51V MAS NMR Investigation of 51V Quadrupole Coupling and

51V MAS NMR Investigation of 51V Quadrupole Coupling and Chemical Shift ..... of 51V Solid-State NMR Parameters of Vanadium(V) Model ...
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J. Phys. Chem. B 2001, 105, 420-429

51V

51V

MAS NMR Investigation of Divalent Metal Pyrovanadates

Quadrupole Coupling and Chemical Shift Anisotropy in

Ulla Gro Nielsen, Hans J. Jakobsen, and Jørgen Skibsted* Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, UniVersity of Aarhus, DK-8000 Aarhus C, Denmark ReceiVed: August 9, 2000

Magnitudes and relative orientations of 51V quadrupole coupling and chemical shift tensors have been determined from 51V magic-angle spinning (MAS) NMR spectra at 14.1 T for seven divalent metal pyrovanadates: R- and β-Mg2V2O7, Ca2V2O7, R-Zn2V2O7, Cd2V2O7, BaCaV2O7, and R-BaZnV2O7. This has been accomplished by least-squares fitting of the integrated spinning sideband intensities observed for the central and satellite transitions employing spectral widths up to 4 MHz. Numerical error analysis of the optimized data reveals that the five NMR parameters characterizing the magnitudes of the quadrupole coupling and chemical shift tensors are obtained with high precision while somewhat larger error limits are observed for the three Euler angles, describing the relative orientation of the two tensors. The optimized data exhibit a significantly higher precision when compared to earlier reported parameters for some of the pyrovanadates, determined from 51V static-powder or MAS NMR of the central transition only. The 51V chemical shift parameters indicate that the different conformations for the V2O74- ion in thortveitite-type and dichromatetype pyrovanadates can be distinguished by the sign for the chemical shift anisotropy (δσ ) δiso - δzz), negative and positive, respectively. A linear correlation is observed between the principal elements for the 51 V quadrupole coupling tensors and calculated electric field gradient tensor elements, obtained from pointmonopole calculations. This correlation is used to assign the NMR parameters for the two different crystallographic 51V sites in the asymmetric units for R- and β-Mg2V2O7, Ca2V2O7, BaCaV2O7, and R-BaZnV2O7.

Introduction 51V

NMR spectroscopy For the past two decades solid-state has developed itself into an important analytical tool in studies of inorganic materials. The main application has been directed toward structural investigations of vanadium-based heterogeneous catalysts,1,2 where 51V NMR is used to probe the environment of the 51V sites, a key factor related to the activity and selectivity of these materials. Such 51V NMR investigations include studies of V2O5 supported on Al2O3,1,3,4 TiO2,1,3,5 and SiO2;1,6 vanadium-niobium oxide catalysts;7 and vanadium in molten alkali pyrosulfate melts.8 These catalysts are used in a number of industrially important chemical reactions including selective oxidation of hydrocarbons, reduction of NOx from flue gas, and oxidation of SO2 in the production of sulfuric acid.9 Generally, solid-state 51V NMR investigations have employed static-powder or magic-angle spinning (MAS) NMR to monitor structural modifications by observing changes in the line shape or the spinning sideband (ssb) intensities for the 51V central transition. Since this transition is dominated by the chemical shift interaction, a determination of the 51V chemical shift anisotropy (CSA) parameters has been used to distinguish different vanadium species in some of the studies. Eckert and Wachs3 have shown that the 51V CSA increases with an increasing degree of polymerization for ortho-, pyro-, and metavanadates, while Lapina et al.1,2 have suggested specific chemical shift parameters for the different types of tetrahedrally coordinated vanadium-oxygen species and for vanadium in * Corresponding author. Phone: (+45) 8942 3900. Fax: (+45) 8619 6199. E-mail: [email protected].

octahedral environments. Furthermore, it has been proposed that ortho-, pyro-, and metavanadates may be distinguished on basis of the 51V shift anisotropy (δσ ) δiso - δzz) and the CSA asymmetry parameter (ησ ) (δxx - δyy)/δσ).10,11 A breakthrough in 51V MAS NMR spectroscopy in our laboratory12-14 was introduced by the detection of the complete manifold of ssbs from the central as well as the satellite transitions employing large spectral windows and a carefully adjusted spectrometer. In addition to a determination of the CSA parameters, these manifolds of MAS sidebands allow determination of the 51V quadrupole coupling parameters and the relative orientation of the two corresponding tensors employing least-squares optimization of simulated to experimental ssb intensities.13,14 The quadrupole coupling parameters (CQ and ηQ) reflect the electric field gradients at the 51V nuclear site in a sensitive manner and are also required to obtain a precise value for the isotropic chemical shift (δiso), because the second-order quadrupolar interaction may shift the isotropic peak several parts per million to lower frequency. This approach has recently been used to determine the 51V quadrupole coupling and CSA parameters in orthovanadates and for a series of mono- and divalent metal metavanadates.11,15 This work reports the determination of the magnitudes and relative orientation of the 51V quadrupole coupling and CSA tensors for a series of divalent metal pyrovanadates, R- and β-Mg2V2O7, Ca2V2O7, R-Zn2V2O7, Cd2V2O7, BaCaV2O7, and R-BaZnV2O7, from 51V MAS NMR spectra of the central and satellite transitions recorded at 14.1 T. For the pyrovanadates M2V2O7 with M ) Mg, Ca, Zn, and Cd, principal elements of

10.1021/jp002882u CCC: $20.00 © 2001 American Chemical Society Published on Web 12/16/2000

51V

MAS NMR Investigation of

51V

Quadrupole Coupling

the 51V chemical shift tensors have earlier been reported from either 51V static-powder or MAS NMR spectra of the central transition,1,3,16 however, with no report of the 51V quadrupole coupling parameters. Furthermore, these studies did not consider the second-order quadrupolar shift for the central transition, which leads to inaccurate values for the isotropic chemical shifts. 51V quadrupole coupling parameters have earlier been reported for one of the two distinct 51V sites in each of the phases Rand β-Mg2V2O7 from 51V MAS NMR spectra of the central transition17 and for the 51V sites in β-Mg2V2O7, Ca2V2O7, and R-Zn2V2O7 from low-field (νL ) 5.25-8.52 MHz) 51V singlecrystal NMR measurements.18,19 Improved and precise values for CQ and ηQ are obtained in the present study from 51V MAS NMR experiments by consideration of the combined effect from the quadrupole coupling and chemical shift interaction. Experimental Section Materials. All pyrovanadates were synthesized from analytically pure reagents that were used without further purification. Purities and structures of the pyrovanadates were confirmed by powder X-ray diffraction (XRD). r- and β-Mg2V2O7. R-Mg2V2O7 was synthesized using the method described by Sam et al.:20 5.56 g (47.5 mmol) of NH4VO3 was dissolved in 50 mL of a 1% NH3 solution followed by addition of 4.75 g (47.4 mmol) of Mg(OH)2. The suspension was evaporated at 80 °C under stirring until a paste was obtained. This precursor phase was dried at 120 °C overnight and subsequently calcined at 740 °C for 80 h, giving R-Mg2V2O7. The high-temperature β-phase was obtained by heating R-Mg2V2O7 at 850 °C for 48 h, followed by rapid cooling to room temperature. Ca2V2O7 and r-Zn2V2O7. These pyrovanadates were obtained from mixtures of CaCO3 or ZnO with V2O5 in a 2:1 molar ratio by heating at 900 and 600 °C for 48 h, respectively. The heating scheme for R-Zn2V2O7 was repeated once in order to complete the reaction of ZnO with V2O5. Cd2V2O7. A solution of 2.35 g (19.35 mmol) of NaVO3 in 50 mL of water was mixed with 2.01 g (9.7 mmol) of CdSO4 dissolved in 25 mL of water. A yellow precipitate was isolated and dried at 120 °C overnight. BaCaV2O7 and r-BaZnV2O7. These pyrovanadates were obtained similar to the method used for R-Mg2V2O7. BaCaV2O7 was synthesized by dissolving 1.79 g (15.4 mmol) of NH4VO3 in 100 mL of a 1% NH3 solution followed by addition of 0.76 g (7.7 mmol) of CaCO3 and 1.51 g (7.7 mmol) of BaCO3. The isolated precursor was calcined at 700 °C for 24 h. The preparation of R-BaZnV2O7 employed 1.69 g (14.4 mmol) of NH4VO3, 1.42 g (7.2 mmol) of BaCO3, 0.59 g (7.2 mmol) of ZnO, and a calcination temperature of 620 °C (24 h). Powder XRD revealed the structure of R-BaZnV2O721 for the isolated material. NMR Measurements. Solid-state 51V MAS NMR experiments were performed at 157.7 MHz (14.1 T) on a Varian INOVA-600 spectrometer using home-built CP/MAS probes for 4 and 5 mm o.d. rotors. In one case (β-Mg2V2O7) a 51V MAS NMR spectrum was also recorded at 78.5 MHz on a Varian INOVA-300 spectrometer. Spinning speeds in the range of 9-15 kHz employed Si3N4 rotors, a Varian rotor-speed controller, and showed a stability of (2 Hz. The 51V MAS NMR spectra were obtained using spectral widths of 2 or 4 MHz, single-pulse excitation with a pulse width of 0.5 µs (γB1/2π ≈ 70 kHz), and a relaxation delay of 1 s. The exact magic-angle setting was achieved by minimizing the line widths of the spinning sidebands in the 23Na MAS spectrum of NaNO3. Baseline

J. Phys. Chem. B, Vol. 105, No. 2, 2001 421 distortions were suppressed by linear prediction of the first (2545) data points of the FID followed by a baseline correction using the Varian VNMR software. Isotropic chemical shifts are reported relative to neat VOCl3 using a solution of 0.16 M NaVO3 (δiso ) -574.38 ppm) as secondary reference.14 However, for the 51V MAS NMR spectra shown in the figures, a kilohertz scale is shown relative to the isotropic peaks, to better appreciate the asymmetry of the ssb manifolds, while insets are shown on a parts per million scale relative to neat VOCl3. Simulations, least-squares optimizations, and error analyses of the experimental 51V MAS NMR spectra were performed on a SUN ULTRA-SPARC 1 workstation using STARS, a solidstate NMR simulation package developed in our laboratory13,14,22 and incorporated in the Varian VNMR software. The numerical analyses used the same approach as recently employed for the spectra of ortho- and metavanadates.11,13-15 The quadrupole coupling and CSA parameters are related to their principal tensor elements by the following equations

CQ ) ηQ )

eQVzz h

Vyy - Vxx Vzz

(1)

δσ ) δiso - δzz ησ )

δxx - δyy δσ

where δiso ) 1/3(δxx + δyy + δzz). The principal tensor elements of the CSA (δ) and electric field-gradient (V) tensors are defined using the convention

|λzz - 1/3Tr(λ)| g |λxx - 1/3 Tr(λ)| g |λyy - 1/3Tr(λ)|

(2)

where λii ) δii, Vii. The Euler angles (ψ, χ, ξ), describing the orientation of the CSA tensor relative to the quadrupole coupling tensor, correspond to positive rotations about δzz (ψ), the new δyy (χ), and the final δzz (ξ) axis. These angles are defined in the ranges 0 e ψ e π and 0 e χ, ξ e π/2.14 The error limits for the anisotropic NMR parameters are 95% confidence intervals calculated using the method described elsewhere.23 Results and Discussion Generally, pyrovanadates contain two tetrahedrally coordinated vanadium atoms connected by a bridging oxygen in a V2O74- unit. From the conformation of the nonbridging oxygen atoms on the two vanadium tetrahedra, two major classes of pyrovanadates can be distinguished.24,25 These are the so-called “thortveitite” (Sc2Si2O7) and “dichromate” (K2Cr2O7) types of pyrovanadates, which exhibit the staggered and eclipsed conformations illustrated in Figure 1a and 1b for the two tetrahedra, respectively. Ideally the staggered conformation of the V2O74anion for the “thortveitite” pyrovanadates (Figure 1a) has a linear V-O-V bond and D3d symmetry, whereas the eclipsed conformation for the “dichromate” pyrovanadates (Figure 1b) ideally has C2V symmetry.24,25 However, deviations from these symmetries may be present for both classes of pyrovanadates. According to Clark and Morely,24 pyrovanadates classified as the thortveitite-type have a V-O-V bond angle larger than 140°, while those with a smaller bridging angle belong to the dichromate-type of pyrovanadates. The pyrovanadates studied here include both “thortveitite” (R-Zn2V2O7, Cd2V2O7) and “dichromate” pyrovanadates (R-Mg2V2O7, Ca2V2O7, BaCaV2O7,

422 J. Phys. Chem. B, Vol. 105, No. 2, 2001

Figure 1. Illustration of the staggered (a) and eclipsed (b) conformation for the V2O74- ion in the thortveitite-type and dichromate-type of pyrovanadates, respectively. The plots have used the atomic coordinates for (a) Cd2V2O7 and (b) BaCaV2O7, which possess the ideal thortveitite and dichromate structure of the V2O74- ion.

R-BaZnV2O7) but also a pyrovanadate structure (β-Mg2V2O7)26 that deviates from these two conformational classes. The 51V MAS NMR spectra and the determination of the 51V quadrupole coupling and CSA parameters are described below for the individual pyrovanadates. This also includes a discussion of relationships between the NMR parameters and the 51V point symmetries of the crystal structures and, where possible, a comparison of the optimized NMR parameters with earlier reported data. Finally, a relationship between the 51V quadrupole coupling parameters and structural data from reported XRD studies is investigated using point-charge calculations that model the electronic structure at the nuclear V5+ site. The optimized 51V NMR parameters are summarized in Table 1. r-Mg2V2O7. Although, the R- and β-polymorphs of Mg2V2O7 are both stable at room temperature, only the crystal structure for the high-temperature β-phase has been solved by singlecrystal XRD.26 However, powder XRD indicates that R-Mg2V2O7 is isostructural with Co2V2O7,27 which is monoclinic (space group P21/c)28 and belongs to the “dichromate” pyrovanadates. The experimental 51V MAS NMR spectrum of the central and satellite transitions for R-Mg2V2O7 (Figure 2a) exhibits two overlapping manifolds of ssbs. The two manifolds are wellseparated at 14.1 T using a spinning speed of νr ) 13.0 kHz. This is apparent from the expansion of the spectral region for the central transitions (inset of Figure 2a) that is dominated by the isotropic peaks for two distinct V5+ sites. Least-squares optimization of simulated to experimental ssb intensities for the two manifolds of ssbs gives the 51V parameters for the magnitudes and relative orientation of the quadrupole coupling and CSA tensors for the two 51V sites in R-Mg2V2O7 (see Table 1). The optimized data are illustrated by the corresponding simulation in Figure 2b for the two overlapping manifolds of ssbs that convincingly reproduce all spectral features in the experimental spectrum (Figure 2a). Simulations of the separate manifolds of ssbs from the V(1) and V(2) sites in R-Mg2V2O7 are shown in parts c and d of Figure 2, respectively, and illustrate the rather small asymmetries in the ssb intensities that reflect effects from the CSA interaction. R-Mg2V2O7 has previously been examined by solid-state 51V MAS NMR of the central transition by Occelli et al.,17 who reported the CSA parameters δiso ) -605 ppm, δσ ) 99 ppm, and ησ ) 0.60 for V(1) and δiso ) -551 ppm, δσ ) -56 ppm, and ησ ) 0.68 for V(2) from analysis of the ssb intensities observed at 11.7 T. Furthermore, the quadrupole coupling parameters CQ ) 4.6 MHz and ηQ ) 0.60 were determined for the V(1) site from the second-order quadrupolar line shape of

Nielsen et al. the central transition observed at 7.1 T. For both sites the values for δiso and δσ are in good agreement with those determined in this work (Table 1), while minor deviations are observed for the two ησ values and for ηQ of the V(1) site. 51V CSA parameters for R-Mg2V2O7 have also been reported by Lapina et al.1 and determined to be δiso ) -617 ppm, δσ ) 83 ppm, and ησ ) 0.12 for V(1) and δiso ) -555 ppm, δσ ) -45 ppm, and ησ ) 0.33 for V(2). These data differ significantly from those reported by Occelli et al.17 and those determined in this work (Table 1). The high-field shifts of the δiso values demonstrate that the second-order quadrupolar shift of the central transition was not considered in that study.1 β-Mg2V2O7. The 51V MAS NMR spectrum (14.1 T) of the high temperature β-phase of Mg2V2O7 (Figure 3a) exhibits manifolds of ssbs corresponding to two distinct vanadium sites, in agreement with the crystal structure for this phase.26 The individual ssbs for the two manifolds are best separated, and observed without overlap, using a spinning speed of νr ) 15 kHz. The spectral features of a second-order quadrupolar line shape is observed for the centerband around -500 ppm (inset in Figure 3a), which indicates a fairly strong 51V quadrupolar interaction for this site. The spectral regions for the two central transitions observed at 7.1 and 14.1 T (Figure 4a and b) clearly demonstrate an improved resolution at 7.1 T for the secondorder quadrupolar line shape of the two 51V centerbands at -500 and -640 ppm (the V(2) and V(1) site, respectively, vide infra). Least-squares optimization of the simulated to the experimental quadrupolar line shape for the centerband at -500 ppm gives the following precise values for the quadrupole coupling and isotropic chemical shift parameters: CQ ) 10.1 ( 0.2 MHz, ηQ ) 0.44 ( 0.03, and δiso ) -494.4 ( 1.0 ppm. These data are particularly useful for the analysis of the ssb manifold observed at 14.1 T (Figure 3a), since the quite strong quadrupole coupling results in reduced ssb intensities at the edges of the full spectral range for the outer ((5/2, (7/2) transitions. This is due to the fact that CQ ) 10.1 MHz corresponds to a total width of 4.3 MHz for these transitions. Furthermore, simulations show that the individual ssbs for the ((5/2, (7/2) transitions at 14.1 T have line widths that decrease systematically from about 9.7 to 3.5 kHz with increasing order of the ssbs. This observation reflects the fact that the individual crystallites in a powder do not contribute with intensity over the full spectral range, as demonstrated elsewhere for the inner ((1/2, ( 3/2) and outer ((3/2, ( 5/2) satellite transitions in 27Al MAS NMR spectra.29 The intensities of the ssbs for the outer 51V transitions are severely affected by a strong quadrupole coupling and accurate intensity values, which are important for a precise determination of CQ and ηQ, can therefore not be obtained. Thus, the CQ and ηQ values for the V(2) site, obtained from the second-order line shape of the central transition at 7.1 T, are used as fixed parameters in the determination of the CSA parameters and the Euler angles (ψ, χ, ξ) from optimization of simulated to observed ssb intensities at 14.1 T. However, the subsequent error analysis shows that the ssb intensities are insensitive to variations of the angle ψ. In fact, theory shows that this angle is undefined for ησ ≈ 0,13 which is the value actually determined. Furthermore, the error analysis reveals that the angle ξ cannot be reliably determined from the ssb intensities, although a clear minimum for the rms value is observed for ξ ) 0°. Thus, the final optimization employs ψ ) ξ ) 0°, which gives the final data for δσ, ησ, and χ listed in Table 1 for the V(2) site in β-Mg2V2O7. The 51V interaction parameters for the V(1) site, which has a somewhat smaller quadrupole coupling, are obtained from seven-parameter fits to the ssb intensities in the 51V MAS

51V

MAS NMR Investigation of

51V

Quadrupole Coupling

J. Phys. Chem. B, Vol. 105, No. 2, 2001 423

TABLE 1: 51V Quadrupole Couplings (CQ, ηQ), Chemical Shift Anisotropies (δσ, ησ), and Relative Orientations (ψ, χ, ξ) of the Two Tensors and Isotropic Chemical Shifts (δiso) for a Series of Divalent Metal Pyrovanadates from 51V MAS NMR at 14.1 Ta compoundb R-Mg2V2O7 β-Mg2V2O7 Ca2V2O7 R-Zn2V2O7 Cd2V2O7 BaCaV2O7 R-BaZnV2O7

V(1) V(2) V(1) V(2) V(1) V(2)

V(1) V(2) V(1) V(2)

CQ (MHz)

ηQ

δσ (ppm)

ησ

ψ (deg)

χ (deg)

ξ (deg)

δisoc (ppm)

4.82 ( 0.05 3.29 ( 0.03 4.80 ( 0.10 10.1 ( 0.2e 1.58 ( 0.02 7.33 ( 0.32 3.86 ( 0.03 6.0 ( 0.1 2.57 ( 0.07 3.20 ( 0.06 3.81 ( 0.06 5.91 ( 0.09

0.43 ( 0.03 0.69 ( 0.02 0.39 ( 0.04 0.44 ( 0.03e 0.90 ( 0.02 0.43 ( 0.05 0.56 ( 0.02 0.41 ( 0.03 0.32 ( 0.02 0.85 ( 0.02 0.58 ( 0.02 0.86 ( 0.02

103 ( 2 -57 ( 3 -113 ( 7 -262 ( 3 71 ( 3 530 ( 10 -119 ( 2 -173 ( 2 100 ( 2 99 ( 3 143 ( 2 252 ( 2

0.34 ( 0.16 0.91 ( 0.10 0.90 ( 0.10 0.10 ( 0.10 0.54 ( 0.35 0.50 ( 0.03 0.62 ( 0.02 0.27 ( 0.10 0.65 ( 0.11 0.49 ( 0.20 0.10 ( 0.20 0.16 ( 0.08

2 ( 34 80 ( 22 36 ( 40 0f 8 ( 34 28 ( 48 128f 131 ( 38 82 ( 28 95 ( 28 2 ( 30 132 ( 63

89 ( 18 1 ( 20 4 ( 42 25 ( 20 90 ( 17 90 ( 19 0 ( 12 0d 38 ( 7 27 ( 16 36 ( 9 17 ( 12

41 ( 9 0 ( 37 17 ( 38 0f 16 ( 15 69 ( 12 90f 90d 90d 90 ( 35 90 ( 28 50 ( 45

-603.5 ( 0.8 -549.2 ( 0.5 -639.3 ( 0.5 -494.4 ( 1.0e -574.9 ( 0.8 -534.0 ( 1.0 -616.6 ( 0.5 -562.7 ( 0.5 -581.6 ( 0.5 -598.8 ( 0.5 -650.1 ( 1.0 -608.4 ( 1.0

a Optimized data from least-squares fits to the experimental ssb intensities observed in 51V MAS NMR spectra at 14.1 T. The error estimates are based on calculations of the 95% confidence limits using the method described in ref 23. b Assignment of the 51V parameters to the crystallographic 51 V sites employing the correlation between the quadrupole tensors elements and calculated electric field gradient tensor elements using the pointmonopole approach (cf., Figure 10). The 51V sites are indexed according to the structure references (cf., Table 2). c Isotropic chemical shift are relative to neat VOCl3. d Parameters fixed in the optimization due to restrictions on the Euler angles imposed by the crystal symmetry (see text). e Parameters determined from the second-order quadrupolar line shape observed at 7.1 T. The parameters are used as fixed parameters in the least-squares fit to the ssb intensities observed at 14.1 T. f The error analysis shows that this parameter cannot be reliably determined from leastsquares fits to the ssb intensities. The parameter given corresponds to the lowest rms value from the optimization.

spectrum (Figure 3a). This gives the data listed for V(1) in Table 1. The simulation of the central and satellite transitions for the two sites at 14.1 T, employing the optimized parameters, is shown in Figure 3b. Parts c and d of Figure 4 illustrate the corresponding simulations of the regions for the central transitions at 7.1 and 14.1 T, respectively. 51V chemical shift parameters for β-Mg V O have earlier 2 2 7 been reported by Occelli et al.17 (δiso ) -642 ppm, δσ ) -103 ppm, and ησ ) 0.52 for V(1) and δiso ) -497 ppm, δσ ) -235 ppm, and ησ ) 0.29 for V(2)) from 51V MAS NMR spectra of the central transitions at 11.7 T. These values are in reasonable agreement with those given in Table 1. 51V CSA parameters for β-Mg2V2O7 have also been reported by Lapina et al.1 and Hayakawa et al.16 However, these data deviate significantly from those determined in this work and by Occelli et al.,17 which most likely reflect the fact that the quadrupole coupling interaction was not considered in these studies. Quadrupole coupling parameters have also been reported for the V(2) site by Occelli et al.17 (CQ ) 9.5 MHz and η Q ) 0.70) from the second-order line shape observed in a 51V MAS spectrum at 7.1 T and for both sites from low-field (5.34 and 7.72 MHz) 51V single-crystal NMR (C ) 4.73 MHz, η ) 0.42 for V(1) Q Q and CQ ) 10.21 MHz, ηQ ) 0.45 for V(2)).18 The quadrupole coupling parameters determined from the spectra in Figures 3 and 4 (Table 1) are in excellent agreement with those reported from the single-crystal 51V NMR experiments, although the CSA interactions were not considered in the analysis of these lowfield 51V spectra. The 51V quadrupole coupling of CQ ) 10.1 MHz for β-Mg2V2O7 is the largest observed so far for inorganic vanadates and most likely reflects the quite unusual coordination environment for the V5+ sites in β-Mg2V2O7. This polymorph crystallizes in the triclinic space group P1h and contains two inequivalent vanadium sites connected via an oxygen with a V-O-V bond angle of 140.6°.26 The crystal structure of β-Mg2V2O7 deviates from the “thortveitite” and “dichromate” pyrovanadate structures, since both vanadium atoms have an additional weak V-O bond in addition to the four V-O bonds in the V2O74unit. The environment of the V(1) atom includes four V-O bonds in a distorted tetrahedral arrangement, with bond lengths in the range 1.65-1.86 Å, and a fifth weak V-O bond, with a length of 2.87 Å.26 The V(2) site is best characterized as being

pentacoordinated exhibiting four V-O bonds between 1.68 and 1.79 Å and a fifth V-O bond of 2.44 Å.26 On the basis of the observation of very strong 27Al quadrupole couplings for fivecoordinated aluminum, Occelli et al.17 assigned the resonance with strong quadrupole coupling to the V(2) site for β-Mg2V2O7. This assignment is supported by our point-monopole calculations of the electric field gradient tensors (vide infra). Ca2V2O7. Calcium pyrovanadate crystallizes in the triclinic space group P1h30 and has a structure with two distinct V5+ sites and a V-O-V bond angle of 124.0°. This resembles the “dichromate” pyrovanadate structure. The V(1) site in Ca2V2O7 includes four V-O bonds (dV-O ) 1.70-1.74 Å) in a fairly symmetric tetrahedron, whereas the V(2) site may be considered pentacoordinated with five V-O distances in the range 1.652.05 Å.30 In agreement with these structural features, the 51V MAS NMR spectrum of Ca2V2O7 (Figure 5a) displays two overlapping manifolds of ssbs, corresponding to two 51V sites with a quite small and a rather large quadrupole coupling. The centerband for the strong quadrupole-coupling V(2) site exhibits a partly resolved second-order quadrupolar line shape at 14.1 T (inset in Figure 5a). However, 51V MAS spectra recorded at 9.4 and 7.1 T show that the increased width of the line shapes at lower magnetic fields results in overlap of the centerbands for the two V5+ sites. This makes these spectra less attractive for a determination of CQ and ηQ from line-shape simulations. Thus, the quadrupole coupling and chemical shift parameters are extracted for both sites from least-squares optimizations to the ssbs observed at 14.1 T (Figure 5a). This gives the optimized 51V data listed in Table 1 and the corresponding simulation shown in Figure 5b. The quadrupole coupling parameters in Table 1 agree fairly well with those reported from low-field (5.25 and 8.53 MHz) 51V single-crystal NMR (CQ ) 1.68 MHz, ηQ ) 0.74 for V(1) and CQ ) 7.96 MHz, ηQ ) 0.34 for V(2)).19 However, the 51V CSA interaction was not considered in that study, which may affect the parameters derived especially for the V(2) site, since this site possesses a very large CSA (i.e., the largest CSA observed so far for a pyrovanadate). 51V CSA parameters have been reported for a single V5+ site in Ca2V2O7 (δσ ) 62 ppm, ησ ) 0.68, and δiso ) -575 ppm) by Hayakawa et al.16 from 51V static-powder and MAS NMR spectra of the central transition. These data are in good agreement with those determined in this work for the V(1) site (Table 1). This

424 J. Phys. Chem. B, Vol. 105, No. 2, 2001

Figure 2. (a) Experimental and (b) simulated 51V MAS NMR spectra of the central and satellite transitions for R-Mg2V2O7 (14.1 T, νr ) 13.0 kHz). The insets in a and b show the spectral region for the isotropic peaks, which are indicated by a circle and a triangle for V(1) and V(2), respectively, employing the same indexing for the two vanadium sites as in the reported crystal structure for the isostructural Co2V2O7 pyrovanadate.28 The simulation employed the optimized 51V parameters for R-Mg2V2O7 in Table 1 and an intensity ratio of 1:1 for the two manifolds of ssbs. Parts c and d illustrate separate simulations of the spectra for V(1) and V(2), respectively.

indicates that these authors only observed the V(1) site, which reflects the fact that the intensity of the V(1) resonance dominates the spectral region for the two central transitions (cf., inset in Figure 5a), especially at lower magnetic fields and lower spinning speeds. Lapina et al.1 have observed two 51V resonances for Ca2V2O7 for which they reported almost identical CSA data and the chemical shifts, δ ) -574 and δ ) -578 ppm. r-Zn2V2O7. R-Zn2V2O7 is monoclinic (space group C2/c) and belongs to the “thortveitite” pyrovanadates with a V-O-V bond angle of 149.3°.31 The bridging oxygen atom for the V2O74- unit is situated on a mirror plane, implying that the two VO4 tetrahedra in this unit are crystallographically equivalent. In agreement with this structure, the 51V MAS NMR spectrum of R-Zn2V2O7 (Figure 6a) displays ssbs from a single 51V site for the central and satellite transitions. Least-squares analysis of the ssb intensities for this spectrum gives the optimized

Nielsen et al.

Figure 3. (a) 51V MAS NMR spectrum of β-Mg2V2O7 recorded at 14.1 T using νr ) 15.0 kHz and a spectral width of 4.0 MHz. The spectrum in part a illustrates the central part (1.6 MHz) of the wideband spectrum, while the inset shows the spectral region for the centerbands. (b) Optimized simulation of the two manifolds of ssbs observed in part a employing the 51V data for β-Mg2V2O7 in Table 1 and an 1:1 intensity ratio for the two spectra. The isotropic peaks for V(1) and V(2) in the experimental and simulated spectra are indicated by circles and triangles, respectively.

parameters listed in Table 1 and the corresponding simulation shown in Figure 6b. The error analysis reveals that the Euler angles ψ and ξ cannot be reliably determined from the integrated ssb intensities, however, the lowest rms value is obtained for ψ ) 128° and ξ ) 90°. In a recent 51V NMR study of orthovanadates,15 R-Zn2V2O7 was observed as an impurity phase in the studied sample of Zn3(VO4)2, and the 51V NMR parameters for the two zinc vanadates were determined from a 51V MAS spectrum recorded at 9.4 T. These data for R-Zn2V2O7 are in accord with the 51V parameters determined in this work at 14.1 T (Table 1). Furthermore, the 51V chemical shift parameters are in good agreement with earlier reported data from static-powder and MAS NMR spectra of the central transition.3,16 Cd2V2O7. Cadmium pyrovanadate crystallizes in the monoclinic space group C2/m and has a structure with a linear V-O-V bond.32 The vanadium atoms are situated in a mirror plane and the bridging oxygen for the two VO4 tetrahedra of the V2O74- ion constitutes a center of symmetry, implying that the oxygens on the two VO4 tetrahedra of the V2O74- ion exhibit an ideal staggered conformation as observed for thortveitite. The 51V MAS NMR spectrum of Cd2V2O7 (Figure 7a) bears a fairly close resemblance to the spectrum observed for R-Zn2V2O7

51V

MAS NMR Investigation of

51V

Quadrupole Coupling

Figure 4. Comparison of the spectral region for the central transitions observed in 51V MAS NMR spectra of β-Mg2V2O7 at (a) 7.1 T (νr ) 13.0 kHz) and (b) 14.1 T (νr ) 15.0 kHz). The centerbands for V(1) and V(2) in β-Mg2V2O7 are indicated by circles and triangles, respectively. The optimized simulations of the second-order quadrupolar line shapes for the central transitions, employing the 51V NMR parameters for β-Mg2V2O7 in Table 1, are shown in parts c and d for the magnetic fields of 7.1 and 14.1 T.

(Figure 6a), reflecting that the two compounds both belong to the “thortveitite” pyrovanadates. Least-squares optimization for the spectrum of Cd2V2O7 reveals that the Euler angle ξ cannot be reliably determined from seven-parameter fits to the experimental ssb intensities. However, the local symmetry for the VO4 tetrahedra requires that two of the three Euler angles are constrained to the values n × 90° (n ) 0, 1, ...). This is utilized in five-parameter optimizations that employ the different combinations of these values for two of the Euler angles. This gives the lowest rms value for χ ) 0° and ξ ) 90° and the optimized value for ψ and the parameters for the two tensors listed in Table 1. These data are illustrated by the optimized simulation in Figure 7b, which convincingly reproduces all the spectral features for the manifold of experimental ssbs (Figure 7a). The 51V CSA for Cd2V2O7 has earlier been characterized by 51V MAS NMR of the central transition by Eckert and Wachs3, who reported the parameters δiso ) -563 ppm, δσ ) -193 ppm, and ησ ) 0.0. These data are in fair agreement with those listed in Table 1 for Cd2V2O7, which should be of higher precision, since the approach taken here involves analysis of the spectrum for all transitions. BaCaV2O7. The BaCaV2O7 mixed-metal pyrovanadate crystallizes in the orthorhombic space group Pnma.21 As expected, it belongs to the “dichromate” pyrovanadates, since this group includes the single-metal pyrovanadates Ca2V2O7 and Ba2V2O7. The structure of the V2O74- ion in BaCaV2O7 contains a mirror

J. Phys. Chem. B, Vol. 105, No. 2, 2001 425

Figure 5. (a) Experimental 51V MAS NMR spectrum of the central and satellite transitions for Ca2V2O7 (14.1 T, νr ) 13.0 kHz). (b) Optimized simulation using the parameters for V(1) and V(2) of Ca2V2O7 in Table 1 and an 1:1 intensity ratio for the two spectra. The right-hand insets in parts a and b illustrate the spectral region for the isotropic peaks, which are indicated by a circle (V(1)) and a triangle (V(2)). The left-hand inset of part a shows the partly resolved secondorder quadrupolar line shape of the centerband from the central transition for the V(2) site.

plane that includes the two vanadium atoms, the bridging oxygen, and two of the terminal oxygen atoms.21 This implies a constraint for two of the Euler angles (ψ, χ, ξ) to the values n × 90° (n ) 0, 1, ...) for both 51V sites. The 51V MAS NMR spectrum of BaCaV2O7 (Figure 8a) exhibits two overlapping manifolds of ssbs in an intensity ratio of 1:1. These manifolds correspond to very similar quadrupole coupling constants and isotropic chemical shifts. Seven-parameter fits to the integrated ssb intensities for one of the manifolds (V(1)) reveal that the Euler angle ξ cannot be obtained with good precision. Thus, the above-mentioned constraint imposed on the Euler angles is employed in six-parameter fits corresponding to ξ ) 0 or 90°. This gives the lowest rms value for ξ ) 90°. The subsequent error analysis gives the values ψ ) 82 ( 28° and χ ) 38 ( 7° for the ssb manifold from V(1). This shows that the symmetry constraint holds for the Euler angle ψ ()90°). Within the error limits, ψ ) ξ ) 90° is also obtained for the ssb manifold from the V(2) site, since seven-parameter fits and the corresponding error analysis give ψ ) 95 ( 28°, χ ) 27 ( 16°, and ξ ) 90 ( 35°. The remaining parameters for the two tensorial interactions are listed in Table 1, and the final result of the numerical

426 J. Phys. Chem. B, Vol. 105, No. 2, 2001

Figure 6. (a) 51V MAS NMR spectrum (14.1 T, νr ) 9.0 kHz) of the central and satellite transitions for R-Zn2V2O7 and (b) optimized simulation corresponding to the 51V parameters in Table 1. The circle indicates the isotropic peak and the asterisk the centerband from a small impurity of Zn3(VO4)2.

analyses based on the ssb intensities is illustrated by the simulated spectrum in Figure 8b. The observation of rather small variations in the values for δiso, δσ, ησ, and CQ for the two 51V sites in BaCaV2O7 is in good agreement with the fact that the geometries of the VO4 tetrahedra in the V2O74- anion are very similar.21 r-BaZnV2O7. BaZnV2O7 exists in two polymorphic modifications that both belong to the “dichromate” pyrovanadates. The high-temperature R-phase is isostructural with BaCaV2O7 and BaCdV2O7 (orthorhombic, Pnma)21 while the low-temperature β-polymorph is monoclinic with the space group P21/n (or P21/c).21 Powder XRD of the synthesized sample of BaZnV2O7 clearly reveals that only the R-polymorph is present in our sample. The 51V MAS NMR spectrum of R-BaZnV2O7 (Figure 9a) exhibits overlapping manifolds of ssbs from two 51V sites. These possess rather different quadrupole couplings, as also illustrated by the observation of partly resolved ssbs from the individual transitions (left-hand inset in Figure 9a) for the 51V site with the largest quadrupole coupling. Least-squares analysis of the ssb intensities for the two manifolds allows determination of all eight parameters (Table 1), characterizing the magnitudes and relative orientations of the two tensors, however with large error limits observed for the Euler angles ψ and ξ. The optimized parameters are illustrated by the simulated spectrum in Figure 9b, which reproduce the intensities of the two ssb manifolds and the line shapes for the partly resolved ssbs from the individual transitions in a convincing manner. A comparison of the optimized data for R-BaZnV2O7 and BaCaV2O7 reveals significantly larger quadrupolar coupling

Nielsen et al.

Figure 7. (a) 51V MAS NMR spectrum of the central and satellite transitions for Cd2V2O7 recorded at 14.1 T using νr ) 14.0 kHz. (b) Optimized simulation of the manifolds of ssbs in part a corresponding to the 51V parameters for Cd2V2O7 in Table 1.

and CSA for the 51V sites in R-BaZnV2O7. This indicates a higher degree of distortion for the VO4 tetrahedra of the V2O74ion in R-BaZnV2O7 as compared to BaCaV2O7. We note that only unit cell parameters were reported in the single-crystal XRD study of R-BaZnV2O7.21 Assignment of the 51V Quadrupole Coupling Parameters. To investigate correlations between the experimental quadrupole coupling parameters and the geometries for the VO4 tetrahedra of the V2O74- ions in the pyrovanadates, calculations of the electric-field gradient tensors (Vest) based on the pointmonopole approximation have been performed. This should allow assignment of the observed 51V NMR parameters to the individual vanadium sites for R- and β-Mg2V2O7, Ca2V2O7, BaCaV2O7, and R-BaZnV2O7. This approach has recently proven useful in providing approximate EFG tensors for 23Na in a series of sodium compounds,33 for 133Cs in inorganic cesium salts,23 and for 51V in orthovanadates and mono and divalent metal metavanadates.11,15 In analogy to these studies, the estimated principal elements of the EFG tensor (Viiest) have been calculated using the point-monopole model and considering charges of the oxygens within the first coordination sphere of the VO4 tetrahedra only. Effective charges (qi) of the oxygen atoms are used and calculated as qi ) (-2 + ∑fij)e, where fij is the covalence of the oxygen(i) - cation(j) bond. The fij values are calculated using the bond-valence description by Brown and Shannon34 and the chemical-bond data by Brown and Altermatt.35 Moreover, the calculations employ crystal structure data determined from single-crystal XRD, except for R-Mg2V2O7 and R-BaZnV2O7, where only unit cell parameters are available but which are isostructural with Co2V2O727,28 and BaCaV2O7/

51V

MAS NMR Investigation of

51V

Quadrupole Coupling

Figure 8. (a) MAS NMR spectrum (14.1 T, νr ) 12.0 kHz) of the central and satellite transitions for BaCaV2O7. (b) Optimized simulation of the two manifolds of ssbs in part a using the 51V data for the two 51V sites of BaCaV2O7 in Table 1 and an intensity ratio of 1:1 for the two manifolds of ssbs. The insets illustrate the spectral region for the central transitions, where the isotropic peaks are indicated by circles (V(1)) and triangles (V(2)). 51V

BaCdV2O7,21 respectively. Thus, the fractional atomic coordinates for these two phases are used in combination with the unit cell parameters for R-Mg2V2O7 and R-BaZnV2O7 in the calculation of their 51V EFG tensors. The estimated principal elements of the EFG tensors (Viiest) for the studied pyrovanadates are listed in Table 2. These data are correlated with the corresponding principal elements of the quadrupole tensors (Qiiexp), calculated from the experimental CQ and ηQ values (Table 1), using the relations

Qzzexp ) CQ Qyyexp ) -1/2(1 - ηQ)CQ

(3)

Qxxexp ) -1/2(1 + ηQ)CQ and assuming a positive value for CQ. A plot of Qiiexp as function of Viiest is shown in Figure 10 and indicates a linear relationship between these parameters. Linear regression analysis of the data gives the equation

Qiiexp ) 2.34Viiest(1020 V m-2) - 0.04

(4)

with a correlation coefficient R ) 0.95. The slope determined in eq 4 may be compared to those obtained for the VO4 tetrahedra in orthovanadates (2.17 MHz/1020 V m-2),15 monova-

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Figure 9. (a) Experimental 51V MAS NMR spectrum (14.1 T, νr ) 13.0 kHz) of the central and satellite transitions for R-BaZnV2O7 and (b) optimized simulation for the manifolds of ssbs from the two 51V sites in R-BaZnV2O7, employing the 51V data in Table 1 and an 1:1 intensity ratio for the two spectra. The left-hand insets illustrate the partly resolved ssbs from the individual satellite transitions observed for the V(2) site, while the right-hand insets show the spectral region for the central transitions. The circles and triangles correspond to the isotropic peaks for the V(1) and V(2) sites, respectively, employing the same indexing for the two vanadium sites as for the reported crystal structure of the isostructural BaCaV2O7 pyrovanadate.21 The asterisk in part a indicates the isotropic peak from a small impurity in the sample.

lent metal metavanadates (2.09 MHz/1020 V m-2),15 and divalent metal metavanadates (2.45 MHz/1020 V m-2).11 The small variations for these slopes indicate that the point-monopole approach for calculating the EFG tensor elements accounts rather well for differences in the type of polymerization of VO4 tetrahedra and for different types of charge-balancing cations. Thus, the relationships between Qiiexp and Viiest may in general turn out to be valid for inorganic vanadates containing VO4 units. The correlation described by eq 4 is employed in the assignment of the NMR parameters to the specific crystallographic V(1) and V(2) sites for R- and β-Mg2V2O7, Ca2V2O7, BaCaV2O7, and R-BaZnV2O7. The assignment is given in Table 1, where the NMR data for the V(1) and V(2) sites employ the same indexing of these sites as used in the X-ray structure references. Finally, we note that negative chemical shift anisotropies (δσ) are observed for the two pyrovanadates with the “thortveitite” structure (i.e., R-Zn2V2O7 and Cd2V2O7), for β-Mg2V2O7, and for the V(2) site in R-Mg2V2O7, whereas positive δσ values are observed for the “dichromate” pyrovanadates (except for the V(2) site in R-Mg2V2O7). The negative δσ values for β-Mg2V2O7 propose a crystal structure for this phase similar to the

428 J. Phys. Chem. B, Vol. 105, No. 2, 2001

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TABLE 2: Calculated Principal Elements (Viiest) (×1020 V m-2) of the 51V Eletric Field Gradient Tensors for the Series of Divalent Metal Pyrovanadatesa compound R-Mg2V2O7 β-Mg2V2O7 Ca2V2O7 R-Zn2V2O7 Cd2V2O7 BaCaV2O7 R-BaZnV2O7

Vxx V(1) V(2) V(1)c V(2)c V(1) V(2)

V(1) V(2) V(1) V(2)

est

-0.951 -0.841 -1.764 -3.019 -0.365 -2.927 -0.683 -1.090 -0.679 -1.239 -1.147 -1.401-

Vyy

est

-0.545 -0.485 -0.002 -0.885 0.000 -1.349 -0.558 -0.667 -0.172 -0.988 -0.323 -0.386

Vzz

est

1.496 1.326 1.766 3.905 0.365 4.276 1.241 1.757 0.851 2.227 1.470 1.787

structure

refb

27, 28 26 30 31 32 21 21

a Estimated values from point-monopole calculations using effective charges for the oxygen atoms surrounding the vanadium ion. The calculations employed tetrahedral coordination for the V5+ ion in all pyrovanadates and assumed a positive value for the unique tensor element (Vzzest). b References for the structural parameters employed in the calculation and reported from XRD. c The fractional coordinate for O(6) has been changed from y ) 0.375326 to y ) 0.3853, since incorrect bond lengths and angles are calculated employing the original value.

Conclusions High-precision parameters for the 51V quadrupole coupling and chemical shift tensors along with the relative orientations of these tensors have been obtained for a series of divalent metal pyrovanadates employing 51V MAS NMR of the central and satellite transitions at a high magnetic field (14.1 T). A linear correlation between the principal elements of the 51V quadrupole coupling tensor and calculated electric field gradient tensors (point-monopole approach) is obtained and is useful for the assignment of the 51V quadrupole couplings, and thereby the CSA parameters, for the pyrovanadates that contain two distinct vanadium sites in the asymmetric unit. Such correlations, also observed for ortho- and metavanadates,11,15 may serve useful for the 51V NMR spectral assignment of V5+ multiple sites in inorganic vanadates. The 51V CSA data show that the “thortveitite” and “dichromate” pyrovanadates can be distinguished by the sign of the CSA parameter (δσ ) δiso - δzz). Furthermore, the VO4 tetrahedra within the V2O74- ion exhibit CSA values in the range 60 e |δσ| e 260 ppm, whereas pentacoordinated V5+ sites in pyrovanadates possess significantly larger values (i.e., |δσ| > 300 ppm) and also some of the largest quadrupole couplings observed so far for inorganic vanadates. The 51V parameters determined here, along with those reported for orthoand metavanadates,11,15 represent an improved platform in the testing of advanced models and theoretical approaches for the determination of relationships between anisotropic NMR parameters and structural data. Acknowledgment. The use of the facilities at the Instrument Centre for Solid-State NMR Spectroscopy, University of Aarhus, sponsored by the Danish Natural Science Research Council, the Danish Technical Science Research Council, Teknologistyrelsen, Carlsbergfondet, and Direktør Ib Henriksens Fond, is acknowledged. References and Notes

Figure 10. Linear correlation between 51V quadrupole coupling tensor elements (Qiiexp) and estimated EFG tensor elements (Viiest) from pointmonopole calculations for the studied divalent metal pyrovanadates. The calculated values for Viiest are summarized in Table 2, while the result from linear regression analysis of the data is given in eq 4. Open and filled circles correspond to the data for the thortveitite-like and dichromate-like types of pyrovanadates, respectively, while the triangles illustrate the tensor elements for β-Mg2V2O7.

thortveitite structure, although the structure of β-Mg2V2O7 includes a weak fifth V-O bond for both VO4 tetahedra of the V2O7 ion.26 This suggests that negative and positive CSA values are to be found in “thortveitite” and “dichromate” pyrovanadates, respectively, a proposal earlier put forward by Hayakawa et al.16 Generally, |δσ| ranges from about 60 to 260 ppm for the pyrovanadates including tetrahedrally coordinated V5+ ions only (i.e., the pyrovanadates studied in this work except for β-Mg2V2O7 and the V(2) site of Ca2V2O7). In accord with earlier interpretations of 51V CSA parameters,1-3,10,11 this indicates that δσ and ησ can be used to distinguish different types of V5+ species in inorganic vanadates, i.e., 0 < δσ < 100 ppm for VO4 units in orthovanadates,15 60 e |δσ| e 260 ppm for VO4 species in pyrovanadates, 220 < δσ < 320 ppm and 0.65 e ησ e 0.80 for VO4 units in monovalent metal metavanadates,15 and |δσ| > 260 ppm for VO5 sites in divalent metal metavanadates11 and pyrovanadates.

(1) Lapina, O. B.; Mastikhin, V. M.; Shubin, A. A.; Krasilnikov, V. N.; Zamaraev, K. I. Prog. Nucl. Magn. Reson. Spectrosc. 1992, 24, 457. (2) Mastikhin, V. M.; Lapina, O. B. in: Encyclopedia of Nuclear Magnetic Resonance, Wiley: 1996; Vol. 8, p 4892. (3) Eckert, H.; Wachs, I. E. J. Phys. Chem. 1989, 93, 6796. (4) (a) Le Coustumer, L. R.; Taouk, B.; Le Meur, M.; Payen, E.; Guelton, M.; Grimblot, J. J. Phys. Chem. 1988, 92, 1230. (b) Lapina, O. B.; Mastikhin, L. G.; Simonova, L. G.; Bulgakova, Y. O. J. Mol. Catal. 1991, 69, 61. (c) Chary, K. V. R.; Kishan, G. J. Phys. Chem. 1995, 99, 14429. (5) (a) Eckert, H.; Deo, G.; Wachs, I. E.; Hirt, A. M. Colloids Surf. 1990, 45, 347. (b) Fernandez, C.; Bodart, P.; Guelton, M.; Lefebvre, F. Catal. Today 1994, 20, 77. (6) (a) Taouk, B.; Guelton, M.; Grimblot, J.; Bonnelle, J. P. J. Phys. Chem. 1988, 92, 6700. (b) Das, N.; Eckert, H.; Hu, H.; Wachs, I. E.; Walzer, J. F.; Feher, F. J. J. Phys. Chem. 1993, 97, 8240. (c) Lapina, O. B.; Mastikhin, V. M.; Nosov, A. V.; Beutel, T.; Knozinger, H. Catal. Lett. 1992, 13, 203. (7) (a) Pries de Oliveira, P. G.; Lefebre, F.; Eon, J. G.; Volta, J. C. J. Chem. Soc. Chem. Commun. 1990, 1480. (b) Smits, R. H. H.; Seshan, K.; Ross, J. R. H.; Kentgens, A. P. M. J. Phys. Chem. 1995, 99, 9169. (8) (a) Lapina, O. B.; Mastikhin, V. M.; Shubin, A. A.; Eriksen, K. M.; Fehrmann, R. J. Mol. Catal. A. 1995, 99, 123. (b) Lapina, O. B.; Terskikh, V. V.; Shubin, A. A.; Eriksen, K. M.; Fehrmann, R. Colloids Surf. 1999, 158, 255. (9) Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis; VCH Verlagsgesellschaft mbH, Weinheim, Germany, 1997. (10) Hayakawa, S.; Yoko, T.; Sakka, S. J. Solid State Chem. 1994, 112, 329. (11) Nielsen, U. G.; Jakobsen, H. J.; Skibsted, J. Inorg. Chem. 2000, 39, 2135. (12) (a) Ja¨ger, C. In NMR Basic Principles and Progress; Diehl, P., Fluck, E., Gu¨nther, H., Kosfeld, R., Seelig, J., Eds.; Springer: Berlin, 1993; Vol. 31, p 155. (b) Freude, D.; Haase, J. In NMR Basic Principles and

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