J. Phys. Chem. C 2007, 111, 5493-5496
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Molecular Orbital Model for Pyridine/r-Pyridyl Adsorption on Metal Surfaces Travis E. Jones,*,†,‡ Chen Zuo,‡ Paul W. Jagodzinski,‡ and Mark E. Eberhart†,‡ Molecular Theory Group and Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401 ReceiVed: December 8, 2006; In Final Form: February 1, 2007
Electronic density functional calculations were used to explain the proposed formation of R-pyridyl from pyridine on silver, copper, cadmium, and gold surfaces. Our results show that the formation of R-pyridyl is governed by the mixing of metal wavefunctions with pyridine’s near-HOMOs. When the organic orbitals mix with metal sp-character, a bonding interaction results and pyridine is the dominant surface species. If the metal orbitals mixing most strongly with the pyridine near-HOMOs are of d-character then an antibonding or nonbonding interaction results and R-pyridyl is the dominant surface species. A predicted correlation between the ratio of the concentration of the two surfaces species and the applied electrode potential is supported by analysis of surface-enhanced Raman data.
Introduction Surface-enhanced Raman (SER) spectra for pyridine absorbed on chemically clean electroneutral copper and gold 〈111〉 surfaces suggest that upon adsorption pyridine loses an R-hydrogen and R-pyridyl becomes the dominant surface species.1 On silver and cadmium, pyridine is absorbed without significant loss of the R-hydrogen, and on nickel and iron an intermediate situation exists. A quantitative assignment of the ratio of R-pyridyl to pyridine bound on the metal surface is difficult, due to the differences in surface signal enhancement. However, the qualitative trend from predominantly R-pyridyl to predominantly pyridine is believed to proceed through the series Cu > Au > Fe > Ni > Cd ≈ Ag. We know of no satisfactory explanation for this trend. Here we attempt to provide a molecular level explanation for the formation of R-pyridyl. We were unable to explain the interaction of pyridine with metal surfaces to favor the formation of R-pyridyl based on electron count, size, crystal structure, or other simple periodic relationships. Thus, to rationalize the experimental findings, electronic density functional calculations were performed with a goal of elucidating the observed trend in terms of quantum mechanical variables. Calculations All calculations were performed with the Amsterdam Density Functional version 2004.01 (ADF).2-4 Core double-ζ, valence triple-ζ, doubly polarized (TZ2P) basis set, and Zero Order Regular Approximation (ZORA)5-9 relativistic corrections were used. Large frozen cores (Ag.4p, Au.4f, C.1s, Cd.4p, Cu.3p, N.1s) were employed, along with the Vosko-Wilk-Nusair (VWN)10 Local Density Approximation (LDA). While the results reported here are those for large frozen cores no significant differences were observed when using all-electron calculations. The calculations consisted of two parts: (1) building clusters qualitatively representative of a metal close * To whom correspondence should be addressed. E-mail: trjones@ mines.edu. † Molecular Theory Group. ‡ Department of Chemistry and Geochemistry.
packed surface and (2) studying the interaction of these surfaces with pyridine and R-pyridyl. Surface Models. For the sake of computational speed and ease of interpretation, we began by finding the minimum size cluster necessary to model the chemical interactions between a close packed metal surface and pyridine/R-pyridyl. The electronic structures of 48 (12 each for Cu, Au, Cd, and Ag) different metal clusters were calculated. Using the interatomic distance of the bulk metal11,12 and stacking one to four close packed planes, with varying number of atoms in each plane, yielded clusters of 10, 13, 19, 25, 31, 34, 37, 42, 50, and 62 metal atoms. For convenience we will denote these as Metal{number of layers,number of atoms}. Hence, a copper cluster of 2 layers and 10 atoms will be denoted Cu{2,10}. It has been shown13-15 that the appropriate cluster size used to model surface properties depends on both the elements involved and phenomenon of interest. By way of illustration, the determination of an absorbed species’ geometry can generally be found by using a smaller cluster as that required for calculating adsorption energy. Here, we are concerned with the variation of the molecular orbital (MO) interactions between the adsorbing molecule (pyridine/R-pyridyl) and the metal surface. The variation in these interactions may be depicted by way of MO-correlation diagrams. Figure 1 shows the MO correlation diagrams for pyridine interacting with three different copper clusters Cu{2,10}, Cu{2,31}, and Cu{4,62}. In each diagram, the molecular orbital manifold of the copper cluster is shown to the left, the MOs of pyridine to the right, and the combined metal-pyridine MOs in the center. The molecular orbitals of the metal-organic complex are computed as linear combinations of the metal and pyridine MOs. The lines connecting the elements of the center manifold to those on the right and left give a qualitative representation of the relative contributions of metal and pyridine character to the MOs of the combined system. The lowest lying orbitals of the metal-pyridine complex “descend” almost exclusively from orbitals of the metal or those of pyridine. Electrons occupying these MOs will be localized either on the organic or metal substrate. On the other hand, the orbitals descending from pyridine’s HOMO, HOMO-1, and HOMO-2
10.1021/jp068458w CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007
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Figure 1. Correlation diagrams for pyridine on the following clusters: Cu{2,10}, Cu{2,31}, and Cu{4,62}. All three cases show similar bonding.
Figure 2. Pyridine-Ag bonding MO for the Ag{2,10} and Ag{2,37} clusters. Both cases show similar bonding between the pyridine and silver.
mix significantly with the MOs from the metal cluster. It is in these “mixed” MOs that the chemical interactions between pyridine and, in this case, the copper surface are mediated. Inspection of Figure 1 shows that the nature of the orbital interactions between pyridine and the copper cluster qualitatively agree through the series Cu{2,10}, Cu{2,31}, and Cu{4,62}. Examination of the metal-organic orbital wavefunctions through the same series shows this agreement to be quantitative, in that regardless of the size of the cluster used to model the metal surface, respective orbitals are composed of similar amounts of metal and organic character. These observations lead us to conclude that the general interactions driving the adsorption of pyridine to copper are controlled by a local interaction between the organic molecule and the copper atom(s) to which it is bound. While including copper atoms distant from the binding site or incorporating surface relaxations may affect the adsorption energy, we do not believe these will alter the overall character of the chemical interactions. When pyridine absorbs on the other metals, an identical trend to that of copper is observed. In the case of silver, for example, the corresponding MO’s formed by pyridine absorbing on the Ag{2,10} and Ag{2,37} surfaces are shown in Figure 2. The two surfaces show similar pyridine-silver interactions. Hence, for the purposes of this discussion, we will restrict our analysis to the M{2,10}-pyridine/R-pyridyl clusters.
Figure 3. Pyridine and R-pyridyl HOMO overlap with Ag, Cu, Au, and Cd bands.
Surface Interactions. Our results show that the formation of R-pyridyl is governed by the mixing of metal MOs with the pyridine orbitals close to the pyridine HOMO. If the interaction with pyridine’s near-HOMOs is through metal wavefunctions of sp-character, the resulting set of mixed molecular orbitals are typified by amplitude across the surface. They are bonding between the surface and pyridine molecule. We denote these interactions as Type 1. In the second, or Type 2, interactions, the metal orbitals mixing most strongly with the pyridine nearHOMOs are of d-character. In this case, the resulting set of pyridine-metal MOs are characterized by nodes in the density parallel to the surface and by little amplitude on the metal surface. These mixed MOs are thus antibonding or nonbonding between the pyridine and the metal. We have discovered that in those systems where Type 2 mixing predominates, R-pyridyl formation is most pronounced. Thus, a qualitative prediction of the extent of metal adsorbed pyridine to metal absorbed R-pyridyl reduces to understanding the factors that influence s-orbital versus d-orbital mixing with pyridine’s HOMO. Figure 3 shows the energy of the pyridine HOMO relative to the calculated molecular orbital manifolds of Ag{2,31}, Cu{2,31}, Au{2,31}, and Cd{2,31}. A representative orbital descending from pyridine’s HOMO and mixing with silver and copper wavefunctions is shown in Figures 2 and 4, respectively.
Pyridine/R-Pyridyl Adsorption on Metal Surfaces
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ψn
For silver, where the energy of pyridine’s HOMO is coincident with the top of the metal’s sp-band, the resultant metal-organic interaction is of Type 1. In the case of copper, the pyridine HOMO is well within the metal d-band and is of Type 2. Though not shown, the cadmium-pyridine interactions are also of Type 1. Gold is, however, best described as a hybrid of Type 1 and Type 2. Pyridine interacts with gold orbitals, which have substantial d-character, on the lower edge of the sp-band. These observations lead us to speculate that when the energy of the pyridine HOMO lies within the metal sp-band, Type 1 mixing ensues. Type 2 mixing is observed when the energy of pyridine’s HOMO lies within the metal d-band. And for those cases where pyridine’s near-HOMOs straddle the top of the d-band, both Type 1 and Type 2 interactions are present. Also shown in Figure 3 is the energy of the HOMO of R-pyridyl. (The loss of a hydrogen atom from the R-carbon shifts the energy of this orbital up by about 1.5 eV.) Note that this orbital now lies in the sp-band for all metals modeled. Further, the R-pyridyl HOMO-metal interactions are all of Type 1 (as modeled with R-pyridyl-M{2,10} clusters, where M ) Cu, Ag, Au, and Cd). We conjecture that on copper, where the pyridinemetal interaction is Type 2, i.e., nonbonding to antibonding, increased bonding (as seen in Figure 4) will drive the formation of R-pyridyl, while on gold the hybrid mixing will result in the formation of R-pyridyl, though to a lesser extent. On silver, and cadmium, where the pyridine interaction is already of Type 1, there is insufficient driving force (through increased bonding) to form R-pyridyl. Discussion The basis for our conjecture is rooted firmly in perturbation theory where the preference for Type 1 or Type 2 mixing can easily be rationalized.16 For orbitals to mix they must have good spatial and energetic overlap, in the absence of large charge transfer between the pyridine molecule and the metal surface. The energetic requirements can be seen in the first-order perturbation:
+
∑ m*n
〈ψm(0)|H ˆ ′|ψn(0)〉 En
(0)
- Em
(0)
ψm(0)
where ψn is the perturbed wavefunction to first order, H ˆ ′ is the electronic interaction between the metal and adsorbate, and the summation term is the first-order correction to the unperturbed superposition of ψn(0) by ψm(0), the organic and metal wavefunctions, respectively. Inspection of this equation reveals that large changes in the wavefunction require the matrix elements of H ˆ ′ be comparable to the energy difference of the unperturbed orbitals. This energetic criterion for orbital overlap implies that the pyridine MOs can mix strongly with metal MOs of comparable energy. In the case of silver, the pyridine HOMO coincides with the metal sp-band (Figure 2). As the 5s-atomic orbitals of silver are also spatially diffuse, both criteria for good orbital mixing between the pyridine HOMO and the sp-band of silver are satisfied. The same conditions apply to cadmium, resulting in nearly identical behavior to that of silver. From an energy standpoint, pyridine will interact strongly with the d-band of copper and to a lesser extent with the d-band of gold, Figure 3. However, metal 3d orbitals are contracted, and both the 3d and 5d transition metal orbitals are less diffuse than s-orbitals. As a result, bonding of pyridine to copper and gold through d-orbital overlap will be minimal. However, by virtue of the alignment of pyridine’s HOMO with the top of gold’s d-band, pyridine can bind to the metal surface through a limited set of gold s-orbitals, leading to the adsorption of some pyridine. This behavior is evident in the gold SER spectrum, which shows features intermediate with those of copper and silver (cadmium).1 Interactions between organic and metal MOs offer a concise rationale of the tendency for pyridine to form R-pyridyl on metal surfaces. The dominant factor is the interaction between pyridine’s near-HOMOs and the metal. When the mixing involves primarily sp-character on the metal, denoted Type 1, primarily pyridine is adsorbed, while Type 2 mixing is observed when the organic interacts with surface d-character. This yields copper, gold, silver, and cadmium calculations all in agreement with experimental data, which some have postulated to show a change in pyridine orientation, and which we believe is a change from end-on to edge-on,1,17 which is coincident with the shift from Type 1 to Type 2 mixing. Additional support for our model is found in the spectral dependency on applied voltage, which will rigidly shift a metal’s bands. For example, if a sufficient negative voltage is applied to silver, its d-band can be made to straddle pyridine’s HOMO, which should lead to Type 2 mixing and R-pyridyl formation. Similarly, the d-band of gold can be shifted up (down) with an applied negative (positive) potential. This shift should increase (decrease) the amount of R-pyridyl on a gold surface. SER data reported by Brolo and co-workers for a Au(210) surface exposed to aqueous pyridine support this model (632.8 nm excitation).17 The data show a change in the relative intensities of the 1010 cm-1 ring-breathing mode (Wilson number 1) and the 1035 cm-1 ring mode (Wilson number 12) as the potential is varied. As the voltage becomes more positive the relative intensities change from disparate to nearly equal, which reflects a decreased amount of R-pyridyl due to the down shift of the gold d-band. The authors indicate that there is a change in the orientation of pyridine as the potential is varied. Our experimental results suggest that this is a change in average orientation (from edgeon to end-on) due to the decrease in the amount of R-pyridyl on the surface.1 (1)
Figure 4. Molecular orbitals representative of the set of MOs resulting from Cu{2,10} (A) and R-pyridyl mixing with Cu{2,10} (B). Type 2 mixing is seen with pyridine, while Type 1 is observed with R-pyridyl. While changes in these MOs are dramatic, the variation in total charge density is subtle. Hence, the vibrational energies are not altered significantly.1
= ψn
(0)
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Figure 5. Change in intensity ratio of SER bands at 1037 and 1008 cm-1 as a function of applied voltage for gold.17
A further conformation of the model is obtained by plotting the ratio of the intensities of the SER bands at approximately 1037 cm-1 and approximately 1008 cm-1 versus the potential applied to various metal electrodes. The significance of this ratio was discussed in ref 1 where it was argued that, “Adsorbed pyridine is bound end-on and displays a strong enhancement of both the trigonal ring breathing mode (∼1037 cm-1) and the totally symmetric ring breathing mode (∼1008 cm-1). The formation of R-pyridyl is accompanied by a change in orientation, from end-on to edge-on. While this has little impact on the totally symmetric ring breathing mode, the trigonal ring breathing mode converts to another in-plane ring distortion mode, with less dipole along the z-axis, and is not enhanced as strongly.” In other words, the formation of R-pyridyl is accompanied by the disappearance of the trigonal ring breathing mode. As such, the ratio of this breathing mode (∼1037 cm-1) to the totally symmetric ring breathing mode (∼1008 cm-1) represents the relative amounts of pyridine to R-pyridyl species on the electrode surface, which by our model should vary with electrode potential and would explain the observations of Brolo and co-workers.17 We have constructed such plots based on previously reported data. Figure 5 shows gold;17 here the intensity ratio is a linear function of applied potential. Though not presented, SER data obtained with silver,18 nickel,19 and iron20 electrodes also show near-linear relationships. Considering the difficulty in obtaining precise intensity ratios from literature data the relationship is remarkable. Preliminary calculations were performed on nickel and platinum in an effort to test the predictive capabilities of our model. In accordance with the model proposed here, we predict that the trend in R-pyridyl formation will proceed through the series Pt ≈ Cu > Au > Ni > Cd ≈ Ag, with those metals to the right characterized only by pyridine adsorption. In the case of nickel, precursory experimental results support this prediction.1 Conclusion While the calculation of total-binding energy requires 70 surface atoms, or more, for single atom adsorption,14 and extensive computational facilities, by employing a chemically intuitive model, we have been able to rationalize complex experimental findings with modest computational investments.
With the growing availability of first principles methods, we believe that simple models of the sort described here will prove increasingly useful in the interpretation of spectroscopic data. We have applied this approach to explaining surface-enhanced Raman data for pyridine and have proposed a molecular level model for the formation of R-pyridyl on metal surfaces. This has provided support for the previous explanation of observed data and has allowed us to show a relationship between SER intensities and applied electrode potential. Acknowledgment. The authors thank DARPA and ONR for support of this work. References and Notes (1) Zuo, C.; Jagodzinski, P. W. J. Phys. Chem. B 2005, 109, 1788. (2) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Fonseca Guerra, C.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931. (3) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor. Chem. Acc. 1998, 99, 391. (4) ADF2004.01, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands: http://www.scm.com. (5) van Lenthe, E.; Ehlers, A. E.; Baerends, E. J. J. Chem. Phys. 1999, 110, 8943. (6) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99, 4597. (7) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1994, 101, 9783. (8) van Lenthe, E.; Snijders, J. G.; Baerends, E. J. J. Chem. Phys. 1996, 105, 6505. (9) van Lenthe, E.; van Leeuwen, R.; Baerends, E. J.; Snijders, J. G. Int. J. Quantum Chem. 1996, 57, 281. (10) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (11) Kittel, C. Introduction to Solid State Physics, 7th ed.; J. Wiley and Sons: New York, 1996. (12) Cullity, B. D. Elements of X-Ray Diffraction, 2nd ed.; AddisonWesley Publishing Co.: Reading, MA, 1978. (13) Bauschlicher, C. W., Jr. J. Chem. Phys. 1994, 101, 3250. (14) Jacob, T.; Fritzsche, S.; Sepp, W.-D.; Fricke, B.; Anton, J. Phys. Lett. A 2002, 300, 71. (15) Hu, Zhenming; Boyd, R. J. J. Chem. Phys. 2000, 112, 9562. (16) Levine, I. Quantum Chemistry, 5th ed.; Prentice Hall: Upper Saddle River, NJ, 1970. (17) Brolo, A. G.; Irish, D. E.; Lipkowski, J. J. Phys. Chem. B. 1997, 101, 3906. (18) Chase, B.; Parkinson, B. J. Phys. Chem. 1991, 95, 7810. (19) Aramaki, K.; Yamada, M.; Uehara, J.; Nishihara, H. J. Electrochem. Soc. 1991, 138, 3389. (20) Guo, L.; Huang, Q.; Li, X-y.; Yang, S. Phys. Chem. Chem. Phys. 2001, 3, 1661.
