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J. Phys. Chem. C 2007, 111, 11384-11389
A Magic Pd-Ag Binary Cluster on the Fs-Defected MgO(100) Surface Giovanni Barcaro and Alessandro Fortunelli* Molecular Modeling Laboratory, IPCF - CNR, Via G. Moruzzi 1, I-56124 Pisa, Italy ReceiVed: March 21, 2007; In Final Form: May 21, 2007
The structure of Pd1AgN clusters (N ) 1-8), both in the gas-phase and adsorbed on an Fs-center of an MgO(100) terrace, is investigated via a density-functional basin-hopping (DF-BH) approach. A structural transition from planar to noncrystalline 5-fold symmetric configurations is found for both free and adsorbed clusters in this size range. The Pd-Ag clusters are highly fluxional. In addition, Pd1Ag6 is found to be a magic cluster, exhibiting a large HOMO-LUMO gap (both in the gas-phase and adsorbed on the defect) and a peculiar structural stability (when adsorbed on the defect). To our knowledge, this is the first example of a magic metal cluster on an oxide surface. This is rationalized in terms of an electronic shell-closure, involving also the electrons trapped in the oxygen vacancy, coupled with a good adhesion to the defected surface. The low-energy Pd1Ag6 isomers (with excitation energies around 0.1-0.3 eV) adsorbed on the Fs-center also exhibit a large HOMO-LUMO gap.
Introduction Metal clusters supported on oxide surfaces have been intensively studied for a long time, due to their important applications in fields as diverse as catalysis, optoelectronic and magnetic devices, chemical sensors, etc.1-3 Much knowledge has been accumulated on these systems, in particular concentrating attention in recent years on the relationships between surface defects and metal cluster growth. It is by now clear that the presence of defects such as kinks, corners, vacancies, can have a dramatic effect on the metal/surface interaction, and thus on the characteristics of the oxide support with respect to the nucleation process, and for orienting the structure of metal clusters adsorbed on it. In this context, the MgO(100) surface is certainly one of the most studied systems, for its widespread use as an inert support and for its simplicity, MgO(100) being an apolar, simple ionic surface with no surface reconstruction.2,4 Among its possible defects, the neutral oxygen vacancy (or Fscenter) on the regular terrace has often been chosen as a prototypical example in theoretical studies, and much effort has been devoted to its full characterization. 5-12 These studies have shown that the metal/surface interaction around such a defect presents peculiar features, namely rotational invariance and a large basin of attraction with a much reduced equilibrium distance on top of the defect with respect to neighboring sites (“double frustration”).13,14 As a consequence, metal clusters growing on the Fs-center exhibit a highly fluxional character, possibly connected with their catalytic activity,15,46 and often a monotonic behavior of the formation energy as a function of cluster size.16 The situation is completely different in the case of gas-phase metal clusters, for which a number of “magic” clusters are known, i.e., clusters presenting a high stability (a differential formation energy much larger than that of neighboring clusters), as a rule associated with structural and/or electronic shell closure.17-19 This is particular true for binary metal clusters, * Corresponding author. Address: IPCF - CNR, via G. Moruzzi 1, I-56124 Pisa, Italy. Tel: +39-050-3152447. Fax: +39-050-3152442. E-mail:
[email protected]. Home page: http://h2.ipcf.cnr.it/alex/af.html.
for which a wealth of magic clusters has been found in the past few years, both at the theoretical and experimental level.20-24 It is thus of interest to ask whether magic clusters adsorbed on an oxide surface do exist, in particular a defected one.25 In the present article, we explore such a possibility by studying Pd1AgN (N ) 1-8) clusters adsorbed on the Fs-defected MgO(100) terrace. We show that Pd1Ag6 possesses the typical features of a magic cluster, i.e., a differential formation energy remarkably larger than that of neighboring clusters and a large HOMO-LUMO gap, signatures of both structural and electronic peculiar stability, simultaneously exhibiting a definite fluxional character. To our knowledge, this is the first example of a magic cluster on an oxide surface. As for the choice of the system, we note that the type and abundance of defect sites on MgO(100) surfaces appreciably depends on the preparation method. Double vacancies are observed when MgO single crystals are cleaved under UHV.26 Local defects in general seem to be nearly absent when thin MgO(100) films are grown by Mg reactive deposition on metal surfaces in an oxygen background and then annealed at high temperatures,27 but both neutral and charged Fs-centers can be easily generated by postprocessing these films.28 In particular, recent work has shown that Fs-defects (both neutral and charged) are common when thin MgO(100) films are subjected to electron bombardment and subsequent annealing,29 and that these sites are trapping centers for the growth of metal clusters.30 Probably due to the kinetics of the defect diffusion during annealing, Fscenters are mostly found on kinks or steps rather than on regular terraces under such experimental conditions.29,31 Our choice of an Fs-center on a MgO(100) terrace is due to technical reasons (i.e., to significantly speed-up calculations) and to our belief that anyway this is a reasonable structural model for metal clusters adsorbed on a locally defected MgO(100) surface. For example, for the Pd1Ag6 adsorbed magic cluster, we do not expect that its adhesion to the surface (which anyway is not the main driving force to its magicity) is spoiled by the change in the structural environment in passing from an Fs-center on a terrace to an Fs-center on a step, while the electron count (including the electrons in the vacancy) still holds in both cases
10.1021/jp072263m CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007
Magic Pd-Ag Binary Cluster and thus should still produce an electronic shell closure. Charged defects are also possible both when MgO(100) surfaces are prepared by cleaving in UHV26,32 or via reactive deposition,29,31 but in this work we limit ourselves to the neutral Fs defect. The choice of the Pd-Ag system is due to several reasons. First of all, a Pd atom interacts much more strongly than an Ag atom with the Fs-center (about 4 vs 1.7 eV).5,9 Moreover, the surface energy of Pd is larger than that of Ag. It can thus be expected that Pd1AgN clusters will adsorb on an Fs-center with the Pd atom pointing to the defect in a Pd-core/Ag-shell configuration. Second, the Pd atom in the gas phase has a 4d105s0 electronic configuration, and about 0.5 eV is required to promote it to the 4d95s1 state. This can give rise to peculiar electronic bi-stability effects when the Pd atom interacts with Ag clusters and the Fs-center. Third, due to the weak bonding in the Pd2 dimer (related to the mentioned 4d105s0 configuration of the gas-phase Pd atom), the Pd dimerization energy on the Fs-center is very small,5,33 and one can think of exploiting this in MBE experiments through a mechanism of sequential deposition, first saturating the Fs-defects with Pd atoms before depositing Ag atoms on top of these “saturated” defects. This suggests an explicit synthetic route to Pd1AgN clusters which could be exploited experimentally. An alternative route would pass through the deposition of mass-selected binary clusters.34 Finally, the Pd-Ag system is completely miscible in the bulk, and Pd-Ag alloys are known to exhibit peculiar properties, being for example commonly used in catalytic applications, see, e.g., ref35 In this connection, it can be added that magic binary clusters have been suggested to possess interesting catalytic properties.36 Results and Discussion The search for the lowest-energy structures of Ag-Pd clusters is performed via a density-functional basin-hopping (DF-BH) approach,23,37 in which a basin-hopping (BH) algorithm38-39 is implemented by determining energies and forces via a firstprinciples DF method. The DF-BH calculations were carried out using a Gaussian-type-orbital (GTO) basis set, the NWChem software,40 and a cluster model for the MgO(100) surface41 (the results have also been validated through a periodic boundary conditions approach,12 as described in the Supporting Information). More computational details can be found in the Supporting Information. We note that our “static” results are only valid at 0 °K, even though the knowledge of the global minimum and low-energy isomers could provide information about relative populations as a function of temperature. For clusters up to N ) 4 three DF-BH runs were performed, each composed of 15 Monte Carlo steps in the BH algorithm. For clusters with N ) 5-8, five DF-BH runs were performed, each composed of 15 Monte Carlo steps. The starting configurations of each run were generated randomly in a sphere of radius 4 Å around the defect. Previous experience shows that such an approach is adequate to single out the global minimum in this size range. To our knowledge, this is the first time that a DF-BH approach is applied to binary metal clusters adsorbed on a (defected) surface. The DF-BH approach is useful to sample the potential energy surface of these complicated systems in which biased searches easily miss the global minimum already for small sizes. It is convenient to define five quantities: (i) the adhesion energy (Eadh), calculated by subtracting the energy of the oxide surface and of the metal cluster, both frozen in their interacting configuration, from the value of the total energy of the system, and by taking the absolute value; (ii) the binding energy of the metal cluster (Emet), calculated by subtracting the energy of the
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11385 TABLE 1: Energetic Quantities (see Text for Their Definition) for Pd1AgN Clusters Adsorbed on the Fs-Defected MgO(100) Surface Obtained Using GTO Basis Set and the NWChem Software40 a cluster
Eadh
Emet
Edist
Ebnd
Ebnd(N) - Ebnd(N - 1)
Pd1Ag1 Pd1Ag2 Pd1Ag3 Pd1Ag4 Pd1Ag5 Pd1Ag6 Pd1Ag7 Pd1Ag8
3.48 3.35 3.21 3.85 3.64 3.63 3.53 4.26
1.50 3.60 5.20 6.85 8.83 11.47 12.99 14.34
0.05 0.18 0.03 0.50 0.22 0.14 0.40 1.32
4.98 6.95 8.41 10.70 12.47 15.10 16.52 18.60
1.08 1.97 1.46 2.29 1.77 2.63 1.42 2.08
a
Energies in eV.
isolated metal atoms from the total energy of the metal cluster in its interacting configuration, and by taking the absolute value; (iii) the metal cluster distortion energy (Edist), which corresponds to the difference between the energy of the metal cluster in the configuration interacting with the surface minus the energy of the cluster in its lowest-energy gas-phase configuration; (iv) the total binding energy (Ebnd), which is the sum of the binding energy of the metal cluster and of the adhesion energy (Ebnd ) Emet + Eadh); and (v) the incremental formation energy, defined as the energy gain (absolute value) for the addition of an Ag atom to a Pd1AgN-1 cluster, i.e., Ebnd(N) - Ebnd(N - 1). In Table 1 we report all these energy quantities for the adsorbed Pd1AgN clusters obtained using a GTO basis set, the NWChem software,40 and a cluster model for the MgO(100) surface. The total binding energy for the gas-phase clusters can be easily obtained by summing Emet + Edist. Gas-Phase Clusters. The structures of the predicted global minima for Pd1AgN clusters in the gas-phase are shown in Figure 1. The corresponding energy quantities can be derived from Table 1, with the incremental formation energy further displayed in Figure 3a. An inspection of Figure 1 shows a crossover between planar configurations, which are the predicted global minima up to Pd1Ag4, and compact, 5-fold symmetric configurations (pieces of an incomplete 13-atoms icosahedron), which are the predicted global minima for Pd1Ag7 and Pd1Ag8, and (presumably) larger clusters. This is in line with expectations based on previous experience, e.g., pure Ag clusters.42-44 One important remark is however in order. While the ground state of Pd1Ag5 is already compact, that of Pd1Ag6 is planar, with the compact configuration (also shown in Figure 1) showing up as a first excited-state higher in energy by only 0.15 eV. The reason of this behavior lies in the electronic configuration bistability of the Pd atom and a peculiar electronic shell closure. We recall that the electronic ground-state configuration of a Pd single atom is 4d10 5s0, with a 4d95s1 state lying 0.5 eV higher in energy. When the Pd atom combines with AgN clusters, with N being even (and thus S ) 0), it essentially keeps its 4d10 state, accepting electronic density from the AgN systems into its empty 5s orbital. The conduction electron count for Pd1Ag6 is thus 6, and corresponds to a magic number for the 2D jellium model.24 This stabilizes the highly symmetric planar configuration, with a HOMO-LUMO gap of 1.5 eV, with respect to the compact configuration (a pentagonal bipyramid), with a HOMO-LUMO gap of 1.1 eV, thus making the former the ground-state for Pd1Ag6. Viceversa, when a Pd atom combines with an oddnumbered (and thus unpaired spin) AgN cluster, the promotion energy of a Pd d-electron to the conduction band is compensated by the formation of the metallic bond, and the unpaired spin remains essentially localized on the Pd d-system (as can be easily checked from electron spin density plots). The results displayed in Figure 3a further confirm this analysis. One
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Figure 1. Predicted global minima of Pd1AgN clusters in the gas phase. For Pd1Ag6, the first excited state (pentagonal bipyramid) is also shown, together with its excitation energy (∆E). For N ) 4, 6, 8 (closed shell systems), the HOMO-LUMO gas is reported.
observes the usual odd-even oscillation of AgN clusters,16,43-44 further enhanced by the 4d10 5s0 f 4d95s1 promotion energy of the Pd atom. The fact that planar Pd1Ag6 achieves electronic shell closure in a region in which compact arrangements are already favored makes that its structural stability is not dramatically superior to that of the neighboring clusters. In passing, it can be noted that, based on the spherical jellium model, we would expect an electronic shell closure for Pd1Ag8. However, similarly to pure Ag8,16,43 this cluster does not exhibit a remarkable stability, due to the fact that the valence 5s orbital of the silver atom is too diffuse to fit the Ag-Ag interatomic distances, and the energy stabilization associated with the jellium shell closure is frustrated by a destructive interference of the electronic wavefunction.45 The issue of the fluxional character of the clusters deserves a separate comment. Our DF-BH search, in fact, generates not only a putative global minimum, but also a series of low-energy excited states. While an accurate description of these states would require longer and more numerous DF-BH runs, it is interesting to observe that we were able to identify for each size several candidates, often belonging to the same structural family, lying only 0.1-0.3 eV higher than the ground-state. The existence of these low-lying excited states, which can presumably be easily interconverted, makes gas-phase Pd1AgN clusters highly fluxional, a phenomenon common for small metal clusters in the absence of peculiar directionality or electronic effects.45 Adsorbed Clusters. The structures of the predicted global minima for Pd1AgN clusters adsorbed on the Fs-defected surface are shown in Figure 2. Note that the MgO system shown in this figure corresponds to the smaller of the two MgO systems employed in the calculations, and that for metal clusters larger than Pd1Ag4 a bigger MgO system was used, as detailed in the Supporting Information. The corresponding energy quantities are reported in Table 1, with the incremental formation energy
further displayed in Figure 3b. First, we observe that the atom directly bound to the defect is always a Pd atom, in line with the expectations discussed above. We tried to locally optimize selected configurations starting from the ones reported in Figure 2 and switching a Pd and an Ag atom, but this always increased abnormally the energy or resulted in a collapse back to a “regular” Fs/Pd/AgN arrangement. The second point to be observed is that the transition from planar to 5-fold symmetric configurations occurs at Pd1Ag5 instead of Pd1Ag7 as in the gas-phase. In other words, Pd1Ag6 is not “re-entrant” anymore, and the compact, pentagonal bipyramid arrangement lies much lower in energy than any planar or quasi-planar configurations. Planar configurations are still favored for adsorbed clusters up to Pd1Ag4 because they can arrange on the surface without any excessive distortion and moreoversby orienting the cluster plane perpendicular to the surfacestake advantage of the “metal-ontop” effect.12 Larger Pd1AgN clusters (N ) 5-8) then grow according to compact structures exhibiting a pentagonal bipyramid motif, i.e., they correspond to fragments of the 13-atom icosahedron for Pd1AgN, N ) 5-7 (with the Pd atom occupying the center of the icosahedron), or a fragment of the 13-atoms icosahedron plus an external atom on a (111) hcp hollow site for Pd1Ag8. With the exception of Pd1Ag8, these structures thus resemble the corresponding lowest-energy 5-fold symmetric arrangements in the gas-phase. Moreover, Figure 3b shows that the odd-even alternation is confirmed and even enhanced for the adsorbed clusters. In analogy with the gas-phase, the N-even Pd1AgN clusters have a closed-shell electronic configuration, with a Pd atom essentially in a 4d10 5s0 state, and thus somewhat lengthened Pd-Ag distances which better match the requisite of double frustration, whereas those with N odd are spin 1/2 systems in which the spin density exhibits appreciable components on the Pd atom, as it can be checked from electron spin density plots and could
Magic Pd-Ag Binary Cluster
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11387
Figure 2. Predicted global minima of Pd1AgN clusters adsorbed on the Fs-defected MgO(100) surface, as side and top views. For N ) 4, 6, 8 (closed shell systems), the HOMO-LUMO gas is also reported.
be experimentally verified by EPR measurements.29 This further destabilizes the N-odd clusters, in particular Pd1Ag1 and Pd1Ag7: it is known in fact that the interaction with the surface in general tend to quench the electron spin of the metal atoms.6,13 The most interesting observation is however that Pd1Ag6 adsorbed on the defect presents a remarkable structural stability, much larger than that of the neighboring clusters. In fact, it exhibits at the same time the highest incremental formation energy (2.63 vs 2.29 eV for Pd1Ag4) and the lowest incremental
formation energy for the successive N + 1 cluster (1.42 vs 1.46 eV for Pd1Ag3). The pentagonal bipyramid configuration of Pd1Ag6, which was already stable in the gas-phase with respect to neighboring clusters and only 0.15 eV higher than the planar hexagonal global minimum, manages to achieve a fair adhesion to the defected surface by pointing the Pd atom toward the vacancy and adhering with its 5-fold symmetric “crown” to the potential energy well created by the defect.13,14 In comparison, Pd1Ag8 is strongly disfavored by the need to
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Figure 3. Plot of the incremental formation energy, Ebnd(N) - Ebnd(N - 1), for Pd1AgN clusters: (a) in the gas phase, and (b) adsorbed on the Fs-defected MgO(100) surface.
deform to an asymmetric structure to better adhere to the surface, thus presenting a large metal distortion energy (Edist) which appreciably decreases its stability. Moreover, what makes Pd1Ag6 stick out even more in the plot of Figure 3b is an electronic shell closure effect. The HOMO-LUMO gap for this cluster is in fact 1.49 eV, i.e., appreciably larger than that of the pentagonal bipyramid in the gas phase and comparable to the gas-phase planar hexagon. This can be rationalized by thinking that the two electrons in the cavity are strongly involved in the metal-surface interaction (see also the discussion about chargetransfer effects below),7,8 so that the conduction electron count for Pd1Ag6 amounts to 8, which is a classical magic number for the spherical jellium model (as well as many other models). Finally, the fluxional character observed in the gas-phase is kept, and even enhanced, for the adsorbed clusters, which may be important for catalytic activity.46 The difference in energy between the putative global minimum and the first excited-state is always of the order of 0.1 - 0.3 eV. This also holds for Pd1Ag6, for which we were able to single out two low-lying excited states with excitation energies in the range 0.1-0.3 eV. In this connection, it is important to underline that these lowlying isomers also exhibit a large HOMO-LUMO gap, in the range 1.4-1.5 eV, whereas the first excited-state with a smaller HOMO-LUMO gap (about 0.7 eV) lies much higher in energy, about 1.4 eV higher than the ground-state. For Pd1Ag6 we thus encounter the peculiar situation of a cluster characterized by a substantial structural stability (structural magicity), which is simultaneously fluxional, i.e., with several low-lying excited states easily interconverted among each other, and electronically “hard” (electronic magicity, which corresponds to a large HOMO-LUMO gap) in both the ground-state and the lowlying excited states. In connection with the catalytic activity of metal clusters, it is interesting to analyze in more detail the issue of the charge transfer between the electron pair in the vacancy and the metal clusters. A useful indicator of this effect can be given by the core level shift of the 4s orbital of the Pd and Ag atoms. In the adsorbed Pd1Ag6 cluster, for example, we find a core level shift of -0.5 and ∼-1.0 eV for the Pd atom and for the Ag atoms, respectively. These values can be compared with the value of +0.7 ÷ 0.9 eV, which is the core level shift of a single Pd atom on an oxygen site of the MgO(100) regular surface, while for a Pd atom on a magnesium site negligible (∼0.1 eV) values were found.47 It can be noted that the direction of the charge transfer is of opposite sign in the case of Pd-Ag clusters adsorbed on the Fs center and of a Pd atom adsorbed on the
regular surface, but the absolute values are similar, implying a quantitatively comparable (and not dramatic) size of the chargetransfer effect, at variance with the estimates for the case of gold clusters adsorbed on the Fs center. Conclusions The structure of Pd1AgN clusters (N ) 1-8), both in the gas phase and adsorbed on an Fs-center of an MgO(100) terrace, is investigated via a density-functional basin-hopping (DF-BH) approach. A structural transition from planar to noncrystalline 5-fold symmetric configurations is found for both free and adsorbed clusters in this size range. In analogy with gold and silver pure cases, the presence of the Fs-center causes the PdAg clusters to be highly fluxional with the lowest-energy isomers lying at 0.1-0.3 eV above the global minimum. In addition, Pd1Ag6 is found to be a magic cluster, exhibiting a large HOMO-LUMO gap (both in the gas-phase and adsorbed on the defect) and a peculiar structural stability (when adsorbed on the defect) with respect to neighboring clusters. To our knowledge, this is the first example of a magic metal cluster on an oxide surface. This is rationalized in terms of an electronic shell-closure, involving also the electrons trapped in the oxygen vacancy, coupled with a good adhesion to the defected surface. Peculiarly, the low-energy Pd1Ag6 isomers (excitation energies around 0.1-0.3 eV) adsorbed on the Fs-center also exhibit a large HOMO-LUMO gap. An explicit synthetic route to the Pd1Ag6 cluster is proposed, passing through the sequential deposition of Pd and Ag atoms (exploiting the unfavorable dimerization of Pd on the Fs-center) or the deposition of massselected binary clusters (the latter is the more viable as the Pd1Ag6 cluster is magic in the gas phase, too). These features are expected to bring about promising consequences on the properties of this cluster, since it has been suggested, for example, that magic clusters can exhibit interesting catalytic properties.36 Moreover, also the transition from planar to compact configurations is expected to appreciably modify their catalytic properties, such as the interaction of Pd1AgN clusters with gases such as O2 and CO. Acknowledgment. This work was supported by the Italian CNR for the project SSATMN within the framework of the ESF EUROCORES SONS, and by the European Community Sixth Framework Program for the STREP Project GSOMEN. Part of the calculations have been performed at the Cineca Supercomputing Center within an agreement with Italian CNR-INFM.
Magic Pd-Ag Binary Cluster Supporting Information Available: Choice of the cluster model for describing the MgO system, computational details such as GTO basis sets for the description of the molecular orbitals and charge density, effective core potentials, and numerical parameters for the Plane Wave basis set calculations. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Campbell, C. T. Surf. Sci. Rep. 1997, 27, 1. (2) Henry, C. R. Surf. Sci. Rep. 1998, 31, 235. (3) Freund, H. J. Surf. Sci. 2002, 500, 271. (4) Robach, O.; Renaud, G.; Barbier, A. Surf. Sci. 1998, 401, 227. (5) Bogicevic, A.; Jennison, D. R. Surf. Sci. 2002, 515, L481. (6) Markovits, A.; Paniagua, J. C.; Lopez, N.; Minot, C.; Illas, F. Phys. ReV. B 2003, 67, 115417. (7) A. Del Vitto A.; Pacchioni, G.; Delbecq, F.; Sautet, P. J. Phys. Chem. B 2005, 109, 8040. (8) Yang, Z.; Wu, R.; Zhang, Q.; Goodman, D. W. Phys. ReV. B 2002, 65, 155407. (9) Neyman, K. M.; Inntam, C.; Matveev, A. V.; Nasluzov, V. A.; Rosch, N. J. Am. Chem. Soc. 2005, 127, 11652. (10) Yoon, B. Ha¨kkinen, H.; Landman, U.; Wo¨rz, A. S.; Antonietti, J. M.; Abbet, S.; Judai, K.; Heiz, U. Science 2005, 307, 403. (11) Molina, L. M.; Hammer, B. Appl. Catal., A 2005, 291, 21. (12) Barcaro, G.; Fortunelli, A. J. Chem. Theory Comput. 2005, 1, 972. (13) Moseler, H.; Ha¨kkinen, H.; Landman, U. Phys. ReV. Lett. 2002, 89, 176103. (14) Barcaro, G.; Fortunelli, A. J. Phys. Chem. B 2006, 110, 21021. (15) Yan, Z.; Chinta, S.; Mohamed, A. A.; Fackler, J. P., Jr; Goodman, D. W. J. Am. Chem. Soc. 2005, 127, 1604. (16) Barcaro, G.; Apra`, E.; Fortunelli, A. Chem.sEur. J. [Early online access]. DOI: 10.1002/Chem.200601796. (17) Li, J.; Li, X.; Zhai, H.-J.; Wang, L.-S. Science 2003, 299, 864. (18) Cui, L.-F.; Huang, X.; Wang, L.-M.; Li, J.; Wang, L.-S. J. Phys. Chem. A 2006, 110, 10169. (19) Chen, Z.; Neukermans, S.; Wang, X.; Janssens, E.; Zhou, Z.; Silverans, R. E.; King, R. B.; von Rague´ Schleyer, P.; Lievens, P. J. Am. Chem. Soc. 2006, 128, 12829. (20) Rossi, G.; Mottet, C.; Fortunelli, A.; Baletto, F.; Ferrando, R. Phys. ReV. Lett. 2004, 93, 105503. (21) Pyykko, P.; Runeberg, N. Ang. Chem. Int. Ed. 2002, 41, 2174. (22) Li, X.; Kiran, B.; Li, J.; Zhai, H.-J.; Wang, L.-S. Ang. Chem., Int. Ed. 2002, 41, 4786.
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