Structural Evolution and Chemical Bonding - American Chemical Society

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Tetratungsten Oxide Clusters W4On-/0 (n ) 10-13): Structural Evolution and Chemical Bonding Bin Wang, Wen-Jie Chen, Bo-Cun Zhao, Yong-Fan Zhang,* and Xin Huang* Department of Chemistry, Fuzhou UniVersity, Fuzhou, Fujian 350108, People’s Republic of China, and State Key Laboratory of Structural Chemistry, Fuzhou, Fujian 350002, People’s Republic of China ReceiVed: October 9, 2009; ReVised Manuscript ReceiVed: NoVember 13, 2009

Density functional theory (DFT) calculations are carried out to investigate the electronic and structural properties of a series of tetratungsten oxide clusters, W4On-/0 (n ) 10-13). Generalized Koopmans’ theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES). A large energy gap (∼2.9 eV) is observed for the stoichiometric W4O12 cluster, which reaches the bulk value. The calculations suggest that W4O12-/0 have the planar eight-membered ring structures, in which each tungsten atom is tetrahedrally coordinated with two bridging O atoms and two terminal O atoms. W4O10-/0 and W4O11- can be viewed as removing two and one terminal O atoms from W4O12-/0, respectively. The W4O11 neutral is an interesting species, which possesses the pentabridged structure. We show that W4O11- contains a localized W3+ site, which can readily react with O2 to form the W4O13- cluster, whereas the corresponding neutral W4O13 can be viewed as replacing a terminal oxygen in W4O12 by a peroxo O2 unit. Molecular orbital analyses are performed to analyze the chemical bonding in the tetratungsten oxide clusters and to elucidate their electronic and structural evolution. 1. Introduction Tungsten oxides have many industrial applications1–4 and are important acid-base and redox catalysts.5–11 However, the identification of catalytic active sites and the detailed reaction mechanisms in the catalytic processes have still been the challenge for both experimentalists and theorists. Gas-phase cluster studies have been considered as an alternative approach to provide fundamental insight into the bulk oxide materials and catalysts.12–23 Interest in transition-metal oxide clusters has been motivated in particular by the use of these species to aid the elucidation of the mechanisms of catalytic reactions.24,25 State-of-the-art theoretical calculations can also provide molecular-level insight into the nature of active species in catalysis.13,14,22 As a first step in developing a comprehensive understanding of complex catalytic processes on transition-metal oxides, we are interested in studying the electronic structures and chemical bonding of isolated transition-metal oxide clusters. In order to mimic the geometric and electronic properties of tungsten oxide surfaces and defects, relatively large tungsten oxide clusters are of interest. The mononuclear, dinuclear, and trinuclear tungsten oxide clusters as well as the Lindqvist dianionic clusters have been studied using photoelectron spectroscopy (PES) combined with density functional theory (DFT) calculations.26–32 However, there have been few previous studies on the tetratungsten oxide clusters. Li et al. reported theoretical calculations on the tungsten oxide (WO3)n (n ) 1-4) clusters,33–36 in whose work the neutral tetramer cluster (WO3)4 was found to be the ring structure with D4h symmetry. Sun et al. investigated the W4On-/0 (n ) 1-6, 12) clusters using PES and first principles molecular orbital calculations.37,38 In their work, they suggested that the W4O12 cluster was an embryonic form of bulk tungsten oxide and * To whom correspondence should be addressed. E-mail: xhuang@ fzu.edu.cn (X.H.); [email protected] (Y.-F.Z.).

presented an abrupt change in the electronic structures of W4On(n e 6) clusters that occurred at n ) 5. We are interested in probing the geometric and electronic structures and the chemical bonding in early transition metal oxide clusters,24–28,39–42 which will help to provide molecular models and mechanistic insight for the oxide catalysts. In the present work, we investigate the structural and electronic properties and the chemical bonding in the tetratungsten oxide clusters, W4On- and W4On (n ) 10-13), using extensive density functional theory (DFT) calculations. By comparison of our current theoretical data with the previous results,23–27 some correlations of the geometric and electronic structures in the corresponding oxide clusters are found for the multinuclear tungsten oxide clusters. For example, the O-rich clusters W2O7, W3O10, and W4O13 are found to be the peroxo complexes, and their anionic counterparts have similar frameworks with very high electron binding energies. In particular, the ground states of W4O11-/0 are found to be interesting species. The W4O11 neutral is a special case, which possesses the pentabridged structure. Our calculations suggest that W4O11- contains a localized W3+ site, and the three localized d electrons on the W3+ site may be readily transferred to the π* and σ* orbitals of an approaching O2 molecule, making W4O11- an anionic molecular model for O2 activation. 2. Density Functional Calculations The theoretical calculations were performed at the DFT level using the B3LYP hybrid functional.43–45 A number of structural candidates including different spin states and initial geometries were evaluated, and the search for the global minima was first performed using analytical gradients with the Stuttgart relativistic small core basis set and efficient core potential46,47 for W and the 6-31+G(d) basis on O.48 Selected low-lying isomers were further reoptimized at the B3LYP hybrid functional, using the Stuttgart relativistic small core basis set and efficient core

10.1021/jp909676s  2010 American Chemical Society Published on Web 12/31/2009

Tetratungsten Oxide Clusters W4On-/0 (n ) 10-13)

Figure 1. Optimized structures for W4O12 and W4O12-. The bond lengths are in angstroms, and bond angles are in degrees.

potential augmented with two f-type and one g-type polarization functions [ζ(f) ) 0.256, 0.825; ζ(g) ) 0.627] for tungsten as recommended by Martin and Sundermann49 and the aug-ccpVTZ basis set for oxygen.50,51 Scalar relativistic effects, that is, the mass velocity and Darwin effects, were taken into account via the quasi-relativistic pseudopotentials. Since we were mainly interested in explaining the PES spectra, no further effort was devoted to resolve the spin-orbit-coupled fine structures in the calculated spectra. The previous results on tungsten oxides showed that spin-orbit coupling effects would shift the orbital energies by up to a few tenths of an eV, which would not affect the spectral assignment.32 Vibrational frequency calculations were performed at the same level of theory to verify the nature of the stationary points. We tested several exchange-correlation functionals for accuracy and consistency. The optimized structures of selected isomers (∆E < 0.4 eV) at the B3LYP level were reoptimized with several functionals (Table S1, Supporting Information). In general, different functionals showed similar behavior. In Li et al.’s benchmark calculations on MO3- and M2O6- (M ) Cr, Mo, W),31 superior results for M ) W were obtained with both CCSD(T) and many DFT methods. In our previous studies, B3LYP gave superior results in terms of energies when directly compared to experimental results. As discussed below, we used the results with the B3LYP functional for the further discussion. Vertical electron detachment energies (VDEs) were calculated using the generalized Koopmans’ theorem by adding a correction term to the eigenvalues of the anion.52 The correction term was calculated as δE ) E1 - E2 - εHOMO, where E1 and E2 are the total energies of the anion and neutral, respectively, in their ground states at the anion equilibrium geometry and εHOMO corresponds to the eigenvalue of the highest occupied molecular orbital (HOMO) of the anion. All calculations were performed with the Gaussian 03 software package.53 Three-dimensional contours of the molecular orbitals (MOs) were visualized using the VMD software.54 3. Theoretical Results The optimized ground-state structures and selected low-lying isomers of W4On-/0 (n ) 10-13) at the B3LYP/W/Stuttgart+2f1g/ O/aug-cc-pvTZ level of theory are presented in Figures 1-4,

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and the alternative optimized results of W4On-/0 (n ) 10-13) along with their relative energies at the B3LYP/W/Stuttgart/O/ 6-31+G(d) level of theory are shown in the Supporting Information (Figures S5-S12). We first presented the results for the stoichiometric clusters W4O12-/0 because the O-rich and O-deficient species are related to the stoichiometric clusters. In the following discussion, Ot and Ob stand for the terminal and bridging oxygen atoms, respectively. 3.1. Stoichiometric Clusters: W4O12 and W4O12-. The optimized ground-state structures of W4O12 and W4O12- are shown in Figure 1a and b. Both of them hold the planar eightmembered ring framework. Each tungsten atom is tetrahedrally coordinated with two terminal and two bridging O atoms. The ground state of W4O12 is revealed to be a highly symmetric D4h (1A1g) structure (Figure 1a), which is in agreement with the previous studies.30,34 The W-Ob and W-Ot bond lengths are 1.90 and 1.71 Å, respectively. Other structural candidates of neutral W4O12 are much higher in energy and thus not listed in the current paper. For the W4O12- anion, the ground state is found to be C2V (2A1) symmetry (Figure 1b). This result agrees with that identified by Sun et al.34 at the BPW91 level. The other isomer with D2h (2B3u) symmetry (Figure 1c) is only 0.25 eV above the C2V ground state (Figure 1b). More detailed results of W4O12-/0 at the B3LYP/W/Stuttgart/O/6-31+G(d) level of theory are given in the Supporting Information (Figures S9 and S10). 3.2. Oxygen-Rich Clusters: W4O13 and W4O13-. Started from the ground state of the stoichiometric W4O12 cluster (Figure 1a), many structural candidates with different spin multiplicities were first investigated at the B3LYP/W/Stuttgart/O/6-31+G(d) level of theory (Figures S11 and S12, Supporting Information). Then, selected isomers of the W4O13 and W4O13- were reoptimized at the B3LYP/W/Stuttgart+2f1g/O/aug-cc-pVTZ level of theory. We present two isomers each for W4O13 and W4O13-, as shown in Figure 2. The ground state of W4O13 possesses C1 (1A) symmetry (Figure 2a) and can be viewed as replacing a terminal O atom in W4O12 by an O2 unit. The calculated O-O bond length of the O2 unit in the W4O13 (Figure 2a) is 1.493 Å, which is close to that of the free peroxide anion O22- (1.53 Å calculated at the same level). A triplet state with a cleaved O-O bond (Figure 2b) is 1.29 eV higher in energy. This isomer, however, becomes the ground state for the W4O13- anion (Figure

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2c), whereas the isomer with the O2 unit (Figure 2d) is significantly higher in energy, by 1.53 eV above the ground state. 3.3. Oxygen-Deficient Clusters: W4O11 and W4O11-. We started the structural searches for the O-deficient clusters W4O11-/0 by removing one oxygen atom (terminal or bridging oxygen) from the ground state of the stoichiometric W4O12 cluster (Figure 1a). A lot of low-lying isomers are located for both neutral W4O11 and anionic W4O11- clusters (Figure 3). The ground state of neutral W4O11 is predicted to be a closed-shell (C2V, 1A1) species with the pentabridged structure (Figure 3a). In this pentabridged structure, there are two µ-oxo ligands bridged to the same pair of tungsten atoms. The corresponding triplet state is located 0.37 eV (C2V, 3B2) (Figure 3c) higher in energy. Another low-lying isomer (Cs, 1A′) with the eightmembered ring (Figure 3b) is 0.22 eV higher in energy, and it is derived from removing one terminal O atom from the D4h

W4O12 cluster. The singlet seven-membered ring structure (Figure 3d), which adopts C2 symmetry, is 0.55 eV less stable. As for the anion, the structure Cs (2A′′) that can be viewed as removing one terminal oxygen atom from the D4h W4O12 is found to be the most likely candidate for the ground state, as shown in the Figure 3e. The second-lowest isomer (Figure 3f) with the seven-membered ring (C2V, 2A1) is only 0.09 eV higher in energy and originates from removing one bridging O atom from the W4O12 (D4h) cluster. These two B3LYP geometries (Figure 3e and f) were reoptimized with various DFT functionals such as BLYP, BHANDHLYP, and BP86 (Table S1, Supporting Information). It should be mentioned that the C2V (2A1) isomer is close in energy to the Cs (2A′′) structure at the DFT level. Further theoretical calculations with more sophisticated methods may be necessary to resolve the true ground state of W4O11-. The third (Figure 3g) and fourth isomers (Figure 3h) of the anion are located 0.23 and 0.32 eV above the lowest-energy isomer, respectively. More optimized structures of the W4O11 and W4O11- together with their relative energies at the B3LYP/W/ Stuttgart/O/6-31+G(d) level are given in the Supporting Information (Figures S7 and S8). 3.4. Oxygen-Deficient Clusters: W4O10 and W4O10-. To search for the most stable structures of W4O10-/0, various structural possibilities with different spin multiplicities including the cage structure known for the neutral M4O10 cluster (M ) V, Nb, Ta)55 were taken into consideration. Selected optimized structures of W4O10 and W4O10-, along with their relative energies, are presented in Figure 4. For the neutral W4O10, the butterfly-like (C2V, 1A1) structure (Figure 4a) is revealed to be the ground state. The second isomer of W4O10 with a singlet cage structure (Td, 1A1) (Figure 4b) is higher in energy by 0.75 eV above the ground state. The ground state of W4O10- is found to be a doublet with C2V (2A1) symmetry (Figure 4c), similar to that of the neutral. Another doublet C2V (2B2) state is 1.52 eV higher in energy (Figure 4d). Other optimized isomers for both the neutrals and the anions are significantly higher (>1.0 eV) in energy; the optimized results at the B3LYP/W/Stuttgart/O/ 6-31+G(d) level are collected in the Supporting Information (Figures S5 and S6). 4. Discussion 4.1. Structural Evolution of W4On-/0 (n ) 10-13) Clusters. All of the W4On- anions are open-shell with one unpaired electron in their lowest-energy structures, whereas the neutral



TABLE 1: Theoretical Vertical Detachment Energies (VDEs) of Global Minima W4On- (n ) 10-13) Clusters and Selected Low-Lying Isomers (all energies in eV) theorya feature -

W4O10 (C2V 2A1)

X A B C

a

W4O11(Cs 2A′′)

X A B

W4O11(C2V 2A1 0.09 eV)

X′ A′ B′

W4O12(C2V 2A1)

X A B

W4O13(C1 2A)

X

MO

VDE

19b2 (β) 18b2 (β) 24a1 (β) 23a1 (β) 10a2 (β) 15b1 (β) 9a2 (β) 17b2 (β) 14b1 (β)

5.81 7.16 7.16 7.31 7.65 7.73 7.81 7.95 8.06

(T) (T) (T) (T) (T) (T) (T) (T) (T)

43a′ (β) 29a′′ (β) 42a′ (β) 41a′ (β) 28a′′ (β) 27a′′ (β) 40a′ (β) 39a′ (β) 26a′′ (β)

5.30 7.34 7.35 7.39 7.82 7.85 7.95 8.06 8.13

(T) (T) (T) (T) (T) (T) (T) (T) (T)

24a1 (β) 13a2 (β) 14b1 (β) 13b1 (β) 12a2 (β) 21b2 (β) 20b2 (β) 23a1 (β)

6.67 7.15 7.25 7.79 7.80 7.99 8.13 8.17

(T) (T) (T) (T) (T) (T) (T) (T)

18b1 (β) 10a2 (β) 17b1 (β) 20b2 (β) 81a (β) 79a (β) 80a (β) 78a (β)

7.12 7.71 7.72 7.99 7.36 7.58 7.72 7.84

(T) (T) (T) (T) (T) (T) (T) (T)

MO 25a1 (R) 19b2 (R) 18b2 (R) 24a1 (R) 23a1 (R) 10a2 (R) 15b1 (R) 9a2 (R) 17b2 (R) 14b1 (R) 30a′′ (R) 43a′ (R) 29a′′ (R) 42a′ (R) 41a′ (R) 28a′′ (R) 27a′′ (R) 40a′ (R) 39a′ (R) 26a′′ (R) 25a1 (R) 24a1 (R) 13a2 (R) 14b1 (R) 13b1 (R) 12a2 (R) 21b2 (R) 23a1 (R) 20b2 (R) 29a1 (R) 18b1 (R) 10a2 (R) 17b1 (R) 20b2 (R) 81a (R) 80a (R) 79a (R) 78a (R) 77a (R)

VDE 4.59 5.95 7.17 7.18 7.56 7.70 7.79 7.82 8.04 8.10 4.60 5.56 7.36 7.37 7.39 7.83 7.90 7.97 8.09 8.16 4.24 6.93 7.18 7.28 7.81 7.81 8.02 8.23 8.28 4.22 7.11 7.73 7.74 7.95 7.62 7.71 7.83 7.86 8.11

(S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S)

The labels R and β denote majority and minority spins, whereas S and T denote singlet and triplet W4On final states upon photodetachment.

ground states are all closed-shell species. It is suggested that the ground states of tetratungsten oxide clusters, W4On-/0 (n ) 10-13), favor the lowest spin multiplicities. As mentioned earlier, the lowest-energy isomers of anionic W4On- (n ) 10-13) preserved the tetrabridged structures. Starting from the butterfly-like W4O10- (Figure 4c), the additional oxygen atoms (n ) 10-13) are shown to successively occupy the terminal sites in the sequential oxidation. Two very different structures are competitive to be the ground state of the W4O11- anion (Figure 3), that is, the tribridged isomer (Figure 3f) and the tetrabridged isomer (Figure 3e). The tribridged isomer (Figure 3f) is nearly degenerate by 0.09 eV higher than the lowest-energy structure (Figure 3e). This isomer (Figure 3f) contains a direct W-W bond and can be viewed as removing one of the bridging oxygens from the ground state of W4O12- (Figure 1b). However, in the tetrabridged structure (Figure 3e), there is no significant W-W bonding because the three d electrons are from essentially a lone pair and one spin on the same W site (this will be discussed in detail in the subsection Interpretation of the Simulated Photoelectron Spectra and Molecular Orbital Analyses). To resolve the true ground state of W4O11-, the calculations with more sophisticated

methods and the comparison between calculations and experiments would be valuable. The similar structures related to the anionic lowest-energy isomers are observed for the W4On (n ) 10, 12, 13) ground states. It is noteworthy that the global minimum of W4O13 holds C1 (1A) symmetry (Figure 2a) and can be regarded as replacing a terminal O atom in W4O12 by the peroxo ligand. The ground state of neutral W4O11 is predicted to be a closed-shell (C2V, 1 A1) species with the pentabridged structure (Figure 3a). The W4O11 structure (Figure 3b), which corresponds to the anionic lowest-energy isomer (Figure 3e), is 0.22 eV higher in energy than the ground state (Figure 3a). 4.2. Interpretation of the Simulated Photoelectron Spectra and Molecular Orbital Analyses. We calculated the vertical detachment energies (VDEs) of W4On- (n ) 10-13) on the basis of the identified anion ground-state structures using the generalized Koopmans’ theorem. The calculated VDEs and the simulated PES spectra for the global minima and selected lowlying isomers are collected in Table 1 and Figure 9, respectively. In the single-particle picture, photodetachment involves removal of electrons from occupied molecular orbitals (MOs) of an anion. The final states are the ground and excited states of the

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Figure 5. (a) LUMO and HOMO pictures of the neutral W4O12 ground state (Figure 1a). (b) The top two molecular orbitals of the anionic ground state (Figure 1b).

Figure 6. (a) The HOMO picture of the neutral W4O13 ground state (Figure 2a). (b) The SOMO picture of the ground state of W4O13cluster (Figure 2c).

corresponding neutral. The lowest binding energy band in a PES spectrum involves photodetachment transition from the ground state of the anion to that of the neutral. The differences between the higher binding energy bands and the lowest binding energy band in the PES spectrum represent the excitation energies of the neutral cluster. Within the one-electron formalism, each occupied MO for a closed-shell anion will generate a single PES band with the associated vibrational structures governed by the Franck-Condon principle. However, all of the W4On(n ) 10-13) anions are open-shell with a single unpaired electron in their lowest-energy structures. In these cases, detachment from a fully occupied MO would result in two detachment channels due to the removal of either the spin-down (β) or the spin-up (R) electrons, giving rise to triplet (T) and singlet (S) final states, respectively, as given in Table 1. In the following, we will attempt to qualitatively account for the simulated PES features using molecular orbital analyses. Considering the complicated nature of the electronic structures of these systems, many of the assignments should be considered tentative. 4.2.1. W4O12 and W4O12-. The neutral W4O12 (Figure 1a) is the stoichiometric molecule in which each W achieves its favorite oxidation state W6+, that is, all of the valence electrons of W are used to form bonds with the O atoms. The simulated PES spectrum of W4O12- (Figure 9c) is consistent with a stable neutral cluster with a large HOMO-LUMO gap (2.89 eV). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) pictures of the neutral W4O12 are depicted in Figure 5a. All MOs from the HOMO and below are primarily O 2p-based orbitals, and the LUMO is the d-d antibonding orbital. When one extra electron is added to the neutral W4O12, the ground state of W4O12- would still maintain the eight-membered ring structure but with lower symmetry of C2V (2A1) (Figure 1b). Its valence electron configuration is (20b2)2(17b1)2(10a2)2(18b1)2(29a1)1. In this C2V structure, the extra electron is located on one of the four tungsten atoms, as is evidenced from its SOMO (29a1). The frontier MOs of the ground-state W4O12- are shown in Figure 5b. All MOs from 18b1 and below are of O 2p character. Photodetachment from the singly occupied SOMO (29a1) of W4O12- (Figure 5b) yields the first PES band X (Figure 9c), for which the VDE was calculated to be 4.22 eV. This value is in considerable agreement with the previous experiment reported by Sun et al.,34 who pointed out the first VDE to be 4.00 eV. As shown in Table

1, the calculated VDE for detachment from the first primarily O 2p type MO, 18b1 (corresponding to the HOMO of neutral W4O12), is 7.1 eV (band A), followed immediately by the detachment from the 10a2 MO with a calculated VDE of 7.7 eV (band B). A total of eight detachment channels are calculated up to 8.0 eV for the O 2p-type orbitals (Table 1). 4.2.2. W4O13 and W4O13-. The ground state of neutral W4O13 (C1, 1A) is a closed-shell species with an O2 moiety (Figure 2a). It can be viewed as replacing one of the terminal oxygens in W4O12 by an O2 unit. The HOMO of this cluster corresponds to the π* orbital of the O-O moiety, as shown in Figure 6a. Addition of an electron into the σ* orbital in the anion completely breaks the O-O bond in the ground state of W4O13(Figure 2c). The isomer with the O-O unit (Figure 2d), in which the extra electron occupies a higher-lying W 5d-type orbital, is much higher in energy by 1.53 eV. The frontier molecular orbitals of the ground-state W4O13- are of O 2p character, and the SOMO is primarily derived from the σ* of the O2 unit, as shown in Figure 6b. From the simulated PES spectrum (Figure 9d), it is found that the O-rich W4O13- anion has very high electron binding energies, and all detachment transitions are observed above 7.3 eV. 4.2.3. W4O11 and W4O11-. The ground state of W4O11 is the pentabridged C2V structure (Figure 3a), with the tetrabridged Cs structure (Figure 3b) being 0.22 eV higher in energy. There is a competition between W-W and W-O bonding. A direct W-W bond is observed in the C2V structure. On the other hand, in the Cs structure, there is no W-W bonding, and the extra pair of d electrons is essentially a lone pair localized on the tricoordinated W atom, which becomes the ground state in the anion (Figure 3e) with a valence electron configuration of (28a′′)2(41a′)2(42a′)2(29a′′)2(43a′)2(30a′′)1. The SOMO (30a′′) and the first fully occupied (43a′) MO (Figure 7b) are mainly from the W 5d orbitals, where the three 5d electrons are located on the same tungsten site (W3+). Such defect sites are chemically active and may act as catalytic centers in bulk oxides or catalysts. For example, the three localized W 5d electrons on this W3+ site may be readily transferred to the two half-filled π* and one unoccupied σ* orbitals of an approaching O2 molecule, which would gradually weaken and finally break the O-O bond of the O2 unit. In fact, the O-rich W4O13- cluster (Figure 2c) can be viewed exactly as the product of W4O11(Figure 3e) reacting with an O2 molecule.



Figure 8. The frontier molecular orbitals for the anionic W4O10ground state (Figure 4c).

Figure 7. (a, b) The frontier molecular orbitals for the ground state of W4O11-/0 clusters (Figure 3a and e). (c) The top two molecular orbitals of the anionic low-lying isomer W4O11- (C2V 2A1 0.09 eV; see Figure 3f).

The W4O11- species represent the most complicated systems in our current study, primarily because of competitions between the W-W, W-O, and WdO bonding. The two low-lying structures of W4O11- (Figure 3e and f) have very different W-O connectivities, but surprisingly, they are nearly degenerate in energy, with the tribridged structure (Figure 3f) being slightly less stable. The tetrabridged W4O11- cluster (Figure 3e) seems to be the most likely candidate for the ground state, which has Cs (2A′′) symmetry with an electron configuration of (28a′′)2(41a′)2(42a′)2(29a′′)2(43a′)2(30a′′)1. Photodetachment from the singly occupied 30a′′ orbital of W4O11- yields the first PES band X with the calculated VDE of 4.60 eV (Table 1). The next detachment channel is from 43a′(β) and 43a′(R), with the calculated VDEs of 5.30 and 5.56 eV, respectively, which should correspond to band A in the simulated PES spectrum. As shown in Table 1, band B in the simulated photoelectron spectrum can be assigned to electron detachment transitions from the MO group (29a′′, 42a′, 41a′). The higher binding energy features above 7 eV in the simulated PES spectra (Figure 9b) are due to detachment from O 2p-based MOs. However, the tribridged isomer (Figure 3f) is very close in energy to the tetrabridged isomer (Figure 3e) in the anion and would give a similar VDE for the ground-state transition. Thus, we also calculated the VDEs and simulated the spectrum from the tribridged isomer, as compared with that from the tetrabridged isomer in Figure 9b. The tribridged isomer has an electron configuration of (12a2)2(13b1)2(14b1)2(13a2)2(24a1)2(25a1)1. The top two predominantly 5d-type MOs are shown in Figure 7c. The extra electron enters the 25a1 orbital with d-d π-bonding character (Figure 7c). The 24a1 MO is a d-d σ-bonding orbital (Figure 7c), and all MOs from 13a2 and below are of O 2p character. Detachment from the 25a1 orbital results in the X′ band in the simulated photoelectron spectrum with the calculated VDE of 4.24 eV

(Table 1). The next transition is from 24a1(β) and 24a1(R), with calculated VDEs of 6.67 and 6.93 eV, respectively, which should correspond to band A′. The spectral features above 7.1 eV in Figure 9b can all be attributed to detachments from O 2p-based MOs with contributions from 13a2 and below, as given in Table 1. 4.2.4. W4O10 and W4O10-. Both W4O10 and W4O10- ground states possess the butterfly-like structures with C2V symmetry (Figure 4a and c). The electron configuration of the anion C2V (2A1) is (10a2)2(23a1)2(24a1)2(18b2)2(19b2)2(25a1)1. The frontier MOs of W4O10- are shown in Figure 8. In the anion, the SOMO (25a1) is primarily a W-W π-bonding orbital. Therefore, when the electron is added to the W4O10, the W-W bond length would decrease from 2.645 Å in the neutral to 2.593 Å in the anion. Photodetachment from the singly occupied 25a1 orbital of W4O10- yields the first PES band X (VDE: 4.59 eV), followed by detachment (band A) from the doubly occupied 19b2 MO with calculated VDEs of 5.81 (β) and 5.95 eV (R), respectively. The 19b2 MO is also primarily a W-W π-bonding orbital. As shown in Table 1, band B in the simulated photoelectron spectrum can be assigned to electron detachment transitions from the MO group (18b2, 24a1, 23a1). Two MOs of 18b2 and 24a1 are derived from the O 2p-type character of the terminal oxygen. The 23a1 MO is mainly a W-W σ-bonding orbital. Detachment from the 23a1 W-W σ-bonding orbital yields the higher binding energy features at 7.31 (β) and 7.56 eV (R), respectively, which fall in the spectral region where detachments from O 2p-based MOs dominate (Figure 9a). The calculated VDEs from 10a2 and below are beyond 7.65 eV, as shown in Table 1. All MOs below are of O 2p character, not only the terminal oxygen atoms but also the bridging oxygen atoms. 4.3. Comparison between W4On and WxOn (x ) 1-3). The previous theoretical calculations on the monotungsten oxide WOn-/0 (n ) 3-5), ditungsten oxide W2On-/0 (n ) 1-7), and tritungsten oxide W3On-/0 (n ) 7-10) clusters have been carried out at the DFT level using the B3LYP hybrid functional.23,24,26–28 In the following sections, we will compare the current work with the previous studies on WxOn (x ) 1-3). 4.3.1. Stoichiometric Clusters. The previous work on the stoichiometric tungsten oxide clusters, WnO3n (n ) 1-3), showed that W prefers tetrahedral coordination with oxygen. The WO3 molecule is shown to have the C3V symmetry with three equivalent WdO double bonds,23 and the W2O6 and W3O9 clusters favor the ring structure, in which each W atom is tetrahedrally coordinated with two bridging O atoms and two terminal O atoms.24,27 The W2O6 and W3O9 clusters adopt the four-membered and six-membered ring structures with D2h (1Ag)

1970


Figure 9. Simulated photoelectron spectra from the lowest-energy structures (black line) and selected low-lying isomers (red line) for W4On- (n ) 10-13) clusters at the B3LYP/W/Stuttgart+2f1g/O/augcc-pvTZ level. The simulations are done by fitting the distribution of calculated VDEs with unit-area Gaussian functions of 0.1 eV width.

and D3h (1A1′) symmetry, respectively. These rules are still observed for the W4O12 cluster, that is, the ground state of the W4O12 cluster (Figure 1a) is shown to be the eight-membered ring with D4h (1A1g) symmetry. Then, for the anion, one more electron will reduce the symmetries of W2O6- and W4O12- from D2h (1Ag) and D4h (1A1g) to C2V (2A1) and C2V (2A1), respectively. The A-X energy difference in the PES spectra represents an approximate measure of the HOMO-LUMO gap for the neutral molecule. Herein, a large HOMO-LUMO gap of W4O12 (∆Egap ) 2.89 eV) is obtained in our simulated PES spectrum (Figure 9c). This value is substantially close to the reported indirect band gaps of bulk WO3 (indirect gap, ∼2.6 eV; direct gap, ∼3.5-3.7 eV).56 Previously, the HOMO-LUMO gap (∆Egap) of the WO3 cluster has been measured to be 1.4 eV by Zhai et al. using anion photoelectron spectroscopy coupled with quantum chemical calculations,23 and that increased to 2.8 eV for W2O6.24 Furthermore, the magnitude of this value was up to ∼3.4 eV for W3O9.27 Our calculated results (2.89 eV for W4O12) generally agree with that in the earlier article by Sun et al.,34 who estimated the energy gap from transition energies to be 2.76 eV. The band gap of the neutral W4O12 cluster is very similar to that in the bulk, and the W4O12 might be regarded as an embryonic form of bulk tungsten oxide. 4.3.2. Oxygen-Rich Clusters. Adding one oxygen atom to the ground state of the stoichiometric WnO3n clusters will generate the oxygen-rich clusters, for example, WO4, W2O7, W3O10, and W4O13. All of them are “oxygen-excessive” species. Peroxo-type ground-state structures might have been expected for such species, where the presence of an O-O bond could provide further stabilization. Therefore, the structures of W2O7,26 W3O10,27 and W4O13 are observed as replacing a terminal O atom in WnO3n (n ) 2-4) by a peroxo O2 unit, except that the structure of WO423 possesses the delocalized (D2d, 1A2) structure, which is a resonance hybrid of the two localized C2V structures. The C2V (1A1) isomer of WO4 with the peroxo ligand is 0.39 eV above the ground state.23 As for the anionic WnO3n+1- (n ) 1-3) clusters, the doublet isomers without the O2 unit are

Wang et al. predicted to be the global minima.23,26,27 The extra unpaired electron is localized on the terminal oxygen with the longest W-O bond length. For W4O13- (Figure 2c), the isomer (C1, 2 A) without the O2 unit is found to be the ground state, whereas the anionic isomer (Figure 2d) with the peroxo unit is much higher in energy by 1.53 eV. Other oxygen-rich clusters, such as WnO3n+2- (n ) 2-4),25 are best considered as WnO3n(O2-), containing a side-on-bound superoxide ligand. It is suggested that the extra electron in WnO3n- (n ) 2-4) is capable of actvating dioxygen by nondissociative electron transfer (W 5d f O2 π*), and the anionic clusters can be viewed as models for reduced defect sites on tungsten oxide surfaces for the chemisoption of O2. 4.3.3. Oxygen-Deficient Clusters. The electronic structures of tungsten oxide clusters are rather complicated, in particular for the oxygen-deficient clusters due to the W 5d electrons, and are challenging computationally. Tungsten is the group VIB transition-metal element with an electron configuration of 5d46s2. In the stoichiometric and O-rich oxide clusters, all of the valence electrons of W are used to form bonds with the O atoms, and each tungsten atom achieves its chemically saturated oxidation state W6+. However, for the O-deficient oxide clusters, the extra valence electrons may be used to form the W-W bonding, which would make the search for the global minima more challenging. The typical examples are the previous work24,27 on the W2O4-/0 and W3O7-/0 species, as well as the current W4O11-/0 systems. There seems to be a competition between the W-W bonding and W-O bonding and between the W-O single bonds and WdO double bonds. In the case of the neutral W4O11, the pentabridged ground state (Figure 3a) contains a W-W bond (2.574 Å), 6 WdO bonds, and 10 W-O bonds. Another low-lying isomer (Cs, 1A′) with the eight-membered ring (Figure 3b) is 0.22 eV higher in energy, which is very similar to the ground states of WnO3n-1 (n ) 2, 3)24,27 All of them are generated by removing one terminal O atom from the stoichiometric WnO3n cluster. 4.3.4. W4O11- as an Anionic Molecular Model for O-Deficient Defect Sites. In the W4O11- anion, it appears that there is a competition between the W-O single bond and WdO double bond. In the tetrabridged W4O11- structure (Figure 3e), there are seven terminal WdO units, whereas in the tribridged lowlying isomer (Figure 3f), there are eight terminal WdO units. Clearly, two bridging W-O single bonds in the former are favored over the WdO double bond in the latter. Similar phenomena are observed in the W3O8 and W3O7 system.27 The W4O11- anion (Figure 3e) is formed by removing a terminal O atom from the stoichiometric W4O12 cluster, creating a tricoordinated W site. Our MO analysis shows that this W site has a pair of localized d electrons and one unpaired spin (Figure 7b), and it is essentially a localized W3+ site. Such defect sites are chemically active and may act as catalytic centers in bulk oxides or catalysts. For example, the three localized d electrons on the W3+ site may be readily transferred to the π* and σ* orbitals of an approaching O2 molecule, which would gradually weaken and finally break the O-O bond of the O2 unit.

W4O11- + O2 f W4O13- (-119 kcal/mol (-5.2 eV)) (1) Our calculation yielded an O2 chemisorption energy of -119 kcal/mol (eq 1) to the W3+ defect site in W4O11-. We expect that the W4O11- cluster can activate other molecules as well and may be considered as anionic molecular model for oxygendeficient defect sites in tungsten oxides.

Tetratungsten Oxide Clusters W4On-/0 (n ) 10-13) 5. Conclusion We report a systematic theoretical study on a series of tetratungsten oxide clusters, W4On- and W4On (n ) 10-13). A large energy gap is generated in the simulated photoelectron spectrum of W4O12-, yielding a HOMO-LUMO gap of ∼2.9 eV for the stoichiometric W4O12 molecule. High electron binding energies (>7.0 eV) are observed for W4O13-, suggesting that the W4O13 neutral cluster is an unusually strong oxidizing agent. Extensive density functional calculations are carried out to elucidate the geometric and electronic structures and chemical bonding in the W4On-/0 clusters. Our calculations show that W4O12 is a D4h cluster with a W4O4 eight-membered ring and two terminal WdO units on each W site. W4O10-/0 and W4O11- can be viewed as removing one and two terminal O atoms from W4O12-/0, respectively. The W4O11 is an interesting species which possess the pentabridged structure. The O-rich cluster W4O13 can be regarded as replacing a terminal O atom in W4O12 by an O2 unit. The W4O11- cluster contains a tricoordinated W site, which is found to possess three localized 5d electrons and is a localized W3+ defect site. It is suggested that W4O11- can be considered as a general anionic model for O-deficient defect sites in tungsten oxides. Acknowledgment. We gratefully acknowledge support from the Natural Science Foundation of China (20641004, 20771026, 20773024, and 90922022) and the Natural Science Foundation of Fujian Province of China (No. 2008J0151). Supporting Information Available: Relative energies in eV for selected low-lying isomers of W4On-/0 (n ) 11-12) with different density functional methods (Table S1), theoretical vertical detachment energies of global minima W4On(n ) 10-13) clusters and selected low-lying isomers (all energies are in eV) at the B3LYP/W/Stuttgart+2f1g/O/augcc-pVTZ level of theory (Table S2), Cartesian coordinates for the alternative optimized structures of W4On-/0 (n ) 10-13) at the B3LYP/W/Stuttgart+2f1g/O/aug-cc-pVTZ level (Table S3), simulated photoelectron spectra from the lowest-energy structures and selected low-lying isomers for W4On- (n ) 10-13) clusters at the B3LYP/W/Stuttgart+2f1g/ O/aug-cc-pVTZ level (Figure S1-S4), and alternative optimized structures for W4On- and W4On (n ) 10-13) at the B3LYP/W/Stuttgart/O/6-31+G(d) level (Figure S5-S12). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Manno, D.; Serra, A.; Giulio, M. D.; Micocci, G.; Tepore, A. Thin Solid Films 1998, 324, 44. (2) Moulzolf, S. C.; LeGore, L. J.; Lad, R. J. Thin Solid Films 2001, 400, 56. (3) Granqvist, C. G. Solar Energy Mater. Solar Cells 2000, 60, 201. (4) Bessie`re, A.; Marcel, C.; Morcrette, M.; Tarascon, J. M.; Lucas, V.; Viana, B.; Baffier, N. J. Appl. Phys. 2002, 91, 1589. (5) Salvatl, L., Jr.; Makovsky, L. E.; Stencel, J. M.; Brown, F. R.; Hercules, D. M. J. Phys. Chem. 1981, 85, 3700. (6) Horsley, J. A.; Wachs, I. E.; Brown, J. M.; Via, G. H.; Hardcastle, F. D. J. Phys. Chem. 1987, 91, 4014. (7) Gazzoli, D.; Valigi, M.; Dragone, R.; Marucci, A.; Mattei, G. J. Phys. Chem. B 1997, 101, 11129. (8) Bigey, C.; Hilaire, L.; Maire, G. J. Catal. 2001, 198, 208. (9) Ji, S. F.; Xiao, T. C.; Li, S. B.; Xu, C. Z.; Hou, R. L.; Coleman, K. S.; Green, M. L. H. Appl. Catal., A 2002, 225, 271. (10) Mamede, A. S.; Payen, E.; Grange, P.; Poncelet, G.; Ion, A.; Alifanti, M.; Paˆrvulescu, V. I. J. Catal. 2004, 223, 1.

J. Phys. Chem. A, Vol. 114, No. 4, 2010 1971 (11) Weinstock, I. A.; Barbuzzi, E. M. G.; Wemple, M. W.; Cowan, J. J.; Reiner, R. S.; Sonnen, D. M.; Heintz, R. A.; Bond, J. S.; Hill, C. L. Nature. 2001, 414, 191. (12) Zemski, K. A.; Justes, D. R.; Castleman, A. W., Jr. J. Phys. Chem. B 2002, 106, 6136. (13) (a) Fialko, E. F.; Kirkhtenko, A. V.; Goncharov, V. B.; Zamaraev, K. I. J. Phys. Chem. B 1997, 101, 5772. (b) Vyboishchikov, S. F.; Sauer, J. J. Phys. Chem. A 2001, 105, 8588. (c) Tenorio, F. J.; Murray, I.; Martinez, A.; Klabunde, K. J.; Ortiz, J. V. J. Chem. Phys. 2004, 120, 7955. (d) Asmis, K. R.; Santambrogio, G.; Bru¨mmer, M.; Sauer, J. Angew. Chem., Int. Ed. 2005, 44, 3122. (e) Bo¨hme, D. K.; Schwarz, H. Angew. Chem., Int. Ed. 2005, 44, 2336. (14) Waters, T.; O’Hair, R. A. J.; Wedd, A. G. J. Am. Chem. Soc. 2003, 125, 3384. (15) Socaciu, L. D.; Hagen, J.; Bernhardt, T. M.; Wo¨ste, L.; Heiz, U.; Ha¨kkinen, H.; Landman, U. J. Am. Chem. Soc. 2003, 125, 10437. (16) Jackson, P.; Fisher, K. J.; Willett, G. D. Chem. Phys. 2000, 262, 179. (17) Asmis, K. R.; Bru¨mmer, M.; Kaposta, C.; Santambrogio, G.; von Helden, G.; Meijer, G.; Rademann, K.; Wo¨ste, L. Phys. Chem. Chem. Phys. 2002, 4, 1101. (18) Shi, Y.; Ervin, K. M. J. Chem. Phys. 1998, 108, 1757. (19) (a) Justes, D. R.; Mitric´, R.; Moore, N. A.; Bonacˇic´-Koutecky´, V.; Castleman, A. W., Jr. J. Am. Chem. Soc. 2003, 125, 6289. (b) Fielicke, A.; Mitriæ, R.; Meijer, G.; Bonacˇic´-Koutecky´, V.; von Helden, G. J. Am. Chem. Soc. 2003, 125, 15716. (c) Asmis, K. R.; Meijer, G.; Bru¨mmer, M.; Kaposta, C.; Santambrogio, G.; Wo¨ste, L.; Sauer, J. J. Chem. Phys. 2004, 120, 6461. (d) Fielicke, A.; Meijer, G.; von Helden, G. J. Am. Chem. Soc. 2003, 125, 3659. (20) Yoder, B. L.; Maze, J. T.; Raghavachari, K.; Jarrold, C. C. J. Chem. Phys. 2005, 122, 094313. (21) Fielicke, A.; Rademann, K. J. Phys. Chem. A 2000, 104, 6979. (22) Bienati, M.; Bonacˇic´-Koutecky´, V.; Fantucci, P. J. Phys. Chem. A 2000, 104, 6983. (23) Fujiwara, T.; Iizuka, A.; Sato, K.; Yamada, Y. Int. J. Mass Spectrom. 2005, 242, 57. (24) Jang, Y. H.; Goddard, W. A., III. J. Phys. Chem. B 2002, 106, 5997. (25) Fu, G.; Xu, X.; Lu, X.; Wan, H. L. J. Am. Chem. Soc. 2005, 127, 3989. (26) Zhai, H. J.; Kiran, B.; Cui, L. F.; Li, X.; Dixon, D. A.; Wang, L. S. J. Am. Chem. Soc. 2004, 126, 16134. (27) Zhai, H. J.; Huang, X.; Cui, L. F.; Li, X.; Li, J.; Wang, L. S. J. Phys. Chem. A 2005, 109, 6019. (28) (a) Huang, X.; Zhai, H. J.; Waters, T.; Li, J.; Wang, L. S. Angew. Chem., Int. Ed. 2006, 45, 657. (b) Wang, B.; Zhang, X. H.; Huang, X.; Zhang, Y. F. Chin. J. Struct. Chem. 2008, 27, 990. (29) Zhai, H. J.; Huang, X.; Waters, T.; Wang, X. B.; O’Hair, R. A. J.; Wedd, A. G.; Wang, L. S. J. Phys. Chem. A 2005, 109, 10512. (30) Huang, X.; Zhai, H. J.; Li, J.; Wang, L. S. J. Phys. Chem. A 2006, 110, 85. (31) Huang, X.; Zhai, H. J.; Kiran, B.; Wang, L. S. Angew. Chem., Int. Ed. 2005, 44, 7251. (32) Yang, X.; Waters, T.; Wang, X. B.; O’Hair, R. A. J.; Wedd, A. G.; Li, J.; Dixon, D. A.; Wang, L. S. J. Phys. Chem. A 2004, 108, 10089. (33) Li, S.; Dixon, D. A. J. Phys. Chem. A 2006, 110, 6231. (34) Li, S.; Dixon, D. A. J. Phys. Chem. A 2007, 111, 11908. (35) Li, S.; Dixon, D. A. J. Phys. Chem. A 2007, 111, 11093. (36) Li, S.; Hennigan, J. M.; Dixon, D. A. J. Phys. Chem. A 2009, 113, 7861. (37) Sun, Q.; Rao, B. K.; Jena, P.; Stolcic, D.; Kim, Y. D.; Gantefo¨r, G.; Castleman, A. W., Jr. J. Chem. Phys. 2004, 121, 9417. (38) Sun, Q.; Rao, B. K.; Jena, P.; Stolcic, D.; Gantefo¨r, G.; Kawazoe, Y. Chem. Phys. Lett. 2004, 387, 29. (39) Wang, B.; Zhai, H. J.; Huang, X.; Wang, L. S. J. Phys. Chem. A 2008, 112, 10962. (40) Zhai, H. J.; Wang, B.; Huang, X.; Wang, L. S. J. Phys. Chem. A 2009, 113, 3866. (41) Bondarchuk, O.; Huang, X.; Kim, J.; Kay, B. D.; Wang, L. S.; White, J. M.; Dohna´lek, Z. Angew. Chem. 2006, 118, 4904. (42) Waters, T.; Huang, X.; Wang, X. B.; Woo, H. K.; O’Hair, R. A. J.; Wedd, A. G.; Wang, L. S. J. Phys. Chem. A 2006, 110, 10737. (43) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (44) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (45) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (46) Andrae, D.; Ha¨eussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (47) Ku¨chle, W.; Dolg, M.; Stoll, H.; Preuss, H. Pseudopotentials of the Stuttgart/Dresden Group, revision August 11, 1998; http://www.theochem.uni-stuttgart.de/pseudopotentiale (1998). (48) Foresman, J. B.; Frisch, A. E. Exploring Chemistry with Electronic Structure methods, 2nd ed.; Gaussian Inc.: Pittsburgh, PA, 1995.

1972


(49) Martin, J. M. L.; Sundermann, A. J. Chem. Phys. 2001, 114, 3408. (50) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007. (51) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. J. Chem. Phys. 1992, 96, 6796. (52) Tozer, D. J.; Handy, N. C. J. Chem. Phys. 1998, 109, 10180. (53) Frisch, M. J.; et al. Gaussian 03, revision D.01; Gaussian, Inc.: Wallingford, CT, 2004.

Wang et al. (54) VMD (Visual Molecular Dynamics): Humphrey, W.; Dalke, A.; Schulten, K. J. Mol. Graphics 1996, 14, 33. (55) Zhai, H. J.; Do¨bler, J.; Sauer, J.; Wang, L. S. J. Am. Chem. Soc. 2007, 129, 13270. (56) Koffyberg, F. P.; Dwight, K.; Wold, A. Solid State Commun. 1979, 30, 433.

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