J. Phys. Chem. B 2001, 105, 641-645
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Interaction between Metallic p Orbitals and the π Orbitals of Organic Molecules: The Binding between Ethylene and Aluminum Elizabeth M. King,*,† Stewart J. Clark,‡ Claudio F. Verdozzi,† and Graeme J. Ackland† Department of Physics and Astronomy, The UniVersity of Edinburgh, Edinburgh EH9 3JZ, U.K., and Department of Physics and Astronomy, The UniVersity of Durham, Durham DH1 3LE, U.K. ReceiVed: April 3, 2000; In Final Form: September 11, 2000
A set of first principles calculations has been performed to characterize the interaction of a π bonded organic molecule with a simple s-p metal surface. The structures of the Al(C2H4) and [Al(C2H4)]+ complex molecules, and of the adsorption of an isolated ethylene molecule onto Al(100) were calculated. We find a C2H4π fAl3p interaction to be the most significant in Al(C2H4) and [Al(C2H4)]+ with additional Al 3p f C2H4π* backdonation in Al(C2H4). In complete contrast, no chemical binding of ethylene to Al(100) is observed, but physisorption is predicted. These calculations show how the binding between a π bond and the π orbitals of a simple metal atom cannot be extended to describing the binding with a p band of a solid. Our conclusions may be extended to other systems where this interaction could be significant.
1. Introduction An understanding of the chemical binding of organic molecules to metallic ions and surfaces is essential for describing a diverse range of structures such as organometallic complexes and biological macromolecules, and also chemical processes such as catalysis and vapor deposition. To identify the salient features of the chemical binding in these systems, we have used first principles calculations to understand the role played by the metallic p orbitals when conjugated organic molecules are adsorbed on an s-p metal surface. This interaction is well represented by the binding of ethylene to aluminum, where the ethylene C- C bond is thought to interact with the valence p orbitals of aluminum. Although the aluminum surface has been studied extensively (for example, see refs 1-4) and the adsorption thereon of molecules such as hydrogen5 and oxygen6 has been looked at, little attention has been paid to the adsorption of conjugated organic molecules. Most previous work has concentrated on the study of gas phase complex molecules such as Al(C2H4)7-10 and [Al(C2H4)]+,11-12 the structures of which possibly give some indication as to the nature of chemisorption of C2H4 on Al(100). Figure 1 shows five suggested bonding schemes for Al(C2H4) with 1d being favored by Kasai,7 1c being favored by Manceron et al.9 and 1e being favored in a recent DFT study by Alikhani et al.10 The results of previous studies are therefore not entirely in agreement with one another, and a recent study by Duschek et al.13 showing that no chemical bonding takes place between benzene and Al(111) suggests that it may not be possible to extend the results of the complex molecule calculations to chemisorption on an extended surface. We first repeat simulations of Al(C2H4) and [Al(C2H4)]+ using the CASTEP planewave DFT code (ref 19). This technique has previously been successfully applied to characterizing crystal structures,15 surfaces,16 isolated molecules,17 and chemisorp* Corresponding author. E-mail:
[email protected]. Fax: +44 (0)131 650 7165. † The University of Edinburgh. ‡ The University of Durham.
Figure 1. Selection of the possible binding schemes for Al(C2H4). 1a and 1d were put forward by Kasai,7 1b and 1c by Manceron et al.9 and Kasai, and 1e by Manceron et al. 1c,d,e have previously been identified as the most likely bonding schemes.
tion.18 Extended slab calculations are then performed using the same technique to see whether the same bonding schemes can be applied to chemisorption on the surface. A brief description of the calculation technique is followed by the results, and the conclusion summarizes the characterization of the interaction of valence p orbitals with a π molecular orbital. 2. Calculation Method The density functional theory (DFT) of Hohenberg and Kohn14 was implemented using the CASTEP code (Cambridge Sequential Total Energy Package) and the CETEP code (Cambridge-Edinburgh Total Energy Package).19 The many body effects of exchange and correlation between electrons are accounted for using the general gradient approximation (GGA),20,21 and its spin polarized version (GGS)20,21 for systems with an unpaired electron in the valence shell. In this computation the Kohn-Sham (KS) orbitals23 are used to describe the valence electronic structure, an approach that has recently been validated by Stowasser et al.22 The influence of the core states is represented by pseudopotentials.24-26 The KS orbitals are expanded using a basis set of planewaves taken up to a specified kinetic energy cutoff of 700 eV for all
10.1021/jp0012810 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/03/2001
642 J. Phys. Chem. B, Vol. 105, No. 3, 2001
Figure 2. (a) Ground-state geometry of Al(C2H4). The starting configuration consisted of an ethylene molecule with the aluminum atom placed at a height of 2.5 Å above the plane of the ethylene molecule and the center of the C- C bond. (b) Ground-state geometry of [Al(C2H4)]+. The starting configuration consisted of an ethylene molecule with the aluminum atom placed at a height of 3.0 Å above the plane of the ethylene molecule and the center of the C- C bond. The positive charge was not associated with any specific constituent of the complex at the start of the calculation. Distances are given in Å and angles in degrees.
calculations, the basis set being checked for completeness by the convergence of the total energy of the system as the cutoff is increased. The energy is minimized using the method of steepest descents until the energy has converged to within 2 × 10-4 eV. The nuclei are moved under the influence of Hellmann-Feynman forces once the ground-state electronic structure has been determined, until the force acting on each ion does not exceed a certain tolerance, here taken to be 0.05 eV Å-1. The calculation is performed on a system placed in a periodically repeating supercell where periodic boundary conditions are enforced,27 atoms in any one supercell being separated by at least 5 Å of vacuum from atoms in adjacent supercells. To characterize the binding between ethylene and aluminum, calculations were done on five different systems: the isolated complexes Al(C2H4) (Figure 2a) and [Al(C2H4)]+ (Figure 2b), the Al(100) surface, ethylene adsorbed at both top and bridge sites on a fixed Al(100) surface (Figure 3), and a larger calculation where the top surface layers were allowed to relax in response to the adsorption of ethylene. For the calculations involving the Al+ ion, a uniform background negative charge is included to counteract the infinite positive charge generated by the ions in all the supercells. To ensure that we had determined the ground-state structures, we looked at different positions of Al relative to the ethylene molecule in Al(C2H4) and [Al(C2H4)]+, and for the adsorption calculation the initial adsorbatessubstrate distance was varied between 2 Å, corresponding to the length of a covalent bond, and 4 Å, at which no attraction between the species is detected. For the calculations on the complexes, a supercell of dimensions 8 × 8 × 8 Å was required to isolate the molecules. For the isolated aluminum surface the slab was separated from the slabs in supercells above and below by a vacuum region of
King et al.
Figure 3. Supercell contents for calculations of the adsorption of C2H4 on a fixed Al(100) surface (the lighter color Al atoms are on the top layer, the darker ones on the layer below). Here the top site adsorption configuration is shown. The starting configuration had the ethylene molecule in its isolated gas-phase geometry placed at height ranging from 2 to 4 Å above the aluminum surface.
5 Å, the total energy having been checked for convergence with respect to vacuum depth. For convergence of the total energy, five layers of aluminum were needed with the geometry of the top two layers allowed to relax and the bottom three kept fixed in the bulk geometry. As the surface relaxation of Al(100) has been reported to be less than 1%,1-4 adsorption calculations were performed with an isolated ethylene molecule being adsorbed onto a two layer fixed slab. For the convergence of the total energy, the slab had to have dimensions of 2 × 2 unit cells (approximately 8 × 8 Å) in the surface plane in order to separate the ethylene molecules. In the final calculation ethylene was positioned at the top site above a slab of 5 layers in which the geometry of the lower three were fixed and with 2 × 2 unit cells in the surface plane. There was at least 5 Å of vacuum separating the ethylene molecule from the slab in the cell above it in both adsorption calculations. For an isolated molecule the states are determined at one reciprocal space k-point, the Γ point of the supercell. For calculations performed on metallic slabs that are finite and therefore aperiodic in one direction, the states need only be determined using a 2- D grid. For the isolated surface a grid of 13 × 13 × 1 (28 special k-points) was used, for C2H4 on 2 layers of Al(100) and C2H4 on 5 layers of Al(100) a 4 × 4 × 1 grid (4 special k-points) determined according to the scheme of Monkhorst- Pack28 was used. 3. Results and Discussion 3.1. [Al(C2H4)]+ and Al(C2H4)}. The binding energies given in Table 1 show that [Al(C2H4)]+ and Al(C2H4) are stable complexes with the Al atom placed symmetrically above the ethylene molecule in agreement with the previous studies.7-12 The geometry of [Al(C2H4)]+ shown in Figure 2b suggests that ethylene retains its molecular identity in the complex. This suggestion is also supported by the comparison of the C- C and C- H bond lengths with those of isolated ethylene in Table 1. The previous study of [Al(C2H4)]+ by Sto¨ckigt12 using Fourier transform cyclotron resonance mass spectrometry and post
Binding between Ethylene and Aluminum
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TABLE 1: Comparison of Molecule Calculationsa study C2H4 C2H6 Sto¨ckigt12 [Al(C2H4)]+ [Al(C2H4)]+ Alikhani10 Al(C2H4) Al(C2H4)
C-C/Å C-H/Å Al height/Å binding energy/eV 1.312 1.513 1.355 1.328 1.400 1.391
1.084 1.093 1.086 1.088 1.080 1.086
2.774 2.715 2.182 2.111
0.590 0.784 0.577 1.076
a Various factors that are significant in determining the type of interactions present in the complexes using our methods are compared with previous studies. Relevant parameters of the complexes are compared to ethylene and ethane to show how the C- C π bond is influenced by the interaction with aluminum.
Figure 5. (a) Illustration of the interaction between aluminum p orbital and the π antibonding orbital of ethylene. This would give rise to the charge density surface of the KS orbital corresponding to the binding π interaction between aluminum and ethylene in (b).
Figure 4. (a) Plot of the charge density surface of the KS orbital corresponding to a [Al(C2H4)]+ binding σ interaction between aluminum and ethylene. The shape of the charge denstity of this KS orbital is similar to that which would be predicted from the interaction of an aluminum p orbital with the π binding orbital of ethylene, as can be seen in (b) (c) Plot of the charge density surface of a KS orbital corresponding to a Al(C2H4) binding σ interaction between aluminum and ethylene. This surface would also be predicted by the interaction displayed in (b).
Hartree- Fock calculations determined that a binding interaction occurred between Al+ and C2H4 by σ donation from the C- C π bond into the Al 3p orbital perpendicular to this bond. The charge density surface of the KS orbital (Figure 4a) from our study corresponding to the σ binding interaction confirms this analysis, clearly showing the interaction between the ethylene C- C π orbital and an unoccupied 3p orbital, the Al 3s orbital being nonbinding. A schematic diagram showing how the unoccupied aluminum 3p orbital and part of the C- C π orbital combine is shown in Figure 4b. In addition, we performed a Mulliken population analysis,29,30 which gave a charge of 0.88e
on Al, showing that there is electron transfer from ethylene to aluminum consistent with the σ interaction. The C- C and C- H bond lengths of ethylene in Al(C2H4) would also suggest that the ethylene C- C π bond is not broken in this complex, but the C- C π bond must be weaker than in [Al(C2H4)]+ when the bending of the C- H bonds out of the molecular plane is considered (Figure 2a). The greater stability of Al(C2H4) over [Al(C2H4)]+ is also apparent from the aluminum- ethylene distance and the greater binding energy. The geometry of our cluster is in good agreement with that determined by Alikhani10 with a DFT study (Table 1) and electron localization function (ELF) analysis to characterize the binding. However, there are two KS orbitals corresponding to binding interactions, the one shown in Figure 4c being similar to the σ orbital in [Al(C2H4)]+, and the other π interaction shown in Figure 5b consistent with donation from the occupied Al 3p orbital to the ethylene π* bond as described by Alikhani and shown in Figure 5a. By examining the eigenenergies of these orbitals displayed in the energy level diagram in Figure 6, we ascertain that in fact the σ interaction is dominant with a lesser contribution from the π interaction. The charge on aluminum according to the Mulliken population analysis is 0.46, which indicates electron transfer from aluminum to ethylene. This is consistent with our calculated molecular dipole of 2.48 D directed from aluminum to ethylene and agrees with our prediction that the σ bond is dominant. There is a discrepancy of 0.5 eV between our Al(C2H4) binding energy and that reported by Alikhani:10 the experimental binding energy reported by Mitchel8 of greater than 0.694 eV provides a lower bound for the binding energy and lies between the two calculated values, our result being 0.382 eV greater than this bound. 3.2. Al(100). Previous experimental studies1-3 and calculations4 have shown there to be little if any relaxation of the Al(100) surface, which is the open face of fcc Al. As predicted by the previous studies, in our calculation only the top two layers were displaced in the direction perpendicular to the surface plane during surface relaxation. There was no movement of atoms in the surface plane and a barely perceptible surface contraction of 0.6% between the first two layers only, the spacing between surface layers differing from that in the bulk by 0.01 Å. 3.3. Adsorption Calculations. The adsorption calculations were done using a two layer slab with the lattice parameter fixed at its bulk value of 3.952 Å. The robustness of our conclusions
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Figure 6. Energy level diagram showing the bonding KS orbitals of [Al(C2H4)]+ and Al(C2H4). As can be seen, the eigenenergies of the σ binding orbitals of the complex lie below the π binding orbital of the ethylene molecule making the interaction with the aluminum unoccupied 3p orbital energetically favorable. The π molecular orbital of Al(C2H4) lies at a higher energy than the σ orbital and so has a lesser contribution to the binding energy.
TABLE 2: Binding Energies for Adsorption of C2H4 on Al(100)a system top site bridge site
C-C/Å C-H/Å binding energy/eV height of C2H4/Å 1.314 1.315
1.086 1.087
0.0345 0.0206
3.505 3.507
a As can be seen, the top site is preferred. Both the binding energies and distances between substrate and adsorbate correspond to physisorption of C2H4 on Al(100).
were then tested by adsorbing ethylene onto a five layer slab, the top two layers of which are allowed to relax. Table 2 shows the results for the top site, with the center of the C- C bond directly above an aluminum atom in the top layer, and a bridge site with the center of the C- C bond directly above the center of an aluminum atom in the second layer. As can be seen, the top site is favored over the bridge site. As the adsorption geometry of ethylene is virtually identical to the isolated gas phase molecule, there is little interaction with the surface. From the DOS plot shown in Figure 7a six sharp peaks can clearly be distinguished corresponding to the six molecular orbitals of ethylene, undisturbed by the presence of the surface. A plot of the charge density surface shown in Figure 7b shows that there is no increase in density between the ethylene molecule and the surface and that no distortion of the surface takes place. This is supported by the results of a Mulliken population analysis where there was no charge associated with any of the surface aluminum atoms. Our results show conclusively that chemisorption does not take place as the binding energy and adsorbate-substrate separation correspond to physisorption. This result was then confirmed using a five layer slab calculation. For starting heights of the ethylene molecule above the surface ranging from 2.5 to 4.0 Å, the ethylene molecule
Figure 7. (a) Density of states plot of ethylene physisorbed at the top site on Al(100). Six peaks can be distinguished below the Fermi level corresponding to the six molecular orbitals of ethylene. (b) Charge density surface of ethylene physisorbed at the top site on Al(100). No chemical interaction between the two species can be discerned.
always moved to a distance greater than 3.5 Å from the surface despite the rearrangement of the surface atoms in response to the presence of ethylene. This gives us confidence in the results of the fixed slab adsorption calculations. The fact that ethylene does not chemisorb onto aluminum can be understood from the directional p orbitals required to form chemical bonds in the complex molecules not being defined in the band structure description appropriate to the electronic structure of the aluminum surface. As the requirement for the formation of a chemical bond is the presence of an occupied and unoccupied p orbital of the correct energy and symmetry, no bond is formed with the metal surface. 4. Comments and Conclusions We have applied the same DFT planewave code to the study of both the complex molecules Al(C2H4) and [Al(C2H4)]+, and the adsorption of C2H4 on Al(100). A direct comparison could therefore be made between two extremes of adsorption system representation. The cluster calculations predicted stable [Al(C2H4)]+ and Al(C2H4) complexes in agreement with previous studies. The binding in [Al(C2H4)]+ was due to a σ bond between the ethylene π bond and an empty 3p orbital on aluminum. In Al(C2H4) the dominant interaction was due to the same σ bond with an additional contribution from a singly occupied π bond
Binding between Ethylene and Aluminum formed between the occupied aluminum 3p orbital and the ethylene π* orbital. However, the slab calculation predicted that no interaction took place between the ethylene molecule and the Al(100) surface with a low barrier to diffusion across the surface. Our calculations therefore show that the conclusions derived from studying an isolated molecule cannot be extended to a description of adsorption on an extended metal surface in this case. The fact that no interaction takes place in the slab calculation can be understood by the treatment of aluminum as a free electron metal. Here the concept of an empty p orbital is no longer appropriate and so the highly directional binding predicted by the cluster calculations cannot be realized. Instead, only a much weaker physisorption interaction can take place. This explanation could also be extended to the prediction that no adsorption would take place on different crystal faces, higher surface coverage and higher temperatures. Our work supports the conclusions of a recent study of the adsorption of benzene on Al(111),13 which predicted physisorption. For the adsorption of species with conjugated electron systems onto transition metals it would therefore be expected that only the more directional d bands would be involved in the binding between adsorbate and substrate. The results of this study show the limits of considering adsorption to be a localized process for determining the type of binding and geometries at adsorption sites. Acknowledgment. We acknowledge contributions from Dr. J. Crain and especially from Dr. S. Bates for helpful discussions. The computational work was done with support from The U.K. Car-Parrinello Consortium, EPCC and CSAR. E.M.K. acknowledges the support of the EPSRC and DERA. References and Notes (1) van Hove, M. A.; Tong, S. Y.; Stoner, M. Surf. Sci. 1976, 54, 259. (2) Aderdam, D.; Bauding, R.; Ganbert, C. Surf. Sci. 1977, 62, 567.
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