Atoms functional group

DCu−molecule (Å)

QM (e)

QH (e)

QV (e)

ΔEbond

F Cl Br I

3.12 3.19 3.20 3.21

0.02 −0.16 −0.24 −0.34

−0.16 −0.18 −0.22 −0.27

−0.17 −0.19 −0.22 −0.27

−19.3 −28.2 −32.6 −37.6

show that generally the halogens deactivate benzene toward nucleophilic reaction with the copper cluster. Derivitization with the most electronegative element, fluorine, results in less charge donation and a weaker bonding energy than for benzene. Triiodobenzene, on the other hand, is about as good of a donor as the parent molecule presumably because of the decreased electronegativity of the substituent. Comparison of the interaction diagram for 1,3,5-trifluorobenzene in Figure 6 with the one for benzene shows that the energy of the HOMO has been raised and that of the LUMO has been lowered as a result of the derivitization. It is not straightforward to determine how much of the interaction of the HOMO with the metal is due to Pauli repulsion (i.e.,

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CONCLUSIONS Dispersion-corrected density functional theory calculations using a finite cluster model for the Cu(111) surface have been carried out in order to study benzene surface adsorption. Our results confirm that the bonding can be explained using the Blyholder model adapted to unsaturated hydrocarbons. Electron donation from the HOMO of benzene to the metal d bands and back-donation to the LUMO results in a net transfer of −0.27 to the surface as determined using a Hirschfeld scheme. Coulomb repulsion between the positively charged molecules overcomes attractive van der Waals forces so that benzene forms repulsive networks on Cu(111), which only become organized when coverage is high. Computations on trisubstituted benzenes show that functionalization with classic activating groups can be used to increase the amount of charge transferred to the metal surface. Benzenes derivatized with N(CH3)2, NH2, N(OH)2, CCH, CH3, and OH behave as better Lewis bases upon adsorption to

Figure 6. Same as Figure 2a−c except for 1,3,5-trifluorobenzene. The smaller band width of the HOMO/metal and LUMO/metal states as compared to that of benzene is indicative of less orbital overlap. 12641



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surely has an impact on the interaction diagrams we draw. However, we are primarily interested in how the computed properties of the functionalized benzenes compare with those found for benzene, and in particular the trends in the amount of charge transferred.

the Cu(111) surface than does the parent molecule. Provided that the intermolecular dispersion and hydrogen-bonding interactions between these molecules do not exceed the Coulomb repulsion, they are likely to form disorganized patterns when adsorbed on Cu(111). Quintessential activating groups, on the other hand, decrease the amount of charge transferred to the metal surface, so that it may be that benzenes functionalized with NF2, CN, SO3H, BF2, CF3, CCl3, COOH, and COH self-assemble into attractive networks on Cu(111). Because the LUMO and LUMO+1 of the most deactivating species considered falls below the Fermi level, 1,3,5trinitrobenzene is found to behave as a Lewis acid upon interaction with Cu(111). The substantial amount of charge transferred suggests that trinitrobenzene molecules will repel each other on the metal surface. The Lewis basicity of trihalogenated benzenes decreases with increasing electronegativity. Our results show that activating and deactivating groups may be employed in order to control the amount and direction of charge transferred at the surface−adsorbate interface. The interaction of the molecular frontier orbitals with the surface bands perturbs their electronic structure, affecting the supramolecular interactions and concomitantly the molecular selfassembly.

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ASSOCIATED CONTENT

S Supporting Information *

Optimized coordinates, interaction diagrams, and CDD isosurface plots for each system. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: ezurek@buffalo.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the Center of Computational Research at SUNY Buffalo, as well as the useful comments and suggestions of Javid Rzayev and the Zurek Research Group.

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METHODS The DFT-D3 calculations were carried out using the ADF software package,56,63 the revPBE generalized gradient density functional,64−67 and Grimme’s dispersion-correction.51 The basis functions on all of the atoms consisted of a triple-ζ Slatertype basis set with polarization functions (TZP) from the ADF basis-set library. The core shells up to 1s for B, C, N, O, and F, up to 2p for S and Cl, up to 3p for Cu and Br, and up to 4p for I were kept frozen. Generally, the Cu(111) surface was modeled using a 64-atom cluster consisting of two layers with 37/27 atoms in the top/ bottom layers. Because of the ABC packing in copper, adding a third layer would have reduced the symmetry of the systems and substantially increased the computational cost. In all of the calculations the top layer of the surface was free to relax, whereas the bottom remained fixed at the experimental lattice parameter of 3.614 Å.68 In order to reduce the computational expense C3v symmetry was initially maintained in the optimizations of the molecules adsorbed to the cluster, and the center of mass of the benzene ring was placed over a central copper atom. The relaxed geometries were employed as starting structures for optimizations using C1 symmetry. In only a few cases a much more stable geometrical alternative was found during reoptimization. For the trimesic acid dimer a larger copper cluster with 91 and 75 atoms in the two layers was employed, and the coordinates of both layers were kept fixed during structural relaxation in order to reduce the computational expense. The basis-set superposition error (BSSE) was obtained using the Counterpoise method. To clarify the nature of the bonding a fragment orbital analysis56 was performed using the distorted metal surface and the molecule (in the geometry of the optimized metal− adsorbate system) as fragments. This yielded the composition of the MOs in terms of the occupied and unoccupied MOs of the fragments as well as the charge density difference (CDD) plots. We note that overestimation of the dispersion interaction strength by DFT-D3 as well as the well-known tendency of conventional DFT to underestimate HOMO−LUMO gaps

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Substituted Benzene Derivatives on the Cu (111) Surface

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