Transition-Metal-Doped Aluminum Hydrides as Building Blocks for

Oct 29, 2010 - A new electron-counting rule for these clusters was formulated as PSEN (paired skeleton electron number) = 4n, which can characterize b...
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J. Phys. Chem. A 2010, 114, 12318–12322

Transition-Metal-Doped Aluminum Hydrides as Building Blocks for Supramolecular Assemblies Jianjun Liu, Jiamei Yu, and Qingfeng Ge* Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901, United States ReceiVed: July 16, 2010; ReVised Manuscript ReceiVed: September 29, 2010

Density functional theory calculations were carried out to characterize a series of transition-metal-doped aluminum hydrides, forming TMAlnH2n and TMAlnH2n+1 (TM ) Sc, Ti, V; n ) 3,4), in either charged or neutral form. A new electron-counting rule for these clusters was formulated as PSEN (paired skeleton electron number) ) 4n, which can characterize both closed-shell and open-shell clusters. On the basis of this electroncounting rule, the superatomic clusters such as TiAl4H9 and TiAl3H6 were identified and can be used to assemble supramolecular structures. Electronic structure analysis showed that three-centered TM-H-Al bonds largely contributed to the structural stability. Also, the spin state of a wide range of clusters in their ground state can be predicted by the electron-counting rule. 1. Introduction Assembling novel nanostructured materials from finite-sized building units is one of the most attractive and challenging tasks because it requires that the building block possess interesting chemical and physical properties and retain its structural integrity and chemical identity.1-9 The polyhedral skeletal electron pair theory formulated by Wade and further developed by Mingos and others has been extensively used to identify cluster structures as the building units.10-12 The jellium model has also been used to predict some magic clusters and understand their stability. One example is the aluminum cluster, Al13, which was shown to have superatomic properties.3,4 The chemically stable Al13cluster has 40 valence electrons, and its inertness can be understood in terms of a closed electronic shell based on the jellium model. Experimental and theoretical studies have determined that Al13 has a high electron affinity, comparable to that of a Cl atom.3 Subsequent study confirmed the superatomic characteristic by forming ionically bound molecules.5 The previously developed electron-counting rules are applicable to stable clusters with characteristics such as a closedshell electron configuration, a large HOMO-LUMO gap, and a highly symmetric geometry. However, such electron-counting rules are not expected to account for the open-shell clusters. In fact, stable superatomic clusters may be in a high spin state and can be used to assemble novel magnetic molecular materials.13 Therefore, it is highly important to develop a method that can predict the stable structure of open-shell clusters. Recently, aluminum hydrides have gained much attention due to their potential applications as hydrogen storage materials as well as solid-state rocket fuels.14-17 Grubisic et al. performed both experimental and theoretical studies for a series of closoalanes AlnHn+2 (n ) 4-8) clusters which adopted n-vertx polyhedral structures with two extra hydrogen atoms forming opposite hydrogen-bridged three-center bonds. These closo structures were shown to follow Wade’s rule very well.14 A recent study of Al4Hx- showed that Al4H7- has a HOMO-LUMO gap of 2.7 eV and an electron affinity of 2.2 eV.17 In fact, the * To whom correspondence should be addressed. E-mail: qge@ chem.siu.edu.

stability of Al4H7- also can be interpreted by Wade’s rule. Al4H7- was proposed as a potential building block to synthesize novel chemical materials by combining them with cationic atoms or molecules. It has been demonstrated that a single transition-metal atom doped in nanoclusters can change the chemical and physical properties of the nanoclusters. By varying the doped transition metals and their position in the clusters, the structural, electronic, magnetic, and chemical properties of the clusters can be tailored for a specific application.18-25 However, adding a transition metal to a closed-shell cluster not only changes the chemical and physical properties of clusters but also introduces different bonding characteristics and changes electron configurations because of the involvement of the more diffuse d or f orbitals. It also made the previous electron-counting rules based on closed electronic shells not applicable. In the present study, we establish a new electron-counting rule to account for transition-metaldoped clusters based on aluminum hydrides. Since the addition of a transition metal affects the spin state of clusters, our new rule explicitly takes the unpaired electrons into consideration. In the present paper, we studied a series of clusters of aluminum hydrides doped with one of the transition-metal elements (Sc, Ti, V), forming TMAlnH2n and TMAlnH2n+1 (TM ) Sc, Ti, V; n ) 3,4) clusters, in either neutral or charged form. We developed an electron-counting rule to account for the stability of these clusters. Further, we used the electron-counting rule to predict the spin state of clusters in their ground state. In addition, we identified some superatomic clusters with characteristics such as large electron affinity (EA) or small ionization potential (IP). These superatomic clusters may be utilized as building blocks to synthesize novel functional materials. Finally, the bonding nature in the clusters was analyzed to elucidate the structural stability. 2. Computational Methods All calculations were carried out by using the GAUSSIAN 03 program package.26 Geometry optimization and vibrational frequency analysis were performed at the B3LYP level of density functional theory.27-30 Vibrational frequency analyses

10.1021/jp1066296  2010 American Chemical Society Published on Web 10/29/2010

Aluminum Hydrides for Supramolecular Assemblies

J. Phys. Chem. A, Vol. 114, No. 46, 2010 12319 TABLE 1: Spin States, Geometric Symmetries, HOMO-LUMO Gaps (H-L; unit: eV), Relative Energies (RE; unit: eV), Total Electron Numbers (TEN), and Paired Skeleton Electron Numbers (PSEN) for TMAlnH2n and TMAlnH2n+1 (TM ) Sc, Ti, and V; n ) 3, 4) Species

Figure 1. The optimized structures of either charged or neutral clusters, TMAl3H6, TMAl3H7, TMAl4H8, and TMAl4H9, at the level of B3LYP/ 6-311++G(d, p).

confirmed that each structure reported here is at a minimum with no imaginary frequency. We used the standard triple-ζ basis set with diffuse and polarization functions, 6-311++G(d, p).31 The results from B3LYP/6-311++G(d, p) calculations have been validated by the more accurate CCSD(T)/6-311++G(d, p) calculations for selected groups of complex hydrides, including TMAl3H7 and TMAl3H7+ (TM ) Sc, Ti, and V). A comparison of the relative energies of each group from the B3LYP and CCSD(T) calculations is provided in Table S1 of the Supporting Information. The bond order was calculated by using the natural bonding orbital (NBO) method implemented in the program package. 3. Results and Discussion We have examined the highly symmetric structures of TMAl3H6 (C3V), TMAl3H7 (C3V), TMAl4H8 (C4V), and TMAl4H9 (C4V) with different spin states (singlet, doublet, triplet, quartet, and quintet). The typical structures are shown in Figure 1. For initial optimized structures with one or more imaginary frequencies, we examined the normal modes corresponding to the imaginary frequencies and modified the structures accordingly to remove the symmetry constraint. Further optimization leads to structures with a lower symmetry (Cs or C1). It is worth noting that the resulting structures maintain a similar shape and retain the three-centered TM-H-Al bonds. In these structures, the TM atom connects the triangle or square formed by the Al atoms and becomes a vertex of the corresponding pyramidal skeleton structure. Hydrogen atoms bind TM or Al either at the terminal sites or in bridging positions. At the terminal positions, the hydrogen atom forms a largely covalent bond with either Al or TM, sharing two electrons in a terminal Al-H or TM-H bond, while the bridging H together with the TM and Al atoms forms a delocalized three-centered TM-H-Al bond. The electrons in these bonds contribute to the total skeleton electrons. Table 1 lists the molecular symmetries, spin states, HOMOLUMO gaps of the singlet species, relative energies, total electron numbers (TEN), and paired skeleton electron numbers (PSEN) of the optimized species. The total electron numbers are calculated by summing the total valence electrons and net charges. Further, the total skeleton electron number (TSEN) is calculated by subtracting the number of terminal Al-H or TM-H bond electrons from TEN. Then, PSEN can be

species

spin

symmetry

H-L (eV)

RE (eV)

TEN

PSEN

ScAl3H62+ ScAl3H62+ ScAl3H6+ ScAl3H6+ ScAl3H6 ScAl3H6 ScAl3H6ScAl3H6ScAl3H62ScAl3H62ScAl3H62ScAl3H7+ ScAl3H7+ ScAl3H7 ScAl3H7 ScAl3H7ScAl3H7ScAl4H8+ ScAl4H8+ ScAl4H8 ScAl4H8 ScAl4H8ScAl4H8ScAl4H9 ScAl4H9 ScAl4H92ScAl4H92TiAl3H62+ TiAl3H62+ TiAl3H6+ TiAl3H6+ TiAl3H6 TiAl3H6 TiAl3H6TiAl3H6TiAl3H6TiAl3H62TiAl3H62TiAl3H7+ TiAl3H7+ TiAl3H7 TiAl3H7 TiAl3H7TiAl3H7TiAl3H72TiAl3H72TiAl3H72TiAl4H8 TiAl4H8 TiAl4H8TiAl4H8TiAl4H82TiAl4H82TiAl4H9 TiAl4H9 TiAl4H9TiAl4H9VAl3H6+ VAl3H6+ VAl3H6 VAl3H6 VAl3H6VAl3H6VAl3H7 VAl3H7 VAl3H7VAl3H7VAl4H8+ VAl4H8+ VAl4H8 VAl4H8 VAl4H8VAl4H8VAl4H9 VAl4H9 VAl4H92VAl4H92-

S T D Q S T D Q S T 5 S T D Q S T S T D Q S T S T S T D Q S T D Q S T 5 D Q D Q S T D Q S T 5 S T D Q S T D Q S T D Q S T D Q D Q S T S T D Q S T S T S T

Cs C1 C3V C3V C3V C1 C3V C1 C3V C1 C1 C1 C1 C3V C1 C3V C1 C1 C1 C1 C1 C4V C1 C1 C1 C4V C1 C3V C1 C3V C1 C3V C1 C3V C1 C3V C1 C3V C1 C1 C3V C1 C1 C3V C1 C3V C1 C4V C1 C4V C1 C1 C1 C1 C1 C4V C4V C3V C3V C1 C3V C1 C3V Cs C1 Cs C1 C1 C1 C1 C1 C4V C1 C1 C1 C4V C1

3.09

0.00 0.81 0.00 0.75 0.00 1.61 0.00 1.05 0.20 0.00 1.06 0.00 0.83 0.00 1.01 0.00 1.11 0.00 1.29 0.00 1.49 0.00 1.48 0.00 1.34 0.00 1.16 0.00 0.87 0.00 0.47 0.00 0.51 5.03 0.00 1.38 0.38 0.00 0.00 1.25 0.00 0.69 5.03 0.00 0.10 0.00 0.41 0.00 1.49 0.00 0.98 0.60 0.00 0.00 1.36 0.00 1.18 0.00 0.59 0.72 0.00 0.60 0.00 0.00 1.02 0.65 0.00 0.00 0.54 0.00 1.49 0.00 0.64 0.00 1.34 0.76 0.00

16 16 17 17 18 18 19 19 20 20 20 18 18 19 19 20 20 22 22 23 23 24 24 24 24 26 26 17 17 18 18 19 19 20 20 20 21 21 19 19 20 20 21 21 22 22 22 24 24 25 25 26 26 25 25 26 26 19 19 20 20 21 21 21 21 22 22 24 24 25 25 26 26 26 26 28 28

10 8 10 8 12 10 12 10 14 12 10 10 8 10 8 12 10 14 12 14 12 16 14 14 12 16 14 10 8 12 10 12 10 14 12 10 14 12 10 8 12 10 12 10 14 12 10 16 14 16 14 18 16 16 14 16 14 12 10 14 12 14 12 12 10 14 12 16 14 14 12 16 14 16 14 18 16

3.11

0.68 3.27

3.08 3.49

2.99 3.32 1.75

3.50

1.61

3.56

1.20 3.68

0.54

3.52

2.54

2.76 3.88

1.91 4.06 1.83

calculated as TSEN - UPEN, where UPEN is defined as the unpaired electron number (singlet: 0; doublet: 1; triplet: 2; quartet: 3; quintet: 4). Wade’s rule has been successfully used in the past to understand the structural stability of boron and aluminum hydrides and has predicted a number of new clusters. For the nido structures, 2(n+2) electrons (n: vertex number) belong to

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the skeletons formed by the vertices and bridging H atoms. In fact, the electron number of 2(n+2) in Wade’s rule should correspond to TSEN, as defined here. The spin-dependent stability of all calculated TMAlnH2n and TMAlnH2n+1 structures cannot be described by Wade’s rule. Furthermore, the closedshell TMAl4H8 structures with TSEN ) 2(5 + 2) ) 14 should be more stable than the clusters with TSEN * 14 according to Wade’s rule. Our results suggest that the stability of TM-doped nido structures does not follow the Wade’s electron-counting rule. Consequently, we developed a different electron-counting rule to account for the structural stability and used the rule to predict new cluster structures. The TMAl3H6 and TMAl3H7 clusters have three threecentered TM-H-Al bonds. We find that the structures with PSEN ) 12 have relatively lower energies than their isoelectronic structures with a different spin state. The closer the PSEN of the cluster is to 12, the lower energy that it has. Similarly, for the TMAl4H8 and TMAl4H9 clusters with four three-centered TM-H-Al bonds, the structures with PSEN ) 16 have relatively lower energies. As a result, we conclude that the stable structure for charged or neutral TMAlnH2n and TMAlnH2n+1 clusters has PSEN ) 4n, where n is defined as the number of Al atoms, and is also equal to the number of three-centered bonds. It should be pointed out that the n defined here is different from the n in the Wade’s rule, which refers to the number of vertices in the skeleton. The 4n paired skeleton electrons in TMAlnH2n and TMAlnH2n+1 are different from the number of electrons predicted by polyhedral skeleton electron rule developed by Wade and Mingos.10-12 There are 2(n+1) and 2(n+2) electrons (n: vertex number) in the skeletons of closo and nido structures of aluminum hydrides, respectively.14 The different electroncounting rules may be attributed to the hyperbonding nature of transition metals capable of forming more three-centered TM-H-Al bonds than Al due to the more flexible and diffuse d orbitals. For example, tetrahedral Al4H6 and Al4H7- structures have only two three-centered Al-H-Al bonds, whereas the tetrahedral TMAl3H6 and TMAl3H7 structures have three threecentered TM-H-Al bonds. Further, we tested the electron-counting rule of PSEN ) 4n for other TM-doped aluminum hydrides. First, we explored the range of n values for which the rule is applicable. Structures TiAlnH2n and TiAlnH2n+1 with n ) 2, 5, and 6 were examined. We indeed found the minimum-energy structures of singlet TiAl2H42+ and doublet TiAl5H102- with no imaginary frequency. Both structures satisfy the electron-counting rule of PSEN ) 4n. However, our attempt to optimize TiAl6H12 and TiAl6H13 structures with a C6V symmetry failed. Second, we explored clusters containing different transition metals. We optimized the structures where other 3d transition metals, including Cr, Mn, and Fe, were doped in aluminum hydrides. Furthermore, we tested the applicability of the electron-counting rule of PSEN ) 4n to the clusters formed by Be, Mg, B, and Ga. We obtained stable structures for Ti-doped magnesium and boron hydrides, for example, TiBe3H4, TiMg3H3+, TiB4H8, and TiGa3H7, which follow our electron-counting rule. A sample of the predicted structures according the PSEN ) 4n rule has been optimized and provided in Figure S1 of the Supporting Information. All of the structures were stable without any imaginary frequency. In summary, a large number of TMXnH2n and TMXnH2n+1 clusters (TM ) Sc, Ti, V, Cr, Mn, and Fe; X ) B, Al, and Ga; n ) 2, 3, 4, and 5) exist, and the most stable structures can be predicted by the electron-counting rule of PSEN ) 4n.

Liu et al.

Figure 2. The schematic molecular orbital diagram of TiMg3H3+ formed by the combination of TiH3+ and Mg3.

Our results showed that the TiMg3H3+ cluster, shown in the inset of Figure 2, was stabilized by 4.25 eV with respect to TiH3+ and Mg3. The TiMg3H3+ cluster has a relatively large HOMO-LOMO gap (2.57 eV). Removing an electron from its bonding orbitals or adding an electron to antibonding orbitals leads to structural instability. The results validate the PSEN ) 4n (n ) 3) electron-counting rule for the TiMg3H3+ cluster. In the following, we will use this cluster to understand the origin of the electron-counting rule. We chose TiMg3H3+ for this analysis because TiMg3H3+ shares a similar TMXnHn skeleton to the TMXnH2n and TMXnH2n+1 clusters. In the molecular orbital diagram shown in Figure 2, the sdn-1-hybridized Ti orbitals of TiH3+ interact with the n bridging hydrogen atoms to form n bonding orbitals and n antibonding orbitals. In forming the n bridging Ti-H-Mg bonds, the original n antibonding orbitals mix with the s orbitals of Mg and become bonding in the new three-centered bonds. The occupation of these n orbitals by the electrons donated from Mg stabilizes the cluster greatly. Consequently, the cluster would need 4n electrons to fill its 2n skeleton bonding orbitals, which gives rise to PSEN ) 4n. For TMXnH2n and TMXnH2n+1 formed with group 13 elements, the terminal bonding X-H bonds will mix with the Ti-H bonding orbitals, which changes the order of the low-lying orbitals, but the n orbitals immediately below the HOMO remain intact. Therefore, the orbital order in the TMXnHn skeleton dictates the applicability of the PSEN ) 4n rule. We can predict the spin state of ground-state clusters based on the new electron-counting rule. For any of the clusters TMAlnH2n and TMAlnH2n+1, its unpaired electron number can be calculated according to the formula of TSEN - PSEN. As shown in Table 1, the closer PSEN is to 4n, the more stable the cluster is. The cluster with TSEN > 4n prefers a high-spin state in order to make PSEN closer to 4n, whereas the cluster with TSEN < 4n favors a low-spin state for the same reason. The analysis of spin density for selected clusters showed that the unpaired electrons distribute among the Al atoms and transition metal of the skeleton. We conclude that the paired skeleton electrons determine the stability of the cluster. Another important application of the electron-counting rule is to predict the superatomic clusters among TM-doped aluminum hydrides. As mentioned earlier, Al13 is considered as a typical halogen-like superatomic cluster because of its high electron affinity.3 Its anion, Al13-, has a highly symmetrical structure, a large HOMO-LUMO gap, and a closed-shell electronic structure. A simple strategy to search for a superatomic cluster is to look for the ground-state anionic cluster that has a closed-shell configuration and meets PSEN ) 4n. Correspondingly, its neutral cluster is a doublet and has PSEN < 4n. As a result, the neutral cluster may exhibit a large electron

Aluminum Hydrides for Supramolecular Assemblies

J. Phys. Chem. A, Vol. 114, No. 46, 2010 12321 TABLE 2: NBO Charges and Bond Orders in TiAl3H6+, TiAl3H7, TiAl4H8, and TiAl4H9- Clusters at the B3LYP/ 6-311++G(d, p) Levela NBO charge species +

Ti

bond order

Al

H(b)

H(t)

coordinates

total

Ti-Al Al-Al Al-H(b) Ti-H(b) Al-H(t) Ti-Al Al-Al Al-H(b) Ti-H(b) Al-H(t) Ti-H(t) Ti-Al Al-Al Al-H(b) Ti-H(b) Al-H(t) Ti-Al Al-Al Al-H(b) Ti-H(b) Al-H(t) Ti-H(t)

0.85 0.30 0.45 0.47 0.89 0.72 0.54 0.55 0.36 0.78 0.74 0.88 0.36 0.43 0.41 0.87 0.88 0.42 0.48 0.34 0.86 0.68

TiAl3H6

0.13

0.83

-0.26

-0.27

TiAl3H7

0.01

0.63

-0.31

-0.24

TiAl4H8

-0.92

0.77

-0.32

-0.22

TiAl4H9-

-1.06

0.61

-0.34

-0.20

Figure 3. The neutral TiAl3H6AlH4 complex structure is formed by combining AlH4- and TiAl3H6+.

affinity due to the high stability of the anion (PSEN ) 4n) and the instability of the neutral cluster (PSEN < 4n). These neutral clusters may exhibit halogen-like properties and tend to gain an electron to form electron configurations similar to that of an inert gas. Similarly, we can also identify alkaline-like superatomic clusters by looking for a closed-shell cationic cluster with PSEN ) 4n. Using this method, we identified TiAl4H9 as a superatom with halogen-like characteristics. At the B3LYP/6-311++G(d,p) level, the EA of TiAl4H9 and the vertical detachment energy (VDE) of TiAl4H9- were calculated as 3.10 and 3.41 eV, respectively. These values are comparable with the EAs of Br (3.36 eV)25 and I (3.09 eV), indicating that TiAl4H9 has a strong ability to gain an electron.32 The anionic cluster TiAl4H9- has C4V symmetry and closed-shell electronic structure with a 3.53 eV HOMO-LUMO gap. The ionic compounds formed by combining TiAl4H9- with cationic alkali metals, such as Li+, Na+, and K+, confirm the halogen nature of the cluster. In these ionic complexes, the TiAl4H9- structure remains intact. Similarly, TiAl3H6 was identified as a superatomic cluster with alkali metal characteristics. The adiabatic and vertical ionic potentials (AIP and VIP) were calculated as 5.98 and 5.87 eV, respectively. These values show that TiAl3H6 may easily lose its HOMO electron to form a stable ionic cluster. We tested this hypothesis by combining TiAl3H6+ with AlH4-, as shown in Figure 3. The species AlH4-, with a VDE of 4.04 eV and HOMO-LUMO gap of 5.31 eV, can be considered as a superhalogen anion. We therefore expect these two species to combine to form a stable species. Indeed, an optimization at the B3LYP/6-311++G(d,p) level led to the formation of a TiAl3H6AlH4 compound with the C3V symmetry. This result validated our predication that TiAl3H6 would act as a superalkali while AlH4 behaves as a superhalogen. This new compound, TiAl3H6AlH4, with a hydrogen capacity of ∼6 wt %, may be a promising hydrogen storage material. The structural integrity is important in developing novel functional materials because the change of spin and charge could lead to structural transformation during applications. We further explored the bonding nature in selected clusters in order to understand their structural stability. We analyzed the charges and bond orders for a series of clusters including TiAl3H6+, TiAl3H7, TiAl4H8, and TiAl4H9- and listed the results in Table 2. As shown in Table 2, there are 3-4 electrons delocalized in the Ti-H-Al triangle and ∼2 electrons distributed in the Ti-Al bond. Only 1-2 electrons are distributed in the Al3 or Al4 ring. We refer to the Ti-H-Al triangle as a 3c-(3-4)e bond in the following discussion. A recent study showed that bonding in aluminum hydrides is similar to that in boron hydrides because n-vertex polyhedral closo structures with two extra hydrogen atoms occupying opposite bridging positions are both experimentally and theoretically identified to be a very stable species, in agreement with the Wade’s (n+1) rule.14 A detailed analysis of dialane, Al2H6,

a The “t” and “b” in parentheses represent the terminal and bridging hydrogen atoms, respectively.

suggested that the traditional electron-deficient 3c-2e bond also exists in the bridging Al-H-Al bonds. Essentially, there is no bond formed between the two Al atoms in dialane.33 Obviously, the 3c-(3-4)e bonds of TM-H-Al are different and expected to be stronger than the 3c-2e bond in dialane due to the bonding interactions between TM and Al. Such 3c-(3-4)e bonds help to maintain the structural integrity of the TM-Aln pyramidal skeleton structure during hydriding and dehydriding and make TM-doped aluminum hydrides potential building blocks for new functional materials. We hope and expect these structures to be confirmed by experimental techniques such as mass spectrometry and photoelectron spectroscopy. 4. Conclusions In summary, we have carried out DFT studies on a series of TM-doped aluminum hydrides, TMAlnH2n and TMAlnH2n+1 (TM ) Sc, Ti, V; n ) 3,4), in either charged or neutral form. The electron-counting rule of PSEN ) 4n was developed to understand the structural stability. This new rule can be applied to both closed-shell and open-shell clusters, which differentiates it from Wade’s rule. In addition, the electron-counting rule of PSEN ) 4n allows us to predict clusters with high magnetic moment and superatomic characteristics. The predicted clusters may be used as building blocks in assembling new functional materials. Acknowledgment. This work was supported by U.S. Department of Energy, Basic Energy Science Grant DE-FG0205ER46231. Supporting Information Available: Calculated relative energies and optimized structures of selected TM complex hydrides. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Li, X.; Wang, L.-S. Phys. ReV. B 2000, 65, 153404. (2) Khanna, S. N.; Jena, P. Phys. ReV. Lett. 1992, 69, 1664.

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(3) Bergeron, D. E.; Castleman, A. W., Jr.; Morisato, T.; Khanna, S. N. Science 2004, 304, 84. (4) Bergeron, D. E.; Roach, P. J.; Castleman, A. W., Jr.; Jones, N. O.; Khanna, S. N. Science 2005, 307, 231. (5) Reber, A. C.; Khanna, S. N.; Castleman, A. W., Jr. J. Am. Chem. Soc. 2007, 129, 10189. (6) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Christopher, J. A.; Whetten, R. L.; Grönbeck, H.; Häkkinen, H. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157. (7) Bergeron, D. E.; Castleman, A. W., Jr.; Jones, N. O.; Khanna, S. N. Chem. Phys. Lett. 2005, 415, 230. (8) Zhao, J.; Xie, R. Phys. ReV. B. 2003, 68, 035401. (9) Zope, R. R.; Baruah, T. Phys. ReV. A 2001, 64, 053202. (10) Mingos, D. M. Nat. Phys. Sci. 1972, 236, 99. (11) Wade, K. Inorg. Chem. Radiochem. 1976, 18, 1. (12) Williams, R. E. Chem. ReV. 1992, 92, 177. (13) Yang, L.-M.; Ding, Y.-H.; Sun, C.-C. J. Am. Chem. Soc. 2007, 129, 1900. (14) Grubisic, A.; Li, X.; Stokes, S. T.; Cordes, J.; Gantefor, G. F.; Bowen, K. H.; Kiran, B.; Jena, P.; Burgert, R.; Schnockel, H. J. Am. Chem. Soc. 2007, 129, 5969. (15) Kiran, B.; Jena, P.; Li, X.; Grubisic, A.; Stokes, S. T.; Gantefor, G. F.; Bowen, K. H.; Burgert, R.; Schnockel, H. Phys. ReV. Lett. 2007, 98, 256802. (16) Li, X.; Grubisic, A.; Stokes, S. T.; Cordes, J.; Gantefor, G. F.; Bowen, K. H.; Kiran, B.; Willis, M.; Jena, P.; Burgert, R.; Schnockel, H. Science 2007, 315, 356. (17) Roach, P. J.; Reber, A. C.; Woodward, W. H.; Khanna, S. N.; Castleman, A. W., Jr. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 14565. (18) Pyykko¨, P.; Runeberg, N. Angew. Chem., Int. Ed. 2002, 41, 2174. (19) Li, X.; Kiran, B.; Cui, L.-F.; Wang, L.-S. Phys. ReV. Lett. 2005, 95, 253401. (20) Li, X.; Kiran, B.; Li, J.; Zhai, H. J.; Wang, L.-S. Angew. Chem., Int. Ed. 2002, 41, 4768. (21) Ha¨kkinen, H.; Abbet, S.; Sanchez, A.; Heiz, U.; Landman, U. Angew. Chem., Int. Ed. 2003, 42, 1297.

Liu et al. (22) Tanaka, H.; Neukermans, S.; Janssens, E.; Silverans, R. E.; Lievens, P. J. Am. Chem. Soc. 2003, 125, 2862. (23) Janssens, E.; Tanaka, H.; Neukermans, S.; Sileerans, R. E.; Lievens, P. Phys. ReV. Lett. 2003, 90, 033401. (24) Koyasu, K.; Akutsu, M.; Mitsui, M.; Nakajima, A. J. Am. Chem. Soc. 2008, 127, 4998. (25) Gao, Y.; Bulusu, S.; Zeng, X. C. J. Am. Chem. Soc. 2005, 127, 15680. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (27) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (28) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (29) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (30) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (31) Binkley, J. S.; Pople, J. A. J. Am. Chem. Soc. 1980, 102, 939. (32) Lide, D. R. CRC Handbook of Chemistry and Physics, 73rd ed.; CRC Press: Boca Raton, FL, 1992. (33) Goebbert, D. J.; Hernandez, H.; Francisco, J. S.; Wenthold, P. G. J. Am. Chem. Soc. 2005, 127, 11684.

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