Structure and Properties of Acidic Protons in Anhydrous

Department of Chemistry, Graduate School of Science, Osaka UniVersity, ... 1H broad line NMR spectrum showed a bell-type resonance line shape with 4.7...
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J. Phys. Chem. B 2001, 105, 5391-5396

5391

Structure and Properties of Acidic Protons in Anhydrous Dodecatungstophosphoric Acid, H3PW12O40, As Studied by Solid-State 1H, 2H NMR, and 1H-31P Sedor NMR Takahiro Ueda,* Tomomasa Tatsumi, Taro Eguchi, and Nobuo Nakamura Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan ReceiVed: September 23, 2000; In Final Form: February 12, 2001

The binding position and the electronic properties of acidic proton in anhydrous dodecatungstophosphoric acid, H3PW12O40, were studied by means of 1H, 2H NMR, and 1H-31P spin-echo double resonance (SEDOR) NMR techniques. 1H broad line NMR spectrum showed a bell-type resonance line shape with 4.7 kHz line width and MAS NMR spectrum has a single resonance peak, suggesting that the hetero-polyanion PW12O403provides a single site to the acidic proton. 2H NMR spectrum leads to the quadrupole coupling constant (QCC) of 210 kHz and the asymmetry parameter of the electric-field-gradient tensor (η) of 0.15. The value of QCC suggests that the deuterons are bonded to oxygen atoms in the hetero-polyanion very rigidly, and that any motion of the hetero-polyanion does not take place. A remarkable decay of the 1H-31P SEDOR signal was observed, due to the dipolar interaction between the central 31P and three protons strongly bonded to the hetero-polyanion. The P-H distance was evaluated by simulating the decay behavior of the echo to be 0.50 ( 0.02 nm. Comparing this P-H distance with model structures in which protons are bonded to cornershared, edge-shared, or terminal oxygen atom of WO6 unit in PW12O403-, the edge-shared oxygen atom seems to be the most probable binding position for the acidic proton.

Introduction Dodecatungstophosphoric acid, H3PW12O40‚nH2O, has been known to be a kind of famous heterogeneous solid catalyst with high catalytic activity in some organic reactions.1 For instance, H3PW12O40‚nH2O (hereafter denoted by DWPA) oxidizes methacrolein to methacrylic acid.2 The reaction is accelerated with coexistence of DPWA, indicating the high catalytic activity of DWPA. In the catalytic process, DWPA acts as the acidic center or point for oxidization. Furthermore, the dry DWPA without water of hydration shows superacidity, the strength of which measured by Hammett indicator is smaller than pKa ) -13.16.3 The high catalytic activity of DWPA may originate from this strong acidity. To clarify the origin of catalytic activity of DWPA, it is important to identify the acidic center in this compound. The DWPA consists of the polycation (H5O2+, H59O29+, etc., which consists of a proton and a number of the water of crystallization) and the characteristic hetero-polyanion, PW12O403-, which has the Keggin structure as shown in Figure 1.4,5 The hetero-polyanion consists of a PO4 tetrahedron and 12 WO6 octahedrons. Tetrahedral PO4 unit locates at the center of the hetero-polyanion: Each oxygen in PO4 bonds to three tungsten atoms. Each one among 12W bonded to PO4 forms WO6 units which are linked together by sharing the oxygen atoms, resulting in a huge and highly symmetric structure with the Td point symmetry. In the hetero-polyanion, there are three chemically inequivalent oxygen atoms, i.e., a terminal oxygen atom (Oterminal) and two bridging oxygen atoms. One of the bridging oxygens, the edge-shared oxygen (Oedge-shared), links two W atoms which are bonded to the same oxygen atom of PO4 unit in the manner to share the edges of different WO6 octahedrons. Another oxygen, corner-shared oxygen (Ocorner-shared), links two * Author to whom correspondence should be addressed. Phone: +816-6850-5779. Fax: +81-6-6850-5785. E-mail: [email protected].

Figure 1. Schematic representation of Keggin structure of PW12O403in H3PW12O40 crystal. Three available sites to the acidic protons, i.e., associated with terminal(Oterminal), corner-shared(Ocorner-shared), and edgeshared (Oedge-shared) oxygen atoms.

W atoms bonded to the different oxygens of PO4 unit in the manner to share the corners of different WO6 octahedrons. The hetero-polyanion and the polycation are arranged to form a large unit cell, the structure of which depends on the amount of the crystallization water, in other words, the shape and the size of the polycation. The crystal structures of DWPA with varying number of water of hydration, n, have been studied in detail by a number of researchers:5-9 n ) 29 and 6 hydrates form the cubic unit cell, whereas n ) 13 hydrate forms triclinic one.9 It have been reported that anhydrous DWPA crystallizes in a tetragonal unit cell,9 but the atomic positions have not been

10.1021/jp003439m CCC: $20.00 © 2001 American Chemical Society Published on Web 05/19/2001

5392 J. Phys. Chem. B, Vol. 105, No. 23, 2001 determined. The refinement of the crystal structure of anhydrous DWPA is much difficult, because its single crystal cannot be grown. In the anhydrous DWPA the “bare” protons may work as the catalytic center because of its strong acidity. Such strong acidity may be brought about by a specific structure of the hetero-polyanion in which three acidic protons are bonded to a special one among three different oxygens. It is important to establish the structure of the anion by studying the position and the nature of the acidic protons in the anion. The proton positions in the anhydrous DWPA have been studied by IR10 and 17O high-resolution solid-state NMR.11 Lee et al.10 studied the isotope effect on WdOterminal, W-Oedge-sharedW, and W-Ocorner-shared-W stretching bands in H3PW12O40 and D3PW12O40. The stretching band of WdOterminal appears at 1007 cm-1 in both compounds and the bands of W-Ocorner-sharedW at 899 cm-1 in hydride and 903 cm-1 in deuteride, whereas the bands of W-Oedge-shared-W are at 798 and 768 cm-1 in the hydrate and 814 and 744 cm-1 in the deuteride. On the basis of the fact that the largest frequency shift due to the deuteration is observed in W-Oedge-shared-W stretching band. Lee et al. concluded that the acidic protons are bonded to the edge-shared bridging oxygen atoms in PW12O403-. In contrast, 17O highresolution solid-state NMR spectrum of H3W1217O40 measured by Kozhevnikov et al.11 showed that the resonance of Oterminal shifts about -60 ppm from the peak position in the solution spectrum, whereas each of the bridging oxygen atoms (Oedge-shared and Ocorner-shared) gives the resonance peak at similar positions in solid and solution. Hence, they concluded that the terminal oxygen (WdO) atom is the most probable site as the proton acceptor in PW12O403-. Thus, these experimental studies postulate that there are two possible sites for protonation, i.e., Oterminal and Oedge-shared. A molecular orbital calculation using XR method were carried out on PMo12O403- which has structure similar to that of PW12O403-.12 According to the MO calculations, the electron density is the highest on the edge-shared bridging oxygen atom (In other words, Oedge-shared is the most basic.), suggesting that the Oedge-shared is the most probable site for protonation. NMR distance geometry can measure the interatomic distances very precisely, and so can be a suitable analytical tool for the study of the protonation site in the anhydrous DWPA. As the typical techniques to measure the interatomic distances by the use of heteronuclear dipolar interaction, double resonance methods such as spin-echo double resonance (SEDOR),13-15 rotational-echo double resonance (REDOR),15-21 and transferredecho double resonance (TEDOR)22,23 have been developed. The SEDOR method was developed from Hahn’s spin-echo experiment.24 It is the simplest and most basic method to measure the heteronuclear dipolar interaction, and its reliability in the internuclear distance thus determined is high. Especially, the SEDOR method have been applied to inorganic materials such as zeolite,25,26 amorphous silicon,27,28 and adsorbed molecules on the metal surface,14,29 in order to examine the atomic positions and local structures around the resonance nuclei of interest. For example, the Al-H distance in zeolite ZSM-5 was determined by 1H-27Al SEDOR technique to be 0.243 ( 0.003 nm.26 In P-doped a-Si:H two different P-H distances were distinguished and determined to be 0.26 and 0.41 nm.27 We now expect that 1H-31P SEDOR NMR experiment can be applied to anhydrous DWPA for the determination of the P-H distance and hence the location of three protons on PW12O403- hetero-polyanion. In the present work, we observe 1H and 2H broad line NMR spectra and 1H magic angle sample spinning (MAS) NMR

Ueda et al. spectrum to study the local structure around the protons. 1H31P SEDOR NMR measurement is carried out in the partially deuterated anhydrous DWPA to determine the P-H distance. On the basis of P-H distance determined, we establish the structure of hetero-polyanion including the hydrogen positions. Experimental Section Sample Preparation. H3PW12O40‚nH2O (n∼29) was of commercial source (Wako Pure Chemical Industries, Ltd., grade E. R.). The water content was determined by thermogravimetry (Seiko TG-DTA200). The compound was used for the preparation of the anhydrous and/or the deuterated DWPA without further purification. Anhydrous DWPA was obtained by evacuation of sample at 623 and 488 K for 6 h. Each sample was sealed into glass ampules with dimension 5 mm φ × 7 mm. The deuterated DWPA was prepared by repeating three times the recrystallization from D2O solution. The deuterated anhydrous DWPA was prepared by the evacuation of the deuterated sample at 488 K for 6 h, after repeating the absorption and evacuation of D2O vapor alternatively. The specimen partially substituted by 2H was prepared by recrystallization from the mixture of H2O/D2O with appropriate ratio. The H/D ratio in each sample was determined to within (2% by measuring the integrated intensity of 1H resonance by referring to 31P resonance intensity. NMR Measurements. The measurements of NMR spectra were carried out using a Bruker DSX200 spectrometer operating at 200.13 MHz for 1H, 81.01 MHz for 31P, and 30.72 MHz for 2H. 1H NMR spectrum was measured by a single pulse sequence with the 90° pulse length of 4 µs. The MAS experiment was done with a Bruker pneumatic unit with the MAS rate of 1.1 kHz. 2H NMR spectrum was measured using solid-echo pulse sequence with 90° pulse length of 2.4 µs and with the pulse delay of 15 µs. 1H-31P SEDOR measurements were carried out using the typical pulse sequence14,29 with the time interval between 90° and 180° pulse in 31P channel of 500 µs; pulse delay between the 90° pulse in 31P channel and 180° pulse in 1H channel was variable from 0 to 480 µs. Typical 90° pulse length of 31P was 4.5 µs. The repetition time of 1H-31P SEDOR measurement was optimized by referring to the spin-lattice relaxation time for 1H and 31P; the T1 value was 14 s for 1H and 97 s for 31P in the anhydrous DWPA. Therefore, the repetition time was set to 500 s, corresponding to 5T1 for 31P. To check the effect of the imperfection of 1H 180° pulse on the decay of echo signal, the SEDOR measurements were carried out using the pulse widths of 5 µs and 8 µs. The decay behavior of the echo was similar in both pulse widths. All SEDOR measurements described below were carried out using the 1H 180° pulse of 8 µs. The 1H, 2H, and 31P NMR, and SEDOR experiments were carried out at room temperature. The 1H broad line NMR spectrum was measured at 293 and 160 K. The chemical shift of 1H was referred to the 1H resonance of TMS, and that of 31P to 85% H3PO4 solution. Results and Discussion 1H Broad Line and MAS NMR Spectra for Anhydrous DWPA. Figure 2a shows 1H broad line NMR spectrum observed at 293 and 160 K. The powder spectrum shows a bell-like line shape, the full width at half-maximum (fwhm) of which was 4.7 kHz (ca. 1.1 G). The spectrum does not change when the sample was cooled to 160 K, implying that the positions of protons are fixed to the particular oxygen atoms in PW12O403hetero-polyanion and any motion to average out the 1H-1H dipolar interaction does not take place. The line shape corre-

Acidic Protons in Anhydrous H3PW12O40

J. Phys. Chem. B, Vol. 105, No. 23, 2001 5393

Figure 3. 2H NMR spectrum (lower) for anhydrous DWPA and the simulated spectrum (upper) with QCC ) 210 kHz and η ) 0.15.

Figure 2. 1H NMR spectrum for anhydrous DWPA observed at 293 and 160 K under static condition (a) and observed under MAS condition at room temperature (b). MAS rate, νr, is 1.1 kHz. 1H chemical shift in MAS spectrum is refered to the signal for TMS external standard.

sponds to a rigid multi-spin system containing more than 2-spins.30 In the rigid lattice, the fwhm of 4.7 kHz corresponds to the second moment of 1.6 × 108 rad2 s-2, assuming the Gaussian resonance line shape. For the three protons interacting each other with a constant internuclear distance in H3PW12O40, this second moment value leads to the internuclear distance of 0.39 nm. Figure 2b shows 1H MAS NMR spectrum of anhydrous DWPA, in which a narrow peak appears at 9.1 ppm referring to TMS. This chemical shift value δ agrees with the literature value.31 In the anhydrous DWPA, the δ value of 9.1 ppm suggests that the protons are directly bonded to the oxygen atoms in the hetero-polyanion. Furthermore, no split of the peak is observed in DWPA, suggesting that three protons are magnetically and chemically equivalent to each other. This fact suggests strongly that the protons are bonded to only a kind of the oxygen atoms among three inequivalent of oxygen atoms. 2H NMR Spectra for Anhydrous DWPA. Figure 3 shows the 2H powder spectrum observed at room temperature in the deuterated anhydrous DWPA. It consists of a typical powder pattern of 2H with the peak-to-peak separation of 157 kHz. The simulation of the powder pattern reproduces the observed spectrum as shown in Figure 3 using the quadrupole coupling constant (QCC) of 210 kHz and the asymmetry parameter (η) of the electric-field-gradient tensor of 0.15. These quadrupolar interaction parameters may be discussed on the basis of the electronic structure of the deuterium and the acidity of the hetero-polyanion. Deuterium in anhydrous DWPA is considered to form O-D group on the hetero-polyanion. Its acid strength depends on the degree of the bond polarization. We now compare the above QCC with those of the OD group in other materials. The OD group located on Al2O3 surface, which may be regarded as an isolated OD group has the QCC value of

280 kHz.32 The QCC value of the OH- ion in LiOD‚D2O crystal is 271.2 kHz at 82 K, in which the bond length of O-D is 0.094 nm.33 Typical values of QCC of 2H in water of hydration of hydrates of inorganic compounds range between 200 kHz and 220 kHz.34 It is noted that the deuterium in D2O ice Ih have the QCC values of 213.2 kHz and 216.4 kHz at 263 K,35 corresponding to long O-D distances of 0.1011 and 0.1015 nm, respectively, due to the formation of hydrogen bond network. Thus, the observed QCC value in anhydrous DWPA suggests that the O-D distance in the hetero-polyanion is longer than normal O-D distances as seen in the isolated OD group. There are two possibilities for the elongation of the O-D distance; one is the formation of a strong hydrogen bond with an oxygen in the neighboring hetero-polyanions, and the other is the increase in the ionic character of O-D bond due to the large electronic density of oxygen atom. The existence of the hydrogen bond is supported by the low-frequency shift of O-H stretching band in IR spectra.36 On the other hand, the molecular orbital calculation of the electronic state of the hetero-polyanion postulates that the electron density of the “edge-shared” oxygen is large,12 suggesting that the bond polarization is likely to occur if an O-D bond is formed. In such a case the ionic character of O-D bond is increased and leads to decrease of QCC of the deuterium, even if no hydrogen bond is formed to the oxygen of the neighboring anion. 31P NMR Spectrum for Anhydrous DWPA. Figure 4 shows the 1H decoupled 31P powder NMR spectrum of anhydrous DWPA measured at room temperature. The spectrum is asymmetric due to the chemical shift anisotropy. Spectral simulation reproduced the observed spectrum as shown in Figure 4, with the chemical shift parameters, δ11 ) -7.06 ppm, δ22 ) -7.14 ppm, and δ33 ) -19.4 ppm. The isotropic value δiso thus determined is -11.2 ppm, which is in good agreement with -11.0 ppm for anhydrous DWPA measured by 31P MAS NMR.37 The asymmetry parameter η of the chemical shift tensor is of 0.01. The chemical shift anisotropy ∆ of -8.20 ppm is very small in comparison with that in other numerous compounds consisting of PO4 unit. For example, ∆ in Q2 phosphorus in phosphate glasses x(CaO)‚(1 - x)(P2O5) with 0.3 e x e 0.45 ranges from -209 ppm to -260 ppm and η from 0.37 to 0.44.38 Then, it suggests that the symmetry around the P atom in anhydrous DWPA is relatively high but lower than that in hexahydrate, in which the P atom locates at the special position

5394 J. Phys. Chem. B, Vol. 105, No. 23, 2001

Figure 4. 31P powder spectrum (lower) measured under the 1H decoupling for anhydrous DWPA and the simulated spectrum assuming the anisotropy of chemical shift tensor (upper).

Ueda et al.

Figure 6. The decay of 1H-31P SEDOR signal for H3PW12O40 that dehydrated at 623 K (b) and 488 K (O), and for the partially deuterated specimen, H3(1-x)D3xPW12O40, with the deuterium concentration (x) of 0.60 (4), 0.70 (3), and 0.94 (0). The solid lines are obtained as the results of least squares' fitting of eqs 1-7.

the signal in the anhydrous DWPA thus measured is caused by the dipolar dephasing due purely to the 1H-31P heteronuclear interaction. We analyzed the decay rates of the signals in the anhydrous DWPA and the partially deuterated anhydrous DWPA to evaluate the 1H-31P heteronuclear dipolar interaction in a hetero-polyanion. For a polycrystalline sample containing an isolated 1H-31P spin pair, the decay of echo signal in the SEDOR experiment depends on both the 1H-31P dipolar interaction and the time interval, t1, as13, 29 Figure 5. The t1 dependence of the intensity of the 31P echo signal for the fully protonated (a) and 60% deuterated (b) anhydrous DWPA.

with the m3m symmetry.5 Although the details of the crystal structure in the anhydrous DWPA have not yet been studied, it is known that the anhydrous DWPA forms the tetragonal lattice. The crystal structure changes on dehydration of DWPA from the cubic lattice to the tetragonal lattice.9 The asymmetry parameter of 0.01 seems to be reasonable for the tetragonal P-site in the crystal lattice. 1H-31P SEDOR for Anhydrous DWPA. Figure 5a and b show the 31P NMR spectra measured by 1H-31P SEDOR pulse sequence for the fully protonated and the 60% deuterated anhydrous DWPA. These spectra were obtained from the echo signal observed at 2τ ) 1 ms. The echo signal intensity decays with increase in the pulse interval t1 between the 90° pulse in 31P channel and 180° pulse in 1H channel. The rate of the decay depends on the concentration of the proton. In the undeuterated anhydrous DWPA (natural abundant concentration of H and D), the signal intensity is reduced drastically by about 80% as t1 increases from 0 µs to 480 µs, whereas in partially deuterated anhydrous DWPA (containing deuterium by 60%) the signal is reduced by about 50%. The strong 1H-1H homonuclear dipolar interaction contributes also to decay the intensity of the echo signal by a factor of exp(-2 τ /T2HH) when the time interval τ is changed,28 where the T2HH is 1H spinspin relaxation time. Since this interaction causes a change in the intensity of the 31P echo signal, its effect is canceled when one takes the ratio of S(t1) to S(0), i.e., the SEDOR echo signal observed at 2τ with and without the 180° irradiation of 1H, respectively. Therefore, this aspect implies that the decay of

S(t1) S(0)

)

∫0π cos(bt1) sin θ dθ

(1)

where b is the 1H-31P heteronuclear dipolar interaction as represented by

b)

γ H γ Pp r3

(3 cos2 θ - 1)

(2)

Here γH and γP are the gyromagnetic ratios of 1H and 31P, respectively, r is the distance between P and H, and θ is the angle between internuclear vector and the magnetic field. Equation 1 is applicable only to an isolated 1H-31P spin pair. In the anhydrous DWPA, three protons are distributed statistically on one kind of oxygen atoms among three inequivalent oxygen atoms (terminal, edge-shared, and corner-shared). Hence, a 31P nucleus interacts with three protons distributed over the 12 equivalent sites. In this case, the decay of the echo signal is given by27

S(t1) S(0)

)

∫0π [∏cos(bjt1)]sin θ dθ

(3)

j

where the product is taken over the 1H-31P dipolar coupling at 12 sites. On the application of eq 3 to the experimental data, the computation of the products over 12 sites is highly exhausting. Then, to simplify the analyses we assume Gaussian distribution for the 1H-31P dipolar interaction. This distribution can reasonably describe the spread of the 1H-31P dipolar coupling over 12 proton sites. Under this approximation, eq 3

Acidic Protons in Anhydrous H3PW12O40

J. Phys. Chem. B, Vol. 105, No. 23, 2001 5395

TABLE 1: P-H Distance in H3(1-x)D3xPW12O40 Determined from 1H-31P SEDOR Experiment x

rP-H/nm

0 0.60 0.70 0.94

0.51 ( 0.01 0.49 ( 0.01 0.50 ( 0.01 0.59 ( 0.10

ocorner-shared

can be simplified to27

S(t1) S(0)

≈ exp(-2∆ω2HP t21)

(4)

where the ∆ω2HP is the powder average of the second moment of the interaction between 1H and 31P nuclei. In powdered anhydrous DWPA, only a single P-H distance r may be defined in a hetero-polyanion, because three protons are equivalent. The second moment is represented by

∆ω2HP )

γ2Hγ2Pp2 5r6

NH

(5)

where NH is the number of protons interacting with the P nucleus. In anhydrous DWPA, the amount of the P-H dipolar interaction is calculated by summing over the 12 equivalent sites, assuming that each site is occupied by 3/12 protons. In fact, the 1H-31P dipolar coupling with the protons in the nearest neighboring DWPA may also contribute to the decay of the SEDOR echo. However, this contribution can be ignored, because the diameter of a hetero-polyanion is about 1 nm and the distance between the hetero-polyanions will be more than 1 nm. In the partially deuterated specimens, the resident protons are distributed statistically on the hetero-polyanion, giving rise to four different Keggin structures, H3-iDiPW12O40 (i ) 0, 1, 2, 3). The contribution of each Keggin structure, pi, depends on the degree of the deuteration, x, and is given by binomial distribution as follows:

pi ) (3i )(1 - x)3-ixi

(6)

Thus, in 60% deuterated DWPA the pi values for i ) 0, 1, 2, 3 are 0.064, 0.288, 0.432, and 0.216, respectively. Since only the 1H-31P dipole interaction contributes predominantly to the decay of the echo signal in the SEDOR experiments, the net decay of the echo signal is given by the weighted sum of the signal from each configuration as follows:

S(t1)

3

pi exp[-2∆ω2HP (i)t 21] ∑ S(0) i)0

TABLE 2: The O-O and P-O Distances in H3PW12O40 Hexahydrate Crystala

(7)

The decay of the echo signal in the D3PW12O40 can be ignored because of the small gyromagnetic ratio of 2H, and ∆ω2HP may be approximated to be zero. Using eqs 1-7 to the protonated and partially deuterated DWPA we analyzed the decay of the integral intensity of 31P resonance as shown in Figure 6, resulting in the P-H distance as listed in Table 1. The P-H distance determined by the SEDOR experiments on the sample with different x coincide with each other within the experimental error, except for the deuterated specimen by 94%, in which the experimental error is large, because of the very weak dipolar dephasing. Ignoring the value of r for the deuterated specimen by 94%, we adopt the probable value of the P-H distance of 0.50 ( 0.02 nm.

oedge-shared

oterminal

rP-O/nm

0.336

(12)

0.394

(12)

0.524

(12)

rO-O/nm

0.258 0.320 0.502 0.578 0.633

(2) (1) (4) (2) (2)

0.263 0.418 0.594 0.681 0.730

(2) (1) (4) (2) (2)

0.494 0.553 0.741 0.907 1.050

(2) (2) (2) (4) (1)

a The numbers in the parentheses indicate the number of the P-O pair and the number of the O-O pairs between crystallographically equivalent oxygens.

To clarify the binding site of the protons, the P-H distance determined above is compared with probable P-H distance in the hexahydrate crystal, which we expect to contain the same Keggin structure as in the anhydrous DWPA. The crystal of DWPA hexahydrate has the P-O distances for the cornershared, the edge-shared, and the terminal oxygens as listed in Table 2.5 Comparing these values with 0.50 nm for the P-H distance by SEDOR, the edge-shared oxygen gives the most probable O-H distance of 0.11 nm assuming that the P-O-H angle is 180°. This long O-H distance may interpret the QCC value of 2H as discussed in a previous section. If the P-O-H angle is bent, the expected O-H distance becomes longer. For example, the P-O-H angle of 120° becomes 1.6 times the O-H distance in the linear configuration. Such a long O-H distance is physically unacceptable. The other two oxygens also bring about the unacceptable O-H distances under the condition that P-H distance is 0.50 nm; for the corner-shared oxygen the shortest O-H distance should be 0.164 nm, and for the terminal one the P-O distance must be longer than the P-H distance. Assuming that the P-H distance (rP-H) is 0.11 nm, the fwhm of the 1H broad line NMR spectrum can be estimated on the basis of the crystal structure of the DWPA hexahydrate. The O-O interatomic distances (rO-O) for three kinds of the oxygens are listed in Table 2. In the case of the P-O-H angle of 180°, the H-H distances (rH-H) for the protons bonded to a kind of oxygen atom can be evaluated by the simple relation of (rP-H × rO-O)/rP-O. Using the Van Vleck formula, the second moment for the 1H-1H dipolar coupling is estimated to be 4.7 × 107 rad2 s-2 for the protons bonded to the corner-shared oxygens and to be 9.6 × 107 rad2 s-2 for ones bonded to the edge-shared oxygens. Furthermore, the second moment for the 1H-31P heteronuclear dipolar coupling is given by the SEDOR to be 3.6 × 106 rad2 s-2. Then, the fwhm of the 1H broad line NMR spectrum is given by the sum of the 1H-1H and 1H-31P dipolar interactions. The sum of the estimated second moment values for 1H-1H and 1H-31P dipolar interactions is 5.1× 107 rad2 s-2 for the corner-shared oxygens and 1.0 × 108 rad2 s-2 for the edge-shared ones, corresponding to the fwhm of 2.7 kHz and 3.8 kHz, respectively. The calculated fwhm for the protons bonded to the edge-shared oxygens is more reasonable to the experimental fwhm value of 4.7 kHz. Thus, we can conclude that the acidic protons are bonded to the edge-shared oxygen atoms. Our present result that the acidic protons are bonded to the edge-shared oxygen is consistent with the result of the IR study by Lee et al. 10 and with the results of a MO calculation, but incompatible with the conclusion given in 17O solid state high resolution NMR study, which states that the protons are bonded to the terminal oxygen atoms on the basis that the chemical shifts of the terminal oxygen in anhydrous DWPA crystal is largely different from that in solution. This large difference in

5396 J. Phys. Chem. B, Vol. 105, No. 23, 2001 chemical shift may be accounted for by an effect of formation of hydrogen bond such as Oedge-shared-H‚‚‚Oterminal. The present studies cannot give any evidence of the hydrogen bond formation, and so further experimental works other than NMR will be necessary. Conclusion 1H-31P SEDOR technique as well as 1H, 2H, and 31P NMR spectra on H3PW12O40 bring about the information on the binding site as well as the electronic state of the acidic proton. Especially, this work is a novel application of the NMR distance geometry to the study of the structure of anhydrous H3PW12O40. 1H broad line and MAS NMR spectra suggest that the protons bind to one kind of oxygen atom among three inequivalent oxygen atoms on the surface of the hetero-polyanion. 2H NMR spectrum gives rise to the quadrupole coupling constant of 210 kHz and the asymmetry parameter for the principal values of the electric-field-gradient tensor of 0.15, implying that the protons are tightly bonded to an oxygen atom and does not undergo any motion such as the migration over the heteropolyanion. 1H-31P SEDOR experiment determined the P-H distance of 0.50 ( 0.02 nm. On the basis of this P-H distance, the binding site for the acidic proton is assigned to be the edgeshared oxygen atom.

References and Notes (1) Okuhara, T.; Mizuno, N.; Misono, M. AdV. Catal. 1996, 41, 113. (2) Jasinski, R. Nature (London) 1964, 201, 1212. (3) Okuhara, T.; Nishimura, T.; Watanabe, H.; Misono, M. J. Mol. Catal. 1992, 74, 247. (4) Keggin, J. F. Nature (London) 1933, 131, 908. (5) Brown, G. M.; Noe-Spirlet, M.-R.; Busing, W. R.; Levy, H. A. Acta Crystallogr. B 1977, 33, 1038. (6) Pies, W.; Weiss, A. Crystal Structure of Inorganic Compounds, LB New Series; Hellwege, K.-H., Hellwege, A. M., Eds.; Springer-Verlag: Berlin, 1977; III/7f, p 405. (7) Spirlet, M. R.; Busing, W. R. Acta Crystallogr. B 1978, 34, 907. (8) Bradley, A. J.; Illingsworth, J. W. Proc. R. Soc. (London) 1936, A157, 113. (9) Fournier, M.; Jantou, C. F-; Rabia, C.; Herve, G.; Launay, S. J. Mater. Chem. 1992, 2, 971. (10) Lee, K. Y.; Mizuno, N.; Okuhara, T.; Misono, M. Bull. Chem. Soc. Jpn. 1989, 62, 1731.

Ueda et al. (11) Kozhevnikov, I. V.; Sinnema, A.; Jansen, R. J. J.; van Bekkum, H. Catal. Lett. 1994, 27, 187. (12) Taketa, H.; Katsuki, S.; Eguchi, K.; Seiyama, T.; Yamazoe, N. J. Phys. Chem. 1986, 90, 2959. (13) Slichter, C. P. Principles of Magnetic Resonance, 3rd ed.; Springer: Berlin, 1989; p 311. (14) Wang, P.-K.; Slichter, C. P.; Sinfelt, J. H. Phys. ReV. Lett. 1984, 53, 82. (15) Mueller, K. T. J. Magn. Reson. A 1995, 113, 81. (16) Gullion, T.; Schaefer, J. AdV. Magn. Reson. 1989, 13, 57. (17) Pan, Y.; Gullion, T.; Schaefer, J. J. Magn. Reson. 1990, 90, 330. (18) Fyfe, C. A.; Mueller, K. T.; Grondey, H.; Wong-Moon, K. C. Chem. Phys. Lett. 1992, 199, 198. (19) Naito, A.; Nishimura, K.; Tuzi, S.; Saito, H. Chem. Phys. Lett. 1994, 229, 506. (20) Fyfe, C. A.; Lewis, A. R.; Che´zeau, J. M.; Grondey, H. J. Am. Chem. Soc. 1997, 119, 12210. (21) Goetz, J. M.; Schaefer, J. J. Magn. Reson. 1997, 127, 147. (22) Hing, A. W.; Vega, S.; Schaefer, J. J. Magn. Reson. 1992, 96, 205. (23) Hing, A. W.; Vega, S.; Schaefer, J. J. Magn. Reson. A 1993, 103, 151. (24) Hahn, E. L. Phys. ReV. 1950, 80, 580. (25) Hu, S.; Reimer, J. A.; Bell, A. T. J. Phys. Chem. B 1997, 101, 1869. (26) Kenaston, N. P.; Bell, A. T.; Reimer, J. A. J. Phys. Chem. 1994, 98, 894. (27) Boyce, J. B.; Ready, S. E. Phys. ReV. B 1988, 38, 11008. (28) Reimer, J. A.; Duncan, T. M. Phys. ReV. B 1983, 27, 4895. (29) Brunner, E.; Sternberg, U. Prog. Nucl. Magn. Reson. Spectrosc. 1998, 32, 21. (30) Abragam, A. The Principles of Nuclear Magnetism; Clarendon Press: Oxford, 1962; p 223. (31) Uchida, S.; Inumaru, K.; Dereppe, J. M.; Misono, M. Chem. Lett. 1998, 643. (32) Majors, P. D.; Raidy, T. E.; Ellis, P. D. J. Am. Chem. Soc. 1986, 108, 8123. (33) Clifford, J. O.; Smith, J. A. S.; Temme, F. P. J. Chem. Soc., Faraday Trans. 2 1975, 71, 1352. (34) Chihara, H.; Nakamura, N. Nuclear Quadrupole Resonance Data, LB New Series; Chihara, H., Ed.; Springer-Verlag: Berlin, 1997; Vol. 39 Supplement to Vol. III/20 and III/31. (35) Waldstein, P.; Rabideau, S. W.; Jackson, J. A. J. Chem. Phys. 1964, 41, 3407. (36) Knoth, W. H.; Farlee, R. D. Inorg. Chem. 1984, 23, 4765. (37) Kanda, Y.; Lee, K. Y.; Nakata, S.; Misono, M. Chem. Lett. 1988, 139. (38) Losso, P.; Schnabel, B.; Jager, C.; Sternberg, U.; Stachel, D.; Smith, D. O. J. Non-Cryst. Solids 1992, 143, 265.