In Situ IR Spectrum of 12-Tungstophosphoric Acid Hexahydrate with

IR spectrum of HPW‚6.2H2O showed a broad continuum below. 3700 cm. -1 with two strong bands at 3250 and 1710 cm. -1 and weak bands in the range ...
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12368

J. Phys. Chem. B 2004, 108, 12368-12374

In Situ IR Spectrum of 12-Tungstophosphoric Acid Hexahydrate with Planar H5O2+ Masaki Hashimoto, Gaku Koyano, and Noritaka Mizuno* Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: April 1, 2004; In Final Form: June 12, 2004

The in situ IR spectra of H3PW12O40‚nH2O (n ) 0-6.2) in the range 4000-1100 cm-1 were measured, and that of H3PW12O40‚6H2O (i.e., [H5O2+]3[PW12O403-]) with planar H5O2+ was investigated. The spectrum showed a broad continuum below 3700 cm-1, with peaks at 3420, 2720, 2030, and 1640 cm-1, and was deconvoluted into five bands at 3420, 2720, 2030, 1780, and 1640 cm-1. These bands, except for 2030 cm-1, were shifted by the deuteration and are assigned to the vibration modes of the planar structure. The vibration modes of the simplified model H5O2+‚4OW(OH)4(OH2) were calculated by the quantum chemical method with the B3LYP/ 6-31+G* level: The 3420 cm-1 band is assigned to the overlap of the ν(OwHw) of the ν7 mode with the A component of Fermi resonance among ν8 (ν(OwHw)), ν3 (δ(OwHw2)), and ν11 (γ(OwHw2)) with the same species, of which the first mode is broadened by the anharmonic coupling of OwsHw and OasHw vibrations. The bands at 2720 and 1780 cm-1 are probably assigned to separated BC components of the Fermi resonance. The 2030 cm-1 band is assigned to the overlap of overtone and combination vibrations of the skeletal vibration modes of νas(PsO), νas(WdO), and νas(WsOcsW) in the same way to that of Cs3PW12O40‚0H2O: Cs3PW12O40‚ 0H2O showed three bands at 2159, 2074, and 1988 cm-1, and the band positions were not changed by the deuteration and agreed with those of overtone and combination vibrations of skeletal vibration modes of νas(PsO), νas(WdO), and νas(WsOcsW). The 1640 cm-1 band is assigned to the HwsOwsHw bending vibration mode.

Introduction Protonated water molecules such as H3O+, H5O2+, and H7O3+ have attracted much attention as models of hydrogen bonding because they are small molecules containing only hydrogen and oxygen, and the characterization with the vibration spectroscopy or quantum chemical method is feasible.1 The ν(OH) bands in IR spectra of water molecules interacting with acidic proton of liquid acids,2-7 frozen liquid acids,8-10 solid heteropoly acids,11-15 zeolites,16-20 and the gas phase21 were broad and intense, and shifted to lower frequency. Therefore, the spectra were very complicated. The broadening due to the hydrogen bonds has been explained by the anharmonic coupling22 for the weak and medium hydrogen bonds, and by homoconjugate23 for the strong hydrogen bonds. IR spectra of H5O2+ with nonplanar structures have extensively been studied.1,3,24-30 Nonplanar H5O2+ has the strong hydrogen bond (i.e., very short O‚‚‚H+‚‚‚O hydrogen bond) and therefore shows the broad IR bands. Kramer, Kollman, and Brzezinski investigated the broadening of IR bands for nonplanar H5O2+ on the basis of the proton potentials, whereas Bene and Sauer et al. studied nonplanar H5O2+ with ab initio calculation.3,25-30 Therefore, little is known of the nature of a hydrogen bond in the planar H5O2+. Heteropolyacids with the R-Keggin structure such as H3PW12O40 and H4SiW12O40 are strong acids and have been reported to be active for various kinds of acid-catalyzed reactions in both the liquid and solid states.11-15,31-43 A remarkable property of heteropolyacids as solid catalysts is the formation of pseudoliquid phase, which is caused by the absorption of polar molecules such as water and alcohols followed by the

interaction with protons and heteropolyanions. A key structure displaying the interaction is a planar

(R ) H, CH3, C2H5, etc.).11,12,31-33,35-39,43 To understand the nature of the interaction, detailed studies are necessary. It is revealed by the neutron diffraction that H3PW12O40‚6H2O has the planar H5O2+ structure shown in Figure 1.36 The IR spectrum of H3PW12O40 in the skeletal vibration region was extensively investigated because of the intense bands of the Keggin structure.38 On the other hand, less has been investigated for the vibrational modes of H5O2+ in H3PW12O40‚ 6H2O.12-15,32-37,39 One of the present authors has also pointed out the broad continuum for H3PW12O40‚6H2O in the range 4000-1100 cm-1, as shown by the solid line in Figure 2b.13 However, no quantitative investigation has yet been reported so far including our work for the IR spectrum of planar H5O2+. In the present study, we attempt to investigate the in situ IR spectrum of H3PW12O40‚6H2O with planar H5O2+ by using the quantum chemical calculation with the B3LYP/6-31+G* level of theory according to ref 27. Experimental Section Reagents and Sample Preparation. 12-Tungstophosphoric acid (H3PW12O40‚nH2O) was commercially obtained from Nippon Inorganic Color and Chemical Co., Ltd. and used after the purification by the ether extraction and recrystallization from

10.1021/jp0485744 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/24/2004

IR Spectrum of 12-Tungstophosphoric Acid Hexahydrate

J. Phys. Chem. B, Vol. 108, No. 33, 2004 12369

Figure 1. Structure of H5O2+ in H3PW12O40‚6H2O. Red, blue, white, and orange spheres showed oxygen, tungsten, hydrogen, and phosphorus atoms, respectively. Oa, terminal oxygen of PW12O403- heteropolyanion; Ow, oxygen of water molecules; Ha+, proton of H3PW12O40; Hw, hydrogen of water molecules. (a) Structure of H5O2+ in H3PW12O40‚6H2O. (b) View of H5O2+ oriented vertically in (a).

water. The n value of the recrystallized sample was ca. 23. Cs2CO3 (Kanto Chemical Co., Ltd.) and D2O (Aldrich, >99% deuterium) were used without the further purification. The stoichiometric cesium salt (Cs3PW12O40‚4.8H2O) was prepared by slow addition of aqueous solution of Cs2CO3 to that of H3PW12O40‚23H2O, and the suspention was subsequently evaporated to dryness. Measurements of IR Spectra. The H3PW12O40‚23H2O sample (10-40 mg) was dissolved in water and the solution was spread on a Si plate. The sample thus prepared was abbreviated as HPW‚nH2O. Because Cs3PW12O40‚4.8H2O was insoluble in water at room temperature, the Cs3PW12O40‚4.8H2O sample (10-40 mg) was dispersed in water and the suspention was spread on a Si plate in the same way to that of 12tungstophosphoric acid. These samples were dried in air at room temperature and placed into the IR cell. The IR spectra were measured at 298 K with a JASCO FT-IR 550 spectrometer. Prior to the measurements, HPW‚nH2O was evacuated at elevated temperatures in an IR cell for certain periods to control the number (n) of water molecules in HPW‚nH2O.

Deuteration of HPW‚nH2O was performed as follows: After HPW‚nH2O had been evacuated at 523 K, it was exposed to D2O vapor (ca. 100 Pa) and reevacuated at 343 K. This procedure was repeated several times. IR spectra were deconvoluted with Gaussian and Lorentzian mixed function. The mixing ratios were kept constant for the bands assigned to the same vibration modes. Determination of Values of n in HPW‚nH2O. The n values were carefully estimated in repeated experiments by using a microbalance (Seiko Instruments Industry, SSC/5200) in a N2 flow (150 mL min-1). The TG data showed no weight loss above 723 K. The unchange was in good agreement with the report that H+ in H3PW12O40 reacts with the PW12O403- to form H2O and PW12O38.5 at 673-723 K.39 Therefore, the weight after the treatment at 723 K was taken as that of PW12O38.5 (n ) -1.5), and the n value was calculated on the basis of the weight. The n value was almost 6.0 at 298-343 K and reached precisely to 6.0 after the treatment at 343 K for 1 h. Therefore, the IR spectrum of HPW‚6H2O was measured after the evacuation at 343 K for 1 h.37,42 The n value decreased to 0.5, 0.3, and 0 at

12370 J. Phys. Chem. B, Vol. 108, No. 33, 2004

Hashimoto et al.

Figure 2. IR spectra of (a) HPW‚6.2H2O, (b) HPW‚6.0H2O, (c) HPW‚ 4.8H2O, (d) HPW‚0.5H2O, (e) HPW‚0.1H2O, and (f) HPW‚0H2O.

higher temperatures of 373, 423, and 573 K, respectively. These values were in good agreement with those obtained in the vacuum system. In addition, the ν(WdO) band at 975 cm-1 observed for HPW‚6H2O was shifted to 1002 cm-1 by the evacuation at 573 K for 1 h. According to Lee et al.,37 this shift corresponds to the change in the water content from n ) 6 to 0 and these n values were in good agreement with the TG/ DTA data. These agreements show the validity of the treatment to prepare HPW‚6H2O sample in the in situ IR cell. The IR sample of D3PW12O40‚6D2O (abbreviated as DPW‚6D2O) was obtained in the same way as that of HPW‚6H2O. Cs3PW12O40‚4.8H2O was completely dehydrated by the treatment in N2 at 423 K for 1 h. Therefore, the IR sample of Cs3PW12O40‚0H2O was obtained by the evacuation at 423 K for 1 h. Quantum Calculation. The ab initio calculation of vibration modes and the frequencies for planar and trans H5O2+ was carried out with an aid of computer programs Gaussian 03.44 The calculation of the whole system of H5O2+‚4PW12O403could not be done because of a large number of atoms. Therefore, the PW12O403- heteropolyanions were simplified by the replacement with Oa′W(OH)4(OH2). The calculated Mulliken charge of Oa′ in the simplified model (H5O2+‚ 4Oa′W(OH)4(OH2), Figure 3) at the B3LYP level with LanL2DZ (W) and 6-31+G* basis sets was -0.81 in agreement with the charge (-0.76 to -0.83) of Oa in H3PW12O40‚6H2O.45 This shows the validity of usage of H5O2+‚4Oa′W(OH)4(OH2) as a model for the following calculations. The vibrational frequencies were computed by determining the Hessian matrix, which held the second partial derivatives of the potential with respect to displacement of the atoms in Cartesian coordinates and then transforming to mass-weighted coordinates. The geometry of calculated models were optimized. The calculation of vibration frequencies of trans H5O2+ (Figure

Figure 3. Structure of optimized simplified model of H5O2+‚ 4Oa′W(OH)4(OH2) for H3PW12O40‚6Hw2Ow at the B3LYP level with LanL2DZ (W) and 6-31+G* basis sets. Red, blue, and white spheres were the same as those in Figure 1. The structural parameters for OwOa distance (2.67 Å) and Ow-Ow distance (2.37 Å) in H5O2+‚4Oa′ were fixed as same as those for H5O2+‚4Oa in Figure 1. (a) Optimized structure of H5O2+ in H5O2+‚4Oa′W(OH)4(OH2). (b) View of H5O2+ oriented vertically in (a).

S1) in HClO4‚2H2O was carried out at the B3LYP level with 6-31+G* basis sets to confirm the applicability of B3LYP theory to H5O2+ systems. The results with the scaling factor of 0.96 showed IR bands at 3300, 3200, 1830, 1750, 1110, and 1020 cm-1, and these values showed fairly good agreement with the experimental data (3300, 3200, 1900-1700, 1080, and 1040 cm-1, respectively) of HClO4‚2H2O in ref 10. Therefore, the ab initio calculation of vibration modes and the frequencies for planar and trans-H5O2+ were carried out with the aid of computer programs Gaussian 03 at the B3LYP level with 6-31+G* basis sets. The same scaling factor of 0.96 was used in both cases. The potential energy surfaces of trans and planar H5O2+ were determined with changes in bond lengths of Ow-Ha+ and OwHw at the intervals of 0.001 Å, freezing the optimized geometries in the ground states. Results and Discussion IR Spectra of HPW‚nH2O (0 e n e 6.2). Figure 2 shows changes in IR spectra of HPW‚nH2O with n. The same spectra were observed in the repeated experiments46 and in ref 13. The

IR Spectrum of 12-Tungstophosphoric Acid Hexahydrate

J. Phys. Chem. B, Vol. 108, No. 33, 2004 12371 TABLE 1: IR Bands Obtained by Deconvolution of Spectra for HPW‚6H2O and DPW‚6D2O in the Range 4000-1100 cm-1 HPW‚6H2O DPW‚6D2O D/Ha 3420

2520

2720

1990

2030

2030

1780

1300

1640

1200

assignmentb

0.737 OwHw stretching vibration mode (ν7) and A component of Fermi resonance (ν8 + 2ν3 + 2ν11) 0.732 B component of Fermi resonance (ν8 + 2ν3 + 2ν11) 1.0 Overtones of νas(P-O) and νas(WdO), and combination of νas(P-O) and νas(WdO) or νas(W-Oc-W) 0.730 C component of Fermi resonance (ν8 + 2ν3 + 2ν11) 0.732 ν3

a

Figure 4. IR spectra of (a) HPW‚6H2O and (b) DPW‚6D2O. The dotted lines showed the deconvoluted bands. The observed spectra were well reproduced by the sum shown by the dashed lines and could not be reproduced by using fewer bands.

IR spectrum of HPW‚6.2H2O showed a broad continuum below 3700 cm-1 with two strong bands at 3250 and 1710 cm-1 and weak bands in the range 2120-2006 cm-1. HPW‚6H2O showed a broad continuum below 3700 cm-1 with bands at 3420, 2720, 2030, and 1640 cm-1, as shown in Figure 2b. The bands around 3420, 2720, 2030, and 1640 cm-1 became sharper and stronger when HPW‚6H2O was cooled to 100 K. The spectrum in Figure 2b is different from that for HClO4‚2H2O with trans H5O2+, which showed bands at 3300-3200, 1900-1700, and 10801040 cm-1.10 When n was decreased to 4.8, the intensities of bands at 3420, 2720, and 1640 cm-1 decreased with an increase in the intensity of the band around 3250 cm-1. According to ref 15, the 3250 cm-1 band ranging from 3500 to 2750 cm-1 is assigned to the OH vibration mode for dehydrated H3PW12O40. This fact supports the assignment. The IR spectrum of HPW‚0.5H2O showed bands at 3250 and 2120-2006 cm-1. A further decrease in n from 0.5 to 0 did not much change the IR spectrum. The 3420, 2720, and 1640 cm-1 bands observed for HPW‚6H2O were hardly observed for HPW‚nH2O (n < 1, Figure 2d-f). This fact is consistent with the report that H5O2+ was hardly observed for HPW‚nH2O (n < 1) by 31P MAS NMR.47 The weak band around 1400 cm-1 in Figure 2a-f may be assigned to the combination of the 796 and 600 cm-1 bands of the PW12O403- heteropolyanion. The IR bands of skeletal vibrations for HPW‚nH2O changed as follows: Four bands at 1079 (νas(PsO)), 975 (νas(WdO)), 886 (νas(WsOcsW); Oc, corner-sharing oxide ion), and 796 cm-1 (νas(WsOesW); Oe, edge-sharing oxide ion) were observed for HPW‚23H2O. The νas(PsO) and νas(WdO) bands splitted at n < 6 as has been reported in refs 15, 32, and 37. IR Spectra of HPW‚6H2O and DPW‚6D2O. First, the deconvolution of IR spectrum of HPW‚6H2O (Figure 2b) was attempted with four bands at 3420, 2720, 2030, and 1640 cm-1. The spectrum in Figure 2b could not be reproduced with four bands, but with five bands. The best fit was obtained with bands centered at 3420, 2720, 2030, 1780, and 1640 cm-1, as shown in Figure 4a, where the deconvoluted bands were shown by the dotted lines and the sum was shown by the broken line. The broken line well reproduced the observed spectrum (solid line). The IR spectrum of DPW‚6D2O is shown by the solid line in Figure 4b. The spectrum could also be deconvoluted with five bands at 2520, 2030, 1990, 1300, and 1200 cm-1, as shown by the dotted lines. Table 1 summarizes the IR bands obtained

The ratios of wavenumbers of the bands for DPW‚6D2O to those for HPW‚6H2O. b νn(n ) 1-15) are the vibrational modes calculated for H5O2+ in HPW‚6H2O shown in Figure 8.

Figure 5. IR spectrum of Cs3PW12O40‚0H2O in the range 4000-1100 cm-1. The expanded spectrum in the range 2200-1900 cm-1 is shown in the inset.

by the deconvolution of the spectra of HPW‚6H2O and DPW‚ 6D2O. The bands at 3420, 2720, 1780, and 1640 cm-1 for HPW‚ 6H2O were shifted to 2520, 1990, 1300, and 1200 cm-1, respectively, by the deuteration, and reasonable isotope shifts were observed for these bands. The 3420, 2720, and 1640 cm-1 bands were not observed for dehydrated Cs3PW12O40 without protons, as shown in Figure 5. These facts show that the bands at 3420, 2720, 1780, and 1640 cm-1 come from the planar H5O2+ in Figure 1.17 The 2030 cm-1 band was observed for both HPW‚6H2O and DPW‚6D2O and was not shifted by the deuteration. Energy Surface of H5O2+. The energy surface of H5O2+ was calculated as a function of the difference (X) of Ha+ from the center of Ow-Ha+-Ow hydrogen bond with a model in Figure 3. It was reported that the energy surface of nonplanar trans H5O2+ with the Ow-Ha+-Ow distance of 2.50 Å has the double minimum on the basis of SCF-MO-LCGO calculation.23 The same results were also obtained for trans form H5O2+ with our method. A double minimum potential similar to that of transH5O2+ was observed for the planar H5O2+ at the Ow-Ha+-Ow distance of 2.50 Å, and the energy barrier of double minimum became lower with decrease in the Ow-Ha+-Ow distance (Figure 6), as has been reported for nonplanar form H5O2+.19,20,36 The energy barrier was hardly observed for the planar H5O2+ with the 2.37 Å Ow-Ha+-Ow distance, and one broad potential was observed (Figure 6). The energy surface of H5O2+‚4Oa′W(OH)4(OH2) as a function of the difference (X) of Hw from the center of the Ow-Hw-Oa′ hydrogen bond is shown in Figure 7. The sharper and lower energy surface was observed for the Ow-Hw hydrogen bond in comparison with that of the Hw-Oa′ hydrogen bond. It follows

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Hashimoto et al. TABLE 2: Vibrational Frequencies Calculated for H5O2+ in H5O2+‚4Oa′W(OH)4(OH2) and D5O2+ in D5O2+‚4Oa′W(OH)4(OH2) H5O2+‚ 4Oa′W(OH)4(OH2) b

3349 (0.00) 3343 (0.00) 1611 (0.38) 820 (0.02) 721 (0.14) 662 (0.04) 3386 (1.00) 3289 (0.85) 1779 (0.01) 1705 (0.02) 953 (0.19) 852 (0.18) 700 (0.13) 541 (0.08) 535 (0.02) Figure 6. Energy surface of H5O2+‚4Oa′W(OH)4(OH2). X, Hw displacement from the center of the Ow-Ha+-Ow hydrogen bond. (a) Trans H5O2+ with the Ow-Ow distance of 2.50 Å. (b) Planar H5O2+ in H5O2+‚ 4Oa′W(OH)4(OH2) with the Ow-Ow distance of 2.50 Å. (c) Planar H5O2+ in H5O2+‚4Oa′W(OH)4(OH2) with the Ow-Ow distance of 2.37 Å.

Figure 7. Energy surface of H5O2+‚4Oa′W(OH)4(OH2). X, Hw displacement from the center of the Ow-Hw-Oa′ hydrogen bond.

that the whole potential surface has the asymmetric single minimum and is broad in comparison with those for nonperturbed hydroxyl groups so far reported.49 Therefore, the anharmonic coupling of Ow-Hw and Oa′-Hw vibrations causes the broadness of the ν(OwHw) band. Assignments of 3420, 2720, 1780, and 1640 cm-1 Bands. The calculation of vibration frequencies of H5O2+‚4Oa′W(OH)4(OH2) were carried out, and the fifteen vibrational modes were related to H5O2+ as summarized in Figure 8. The frequencies for the calculated IR bands are summarized in Table 2. The ν3-ν15 modes were IR active whereas those of ν1 and ν2 were IR inactive. ν7 and ν8 are IR-active Ow-Hw stretching modes.50 ν3 is a Hw-Ow-Hw bending mode, and ν9 is a out-of-plane Hw-Ow-Hw bending mode. The ν10 mode has mixed characters of Ow-Ha+ stretching and Hw-Ow-Hw bending modes. The calculated intensities of the ν9 and ν10 modes were much weaker than those of the other IR active modes. Therefore, it is probable that the bands of ν9 and ν10 modes are probably overlapped with the intense ν3 band and could not be detected by the deconvolution, as shown in Figure 4a. The bands of ν4-ν6 and ν11-ν13 modes in the range of 1100-600 cm-1 could not be

D5O2+‚ 4Oa′W(OH)4(OH2)

D/Ha

mode

2444 2374 1160 599 534 503 2540 2368 1299 1211 686 622 525 384 396

0.73 0.71 0.72 0.73 0.74 0.76 0.75 0.72 0.73 0.71 0.72 0.73 0.75 0.71 0.74

ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 ν13 ν14 ν15

a The ratios of wavenumbers of the bands calculated for D5O2+‚ 4Oa′W(OH)4(OH2) to those calculated for H5O2+‚4Oa′W(OH)4(OH2). b Relative intensities with respect to the most intense band were given in parentheses.

successfully investigated because of the overlap of intense bands of skeletal vibrations for the PW12O403- heteropolyanion. The ν7 mode was calculated to appear at 3386 cm-1 and close to 3420 cm-1 in Figures 2b and 4a. Therefore, the ν7 mode much contributes to the 3420 cm-1 band. The ν3 mode was calculated to appear at 1611 cm-1 and was close to 1640 cm-1 in Figures 2b and 4a. Therefore, the 1640 cm-1 band assigned to the ν3 mode. The calculation results were not able to explain the broad intense bands at 2720 and 1780 cm-1 in Figure 4a. The bands can be explained by the Fermi resonances caused by the two very close vibration levels of proper symmetry to generate three bands A, B, and C, whose relative intensity changes from IA ≈ IB > IC to IA < IB ≈ IC for the medium-strong hydrogen bond: 4,17,51 The ν mode (ν(O H )) is broadened by the anharmonic 8 w w coupling of Ow-Hw and Oa-Hw vibrations as described, and the Fermi resonance among the modes of ν8 (ν(OwHw), 3289 cm-1), ν3 (δ(OwHw2), 1611 cm-1), and ν11 (γ(OwHw2), 953 cm-1) with the same species52 generates three bands of A (>3289), B (2720), and C (1780) with the intensity of IA < IC < IB. The position of the minimum intensity between bands B and C was around 2000 cm-1, in approximate agreement with that of 2ν11 (1906 cm-1), supporting the presence of Fermi resonance. The A band probably overlaps with 3420 cm-1 band. The IR spectrum of protonated water in the acidic solution has been explained by the Fermi resonance among the ν(OwHw), δ(OwHw2), and γ(OwHw2) modes in the same way.17 Assignment of the Bands in the Range 2240-1820 cm-1 for HPW‚6H2O. The IR spectrum of Cs3PW12O40‚0H2O in Figure 5 showed three weak bands at 2159, 2074, and 1988 cm-1 in the region of 2200-1920 cm-1, and these bands were not shifted by the deuteration. The IR bands of skeletal vibration modes of Cs3PW12O40‚0H2O appeared at 1080 (νas(PsO)), 985 (νas(WdO)), 891 (νas(WsOcsW)), and 803 cm-1 (νas(WsOes W)). The bands for overtone of νas(PsO), combination of νas(PsO) and νas(WdO), overtone of νas(WdO), and combination of νas(PsO) and νas(WsOcsW) were calculated to appear at 2160, 2065, 1971, and 1970 cm-1, respectively. These respective values were in good agreement with 2159, 2074, 1988, and 1988 cm-1 observed for Cs3PW12O40‚0H2O. Therefore, the bands at 2159, 2074, and 1988 cm-1 for Cs3PW12O40‚

IR Spectrum of 12-Tungstophosphoric Acid Hexahydrate

J. Phys. Chem. B, Vol. 108, No. 33, 2004 12373

Figure 8. Calculated vibrational modes for planar H5O2+ in H5O2+‚4Oa′W(OH)4(OH2): (b) Ow; (O) Hw; (*) Ha+.

0H2O can be explained by the overtones and combinations of skeletal vibration of the PW12O403- heteropolyanion. The bands for HPW‚6H2O in the range 2240-1820 cm-1 were not shifted by the deuteration. These bands were still observed after n in HPW‚nH2O reached zero, as shown in Figure 2f. These facts show that the bands cannot be assigned to vibration modes of planar H5O2+. Therefore, the bands in the range 2240-1820 cm-1 for HPW‚6H2O can be assigned in the same way to that for Cs3PW12O40‚0H2O. Because νas(PsO), νas(WdO), and νas(WsOcsW) bands for HPW‚6H2O were observed at 1079, 975, and 886 cm-1, respectively, two overtone vibrations and one combination of νas(PsO) and νas(WdO), and one combination of νas(PsO) and νas(WsOcsW) must be observed in the range 2240-1820 cm-1; 2158 (2νas(PsO)), 2054 (νas(PsO) + νas(WdO)), 1950 (2νas(WsO)), and 1965 cm-1 (νas(PsO) + νas(WsOcsW)). Therefore, the four bands exist in the range 2240-1820 cm-1. Because these bands are broad in comparison with those for Cs3PW12O40‚0H2O and the positions are close, they overlap with one another to form one broad band in the range 2240-1820 cm-1. In conclusion, the broad continuum below 3700 cm-1 for H3PW12O40‚6H2O with planar H5O2+ was deconvoluted into five bands at 3420, 2720, 2030, 1780, and 1640 cm-1. The bands are assigned on the basis of the results of the isotope shifts and vibration modes of the simplified model of H5O2+‚4OW(OH)4(OH2) with the quantum chemical method.

Acknowledgment. We acknowledge financial supports from Ministry of Education, Culture, Sports, Science, and Technology of Japan and the Core Research for Evolutional Science and Technology (CREST) program of Japan Science and Technology Agency (JST). We thank Messers. T. Saito and Y. Ogasawara for the help of IR experiments and Profs. S. Hikichi and Emeritus M. Misono for discussions. Supporting Information Available: Structure of transH5O2+. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48. (2) Nash, K. L.; Sully, K. J.; Horn, A. B. Phys. Chem. Chem. Phys. 2000, 2, 4933. (3) Brzezinski, B.; Rozalski, B.; Schro¨der, G.; Bartl, F.; Zundel, G. J. Chem. Soc., Faraday Trans. 1998, 94, 2093. (4) Stoyanov, E. S. J. Chem. Soc., Faraday Trans. 1997, 93, 4165. (5) Leuchs, M.; Zundel, G. J. Chem. Soc., Faraday Trans. 2 1978, 74, 2256. (6) Scho¨berg, D.; Zundel, G. Z. Phys. Chem. 1976, 102S, 169. (7) Blinc, R.; Hadzi, D. Sperctrochim. Acta 1960, 16, 852. (8) Green, M. E. J. Phys. Chem. A 2002, 106, 11221. (9) Gilbert, A. S.; Sheppard, N. J. Chem. Soc., Faraday Trans. 2 1973, 69, 1628. (10) Pavia, A. C.; Giguere, P. A. J. Chem. Phys. 1970, 52, 3551. (11) Misono, M. Chem. Commun. 2001, 1141. Hill, C. L., Ed. Polyoxometalate. Chem. ReV. 1998, 98, 1. Okuhara, T.; Mizuno, N.; Misono,

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