On the Structure and Chemical Bonding of Tri-Tungsten Oxide ...

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86 J. Phys. Chem. A, Vol. 110, No. 1, 2006 Huang et al.chemisorption of O 2 on W 3 O 8 and may serve as importantintermediate in catalytic oxidation reactions.2. Experimental and Theoretical Methods2.1. Photoelectron Spectroscopy. The experiments werecarried out using a magnetic-bottle photoelectron spectroscopy(PES) apparatus equipped with a laser vaporization supersoniccluster source. 30,31 Briefly, W m O n - cluster anions were producedby laser vaporization of a pure tungsten target in the presenceof helium carrier gas seeded with 0.5% O 2 and were analyzedusing a time-of-flight mass spectrometer. The W 3 O n - (n )7-10) clusters of current interest were each mass selected anddecelerated before being photodetached with 157-nm photonsfrom an F 2 excimer laser. Photoelectrons were collected at nearly100% efficiency by the magnetic bottle and analyzed in a 3.5m long electron flight tube. Photoelectron spectra were calibratedusing the known spectrum of Rh - , and the energy resolutionof the apparatus was ∆E k /E k ≈ 2.5%, that is, ∼25 meV for 1eV electrons.2.2. Density Functional Calculations. The theoretical calculationswere performed at the DFT level using the B3LYPhybrid functional. 32-34 A number of structural candidates wereevaluated, and the search for the global minima was performedusing analytical gradients with the Stuttgart 14-valence-electronpseudopotentials and the valence basis sets 35,36 augmented withtwo f-type and one g-type polarization functions [ζ(f) ) 0.256,0.825; ζ(g) ) 0.627] for tungsten as recommended by Martinand Sundermann 37 and the aug-cc-pVTZ basis set for oxygen.38,39 Scalar relativistic effects, i.e., the mass-velocity andDarwin effects, were taken into account via the quasirelativisticpseudopotentials. Since we were mainly interested in explainingthe experimentally observed PES spectra, no further effort wasdevoted to resolve the spin-orbit coupled fine structures in thecalculated spectra. Our previous results on tungsten oxidesshowed that spin-orbit coupling effects would shift the orbitalenergies by up to a few tenth of an eV, which would not affectthe spectral assignment. 40 Only a selected set of optimizedstructures (the ground state and a few low-lying isomers),considered to be important in interpreting the experimental data,are reported. Vibrational frequency calculations were performedat the same level of theory to verify the nature of the stationarypoints. The vertical detachment energies (VDEs) were calculatedusing a combined ∆SCF-TDDFT approach, as we previouslyoutlined. 41,42 In this approach, the ground-state energies of theanions and the neutrals were calculated from the ∆SCF energydifference at the B3LYP level, whereas the excited states ofthe one-electron-detached species were obtained from TDDFTcalculations of the neutrals. For all calculations, the extra fineintegration grid was used to obtain highly accurate DFT results.All calculations were accomplished using the NWChem 4.6program and the Molecular Science Computing Facility locatedat the Environmental Molecular Sciences Laboratory. 43 TheExtensible Computational Chemistry Environment software wasused to generate the three-dimensional contours of the calculatedKohn-Sham orbitals. 443. Experimental ResultsThe PES spectra of W 3 O n - (n ) 7-10) measured at 157 nmare shown in Figure 1. The observed spectral bands are labeledwith letters (X, A, B, C). The low binding energy features inW 3 O n - (n ) 7-9) are expanded to illustrate the spectral details.The obtained VDEs and ground-state adiabatic detachmentenergies (ADEs) are given in Table 1.Figure 1. Photoelectron spectra of W 3O n- (n ) 7-10) at 157 nm(7.866 eV).The spectrum of W 3 O - 7 (Figure 1a) shows two relativelyweak bands (X and A) at low binding energies. More intensesignals above 6 eV appear to be congested and continuous. Theground-state band X has a VDE of 3.9 eV with a long tail downto ∼3.3 eV either due to vibrational hot bands or a largegeometry change between the anion and neutral ground states.The ADE of the X band was estimated to be 3.5 ( 0.1 eV,which defines the electron affinity (EA) of neutral W 3 O 7 . Thelarge uncertainty was a result of the low energy tail, which madeit difficult to define the ADE. Band A with a VDE of 5.04 eVis relatively sharp, which should correspond to the first excitedstate of W 3 O 7 . A shoulder seems to be present at the lowerbinding energy side of band A. The spectrum beyond ∼6.0 isfeatureless, which is labeled with B and C simply to denotetwo spectral regions.The spectrum of W 3 O - 8 (Figure 1b) displays relatively sharpfeatures. A weak band X with a VDE of 4.62 eV is followedby a large energy gap and three well-defined spectral features(A, B, C) at high binding energies. The ground-state band Xyields an ADE of 4.37 eV, which defines the EA of neutralW 3 O 8 . The first excited-state band (A) at a VDE of 6.78 eVgives a X-A gap of 2.16 eV. The VDEs of bands B and C aremeasured to be 7.23 and 7.65 eV, respectively.The PES spectrum of the stoichiometric W 3 O - 9 cluster isshown in Figure 1c. The ground-state X has weak intensity andis very broad, suggesting a large geometry change between theanion and neutral ground state. The X band yields a VDE of4.2 eV and an ADE of 3.55 eV. The X band is followed by alarge energy gap and the first excited-state band (A) is locatedat a high binding energy with an estimated onset of ∼7 eV. Nodistinct features were observed in the higher binding energy
Tri-Tungsten Oxide Clusters W 3 O n - and W 3 O n J. Phys. Chem. A, Vol. 110, No. 1, 2006 87TABLE 1: Experimental VDEs of W 3 O n- (n ) 7-10) andComparison with Theoretical VDEs Calculated from theLowest-Energy Anions (All Energies Are in eV)exptl atheor b,cADE VDE MO VDE MO VDEW 3O - 7 X ∼3.5 ∼3.9 29a′ (R) 4.01 (S)A 5.04 (5) 28a′ (β) 5.04 (T)B ∼6.0-7.0 27a′ (β) 6.40 (T) 28a′ (R) 6.23 (S)21a′′ (β) 6.65 (T) 27a′ (R) 6.76 (S)26a′ (β) 6.74 (T) 21a′′ (R) 6.84 (S)26a′ (R) 6.90 (S)C >7.0 20a′′ (β) 7.42 (T) 20a′′ (R) 7.57 (S)25a′ (β) 7.57 (T) 25a′ (R) 7.78 (S)W 3O - 8 X 4.37 (5) 4.62 (5) 19a 1 (R) 4.58 (S)A6.78 (3) 18a 1 (β) 6.89 (T)B 7.23 (5) 7a 2 (β) 7.09 (T) 7a 2 (R) 7.20 (S)12b 1 (β) 7.11 (T) 12b 1(R) 7.23 (S)11b 1 (β) 7.19 (T) 11b 1(R) 7.31 (S)C 7.65 (5) 14b 2 (β) 7.58 (T) 18a 1 (R) 7.59 (S)6a 2 (β) 7.64 (T)W 3O - 9 X 3.55 (10) ∼4.2 10a 1′ (R) 4.39 (S)A ∼7.4 6a 2′′ (β) 7.18 (T) 6a 2′′ (R) 7.32 (S)7e′′ (β) 7.20 (T) 7e′′ (R) 7.34 (S)12e′(β) 7.66 (T) 12e′ (R) 7.81 (S)6e′′ (β) 7.70 (T) 6e′′ (R) 7.83 (S)W 3O - 10 X 7.1 (1) 7.1-7.8 61a (β) 7.09 (T) 62a (R) 7.90 (S)58a (β) 7.45 (T)60a (β) 7.54 (T)57a (β) 7.64 (T)aNumbers in the parentheses represent experimental uncertainty inthe last digits. b The ground-state electronic configurations for the anionsare: W 3O 7- , (25a′) 2 (20a′′) 2 (26a′) 2 (21a′′) 2 (27a′) 2 (28a′) 2 (29a′) 1 ;W 3O 8- ,(6a 2) 2 (14b 2) 2 (11b 1) 2 (12b 1) 2 (7a 2) 2 (18a 1) 2 (19a 1) 1 ;W 3O 9- , (6e′′) 4 (12e′) 4(7e′′) 4 (6a 2′′) 2 (10a 1′) 1 ;W 3O 10- , (57a) 2 (58a) 2 (59a) 2 (60a) 2 (61a) 2 (62a) 1 .cThe labels “R” and “β” denote majority and minority spins, whereasS and T denote singlet and triplet W 3O n final states upon photodetachment.Figure 3. Optimized structures and their relative energies for W 3O 8and W 3O 8- . The bond lengths are in angstroms and bond angles indegrees.Figure 2. Optimized structures for W 3O 9 and W 3O 9- . The bond lengthsare in angstroms and bond angles in degrees.part of the spectrum. The PES spectrum in Figure 1c yields anapproximate energy gap of ∼3.4 eV, which is nearly identicalto the bulk band gap of WO 3 . In the previous work by Sun etal. at 193 nm, 29 only a lower limit for the highest occupiedmolecular orbital (HOMO)-lowest unoccupied molecular orbital(LUMO) gap was given (>1.5 eV). Clearly the lowerphoton energy used could not have accessed the band gap.The W 3 O 10 - cluster possesses a high electron binding energyand its PES spectrum at 157 nm only gave an onset at 7.1 eV,which yielded a very large EA for neutral W 3 O 10 .4. Theoretical ResultsThe optimized ground-state geometries of W 3 O n and W 3 O- nand selected low-lying isomers are presented in Figures 2-5.We first present the results for the stoichiometric clusters, W 3 O 9and W 3 O - 9 (Figure 2), because the O-deficient and O-richspecies are related to the stoichiometric clusters. In the followingdiscussion, O t and O b stand for the terminal and bridging oxygenatoms, respectively.Figure 4. Optimized structures and their relative energies for W 3O 7and W 3O 7- . The bond lengths are in angstroms and bond angles indegrees.4.1. Stoichiometric Clusters W 3 O 9 and W 3 O 9 - . Theoptimized ground-state structures of W 3 O 9 and W 3 O 9 - areshown in Figure 2. For W 3 O 9 neutral, we considered two initialstarting geometries with C 3V and D 3h symmetries, which
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