Directly Probing the Metal Center Environment in Layered Zirconium

J. Phys. Chem. C 2008, 112, 8575–8586

8575

Directly Probing the Metal Center Environment in Layered Zirconium Phosphates by Solid-State 91Zr NMR Zhimin Yan, Christopher W. Kirby, and Yining Huang* Department of Chemistry, The UniVersity of Western Ontario, London, Ontario, Canada, N6A 5B7 ReceiVed: NoVember 23, 2007; ReVised Manuscript ReceiVed: February 1, 2008

Layered zirconium phosphates (ZrPs) and their derivatives potentially have many important applications. In the past, these materials have been mainly characterized by X-ray diffraction and 31P MAS NMR. Their metal centers have not been directly probed by solid-state NMR spectroscopy. In this work, we present solidstate 91Zr NMR spectra acquired at several magnetic fields for several representative layered zirconium phosphates including R-Zr(HPO4)2 · H2O, γ-Zr(PO4)(H2PO4) · 2H2O, Zr(NH4PO4)2 · H2O, and Zr2(NaPO4)4 · 6H2O. The NMR interaction tensors were extracted from the spectra. The results indicate that the 91Zr spectra are sensitive not only to the relatively small distortion in ZrO6 polyhedron but also to the difference in the geometry of Zr(OP)6 units in these materials. We show that 91Zr quadrupolar coupling constants (CQ) correlate well with several angular distortion parameters reflecting the deviation from a perfect ZrO6 octahedron such as distortion index, shear strain, and mean O-Zr-O angle. The relationships between CQ and structural parameters related to the Zr(OP)6 unit including the mean Zr-P distance and Zr-O-P angle also appear to exist. The theoretical calculations at both restricted Hartree-Fock and density functional levels were performed on model clusters to establish the relationships of various structural parameters with 91Zr EFG tensors, and the calculation results are consistent with the empirical correlations. For the related layered zirconium phosphates whose structures are unknown or poorly described, we have shown that 91Zr NMR can be used to directly obtain structural information on the local environment around the metal centers, which is complementary to that obtained from powder XRD and 31P MAS NMR, as demonstrated by a novel meso-lamellar ZrP as an example. Introduction Zirconium plays an important role in a number of materials with applications in the areas of catalysis,1 ceramics,2 bioceramic materials,3 and fuel cells.4 In particular, layered zirconium phosphates (ZrPs) have attracted considerable interest for their wide use in the areas of ion exchange,5 intercalation,5a,b,6 catalysis,1a,5a–c,7 sorption,5b,c protonic conductors,7b,c solar energy storage,7b,8 and crystal engineering.5b,6,9 Characterization is important because understanding the relationship between the novel properties of the materials and their structures is crucial for developing new applications and for improving their performance in current uses. However, due to the difficulty in obtaining suitable crystals for single-crystal X-ray diffraction studies, structural determinations for many layered ZrP derivatives have been attempted from much more limited powder X-ray diffraction data. Solid-state (SS) NMR is a complementary technique to X-ray diffraction. Indeed, 31P MAS NMR has been successfully used to characterize various layered ZrP materials.7d,10 On the other hand, the zirconium environments in layered ZrPs have never been directly probed by 91Zr SS NMR. This is because 91Zr, the only NMR-active isotope, has a half-integer quadrupolar nucleus (I ) 5/2) with a moderately sized nuclear quadrupole moment (Q ) -1.76 × 10-29 m2). This results in significant line broadening in the presence of any appreciable electric field gradient (EFG). 91Zr SS NMR spectra are further hampered by its low magnetogyric ratio (γ ) -2.4975 × 107 rad T-1 s-1) and low natural abundance (11.23%), leading to * Corresponding author. E-mail: [email protected]. Tel: 519-661-2111, ext 86384. Fax: 519-661-3022.

low sensitivity. As a result, there are relatively few reports on 91Zr SS NMR in the literature, mainly dealing with zirconiumcontaining inorganic materials11 and metallic alloys12 (for a recent review, see ref 13). Recently developed sensitivity enhancement techniques such as quadrupolar Carr-PurcellMeiboom-Gill (QCPMG)14 and related sequences15 have provided a new opportunity for studying low-γ quadrupolar nuclei. Indeed, a recent study has demonstrated that high-quality 91Zr spectra of organometallic compounds can be obtained by using QCPMG-based techniques at standard magnetic field strengths.16 In the present study, we examined several representative layered ZrPs by 91Zr SS NMR. We have demonstrated that the 91Zr SS NMR spectra of ZrPs can be obtained by using QCPMG type of techniques and that the 91Zr spectra are sensitive to the relatively small changes in the 91Zr local environments for a number of structurally related ZrPs. We have also shown experimentally and theoretically that the correlations between quadrupolar coupling constants and various structural parameters exist and they can be used to obtain valuable information regarding the Zr local coordination environment of the layered ZrPs with unknown structure. Experimental Section Layered zirconium phosphates with R-structure (R-ZrP) were synthesized using the method described by Clearfield and Stynes17 using 8.4 M phosphoric acid solution as the reflux medium and with refluxing for 140 h. After separation of the solids by centrifugation, the product was carefully dried in air at room temperature and kept in tightly sealed glass vials. The

10.1021/jp711137c CCC: $40.75  2008 American Chemical Society Published on Web 05/14/2008

8576 J. Phys. Chem. C, Vol. 112, No. 23, 2008 ammonium ion-exchanged R-ZrP (abbreviated as NH4-ZrP) was prepared by adding concentrated ammonia dropwise to 0.5 g of an R-ZrP powder in 200 mL of water until a final pH of 11 was achieved.18 The sodium-exchanged R-ZrP (abbreviated as Na-ZrP) was prepared by a reaction of R-ZrP with sodium hydroxide solution.19 Layered zirconium phosphate with γ-structure (γ-ZrP) was synthesized according to the procedure described in the literature.20 A meso-structured lamellar ZrP (LZrP) was prepared in the presence of mono-n-dodecyl phosphate [CH3(CH2)11-O-P(O)(OH)2] as a template and phosphoric source. The source of zirconium was zirconyl(IV) nitrate hydrate (ZN). Typically, 1.15 g of ZrN was dissolved in 5.0 g of distilled water at room temperature. To this solution, 2.66 g of CH3(CH2)11-O-P(O)(OH)2 dissolved in 15.0 g of H2O was added. After 1 h of stirring at room temperature, the mixture was transferred to a Teflon-lined autoclave and heated in an oven at 393 K for 24 h. The product was then filtered, washed with ethanol, and dried at room temperature under a vacuum. The identity of the synthesized materials was confirmed by powder X-ray diffraction (Supporting Information Figure S1). Powder X-ray diffraction patterns were recorded on a Rigaku diffractometer equipped with a graphite monochromator using Co KR radiation (λ ) 1.7902 Å). The step size used was 0.02° and the scan range was 5-65° (2θ). Most of the 91Zr SS NMR experiments were conducted on a Varian/Chemagnetics InfinityPlus instrument and a Varian Inova spectrometer operated at 9.4 T (37.26 MHz) and 14.1 T (55.73 MHz), respectively. Additional 91Zr SS NMR spectra were also acquired at 21.1 T (83.72 MHz) on a Bruker Avance-II spectrometer at the National Ultrahigh-Field NMR Facility for Solids in Ottawa, Canada. Varian/Chemagnetics 5 and 3.2 mm HXY MAS probes and a 5 mm HX (static) probe were used. A Bruker 5 mm single channel static probe was also utilized. For 91Zr static experiments, the tightly packed powdered samples were sealed in glass tubes with appropriate diameters (i.e., 3 and 5 mm o.d.). For 91Zr NMR experiments under MAS conditions, instead of commonly used ZrO2 rotors, 5 mm and 3 mm silicon nitride rotors were utilized to avoid the Zr background. The 91Zr chemical shifts were referenced to BaZrO3 with δiso ) 0 ppm11c–f (note that the chemical shift of BaZrO3 relative to the solution of Cp2ZrCl2, another 91Zr chemical shift reference,21 is +317.2 ppm). 91Zr QCPMG14 and DFSQCPMG15a experiments were used to acquire both static and MAS spectra. The central transition (CT) selective π/2 pulse lengths were determined on the setup compound (a concentrated solution of Cp2ZrCl2 in CH2Cl2) and ranged from 1.33 to 1.70 µs, depending on the spectrometer and the probe used. Proton decoupling was applied during acquisition. Although decoupling does not affect the spectral profile and therefore the simulation results, it does lead to the slight line width narrowing in the individual spikelets. Detailed spectrometer conditions and parameters used for acquisition are given in the Supporting Information Table S1. For all the QCPMG spectra (which are quadrupolar-dominated), the frequency scales are in kiloHertz (kHz) units. The spectra with frequency in part per million (ppm) units are also presented in Figure S5 (Supporting Information). 31P MAS NMR experiments were performed at 9.4 T (161.719 MHz) on the Varian/Chemagnetics InfinityPlus 400 WB spectrometer. A 5 mm triple-tuned magic-angle spinning (MAS) probe was used with the sample spinning at 10 kHz. A π/6 pulse of 1.5 µs and a recycle delay of 120 s were used. The 31P chemical shifts were measured relative to 85% phosphoric acid (δ ) 0 ppm), with NH4H2PO4 (δ ) 0.81 ppm) used as a secondary reference.

Yan et al. Numerical simulations of 91Zr QCPMG static and MAS spectra were carried out by using the SIMPSON (version 1.1.0) software package to obtain 91Zr electric field gradient (EFG) and chemical shielding (CS) parameters.22 The simulations were preformed on a LG computer running Red Hat Linux by the direct method of powder averaging using the zcw4180 crystal file provided by the package. The start and detect operators were set, respectively, to I1Z and I1C, while experimental values were employed for all remaining parameters. The experimental error for each measured parameter was determined by visual comparison of the numerically simulated spectrum with observed one.23 In particular, the parameter of concern was varied bidirectionally starting from the best-fit value and all other parameters were kept constant until changes in spectral singularities exceed the separation of spikelets. All theoretical calculations on 91Zr EFG tensors were carried out using the Gaussian98 program24 on a Pentium IV personal computer. A simplified molecular cluster containing the structural unit of interest was used. The cluster was constructed as the following: (1) The cluster model consists of one ZrO6 octahedron and six PO4 tetrahedra. (2) The cluster was terminated at O in the third coordination sphere, and the dangling bonds were saturated by attaching H to O forming two P-OH bonds at each P site. This leads to a {ZrO6P6O18H18}4+ cluster. (3) The cluster adopts the same geometry of R-ZrP determined from X-ray diffraction25 without further optimization. The calculations were performed using restricted Hartree-Fock (RHF) and hybrid density functional theory (DFT) with the B3LYP functional.26 To examine the basis set dependence, several different basis sets (3-21G, 6-31G**, 6-311G**, 3F (33333/333/33), 5F (43333/433/43)) available within the Gaussian98 package were used. The 3-21G basis set was used for all elements. The basis sets of 6-31G** and 6-311G** were applied to all nuclei except Zr, for which expanded all-electron basis sets (3F or 5F) were used. The principle components of the electric field gradient tensor, Vii (ii ) 11, 22, 33; |V33| g |V22| g |V11| and V33 + V22 + V11 ) 0) were computed in atomic units (1 au ) 9.717365 × 1021 V m-2). The quadrupolar coupling constant (CQ) and asymmetry parameter of the EFG tensor (ηQ) are calculated by the following equations

CQ(91Zr) ) (eV33Q/h) × 9.71736 × 1021 (units V · m-2) (1) ηQ ) (V11 - V22)/V33

(2)

where Q is the nuclear quadrupolar moment [Q(91Zr) ) -2.1 × 10-29 m2],27 e is the elementary charge (1.602188 × 10-19 C), and h is the Planck constant (6.6260755 × 10-34 J · s). The effect of distortion in Zr environment of the cluster on CQ was evaluated by systematically changing a single bond angle or bond distance. More details are given in the text. We realized that there are several programs that can be used to calculate the electronic properties of periodic solids such as WIEN2k, using a full potential linearized augmented plane wave approach,28 and CASTEP, which employs DFT with a plane wave basis set for calculation from first principles.29 However, these software packages are not readily available to us at the present time. Results and Discussion A variety of layered zirconium phosphates have been investigated in the present study to evaluate the sensitivity of 91Zr SS NMR spectra to the Zr local environment and the possible correlations between quadrupolar coupling constant and the structure parameters.

Layered Zirconium Phosphates

Figure 1. Graphical representations of layered ZrP materials. For clarity, the hydrogen atoms of NH4+ in NH4-ZrP and of water in γ-ZrP and Na-ZrP are not shown.

r-ZrP. The most important layered zirconium phosphate is R-Zr(HPO4)2 · H2O (R-ZrP),17 from which numerous layered ZrP derivatives with interesting properties have been obtained by ion exchange, intercalation, and pillaring.5a It is one of the most extensively investigated zirconium phosphates.5a The structure of R-ZrP was determined by Clearfield and Troup.25 It crystallizes in the monoclinic system. Each layer contains a single sheet of octahedral Zr atoms (Figure 1A). Each Zr atom is octahedrally coordinated to six oxygen atoms belonging to six PO4 tetrahedra with each P atom tetrahedrally coordinated to three oxygens shared with three different ZrO6 octahedra arranged in the form of an equilateral triangle and to one hydroxyl oxygen (Figure 1E).25 There is only one Zr lattice site. Figure 2A illustrates the 91Zr QCPMG MAS spectra at the field strengths of 9.4 and 14.1 T. Spinning the sample at the magic angle averages out 91Zr chemical shift anisotropy (CSA). It also partially averages the second-order quadrupolar interaction. The QCPMG MAS spectra exhibit a typical line shape due to the second-order quadrupolar interaction. The CT spectra at both fields are well-simulated numerically with a single set of parameters with CQ ) 5.8 MHz, ηQ ) 0.27, and δiso (isotropic chemical shift) ) -385 ppm. In general, to understand the CT (+1/2, -1/2) spectra of a half-integer quadrupolar nucleus, the second-order quadrupolar and CS interactions should both be considered. To obtain CS tensor parameters (span (Ω), skew (κ)), the static spectra at three different field strengths were acquired (Figure 2B). By use of the CQ, ηQ, and δiso values extracted from the MAS spectra, the static spectra obtained at three fields can be well simulated numerically with the same set of parameters (Figure 2B). The span of the CS tensor (Ω ) 10 ppm) is very small, confirming that the observed patterns are quadrupolar-dominated. However, this small CSA is necessary for fitting the spectra, especially the one obtained at the highest field. The skew (κ ) 0.2) is nonaxial, suggesting that the CS tensor does not have a pseudounique component. The good agreement between simulated and experimental patterns

J. Phys. Chem. C, Vol. 112, No. 23, 2008 8577 reveals that the measured CS and quadrupolar parameters are reliable. The CS tensor parameters and the Euler angles are given in Table 1. Observing a sizable CQ is not surprising since the Zr is located at a general position and the ZrO6 is not a perfect octahedron. However, although the observed spectra are dominated by the second-order quadrupolar interaction, the value of CQ (5.8 MHz) is actually rather small compared with the values of many inorganic Zr compounds reported in the literature (for example, zirconia (CQ ) 16-19.1 MHz),11d,g Na2ZrO3 (CQ ) 14.6 MHz);11e Na2ZrSiO5 (CQ ) 29.4 MHz);11e ZrSiO4 (CQ ) 20.5 MHz);11f Al3Zr (CQ ) 7.3 MHz)12e). This relatively small CQ (for 91Zr) is due to the fact that the distortion in ZrO6 is small.25 The largest deviation of the O-Zr-O bond angles from 90° is only 1.14°, and the variation in the O-Zr-O bond angle is only within 2.26°. The largest difference in Zr-O bond length is merely 0.027 Å. The value of ηQ is 0.27, implying that the V33 component may be viewed as the pseudounique component of the EFG tensor. As mentioned earlier, the deviation of ZrO6 in R-ZrP from a perfect octahedron is small. If only ZrO6 is considered, several approximate local C4 and C3 axes exist. However, if the P atoms in the second coordination sphere are included (i.e., considering the Zr(OP)6 unit), there is only one unique pseudo-three-fold axis (perpendicular to the two triangle faces defined by the six P atoms bound to O5, O6, and O11 and O4, O8, and O9 (Figure 1F)). It seems that the V33 direction may lie approximately along this unique pseudo C3 axis. NH4-ZrP. Starting from R-ZrP, one can prepare Zr(NH4PO4)2 · H2O where the acidic protons in R-ZrP are replaced by NH4+ ions. Although this derivative has been frequently referred to as “ammonium-exchanged phase”, it actually cannot be prepared by direct ion-exchange with NH4+ ions simply because ammonium ions cannot diffuse freely into the interlamellar spacing of R-ZrP and therefore do not exchange with protons other than those on the crystal surface.30 R-ZrP, however, is readily intercalated by ammonia to yield Zr(NH4PO4)2 · H2O. Zr(HPO4)2 · 2NH3 · H2O is a formula which describes the origin of the material more precisely, and this material is a model compound of the intercalated zirconium phosphates. The structure of NH4-ZrP has been determined by single crystal X-ray diffraction and belongs to the space groups P21/c.18 There is only one crystallographic Zr site. 91Zr static QCPMG spectra of NH -ZrP obtained at 9.4 and 4 14.1 T are presented in Figure 3A along with the simulated spectra. The 91Zr NMR line shapes of the static samples of NH4-ZrP look significantly different from those of the parent R-ZrP. Upon intercalation, the resonance became much wider than that of the parent compound at a given field. The breadth of the CT pattern (ca.135 kHz) of NH4-ZrP at 9.4 T is more than 4 times larger than that of R-ZrP (ca. 30 kHz). The patterns at both fields were broad relative to the achievable MAS spinning speeds with the available MAS probes. Spectral simulations indicate that the anisotropic broadening of CT due to the second-order quadrupole interaction is much pronounced after ammonia intercalation (Table 1). Particularly, the quadrupolar coupling constant (10.20 MHz) obtained from simulation is significantly larger than that of parent R-ZrP. Single crystal X-ray structure shows (Figure 1B) that in NH4-ZrP, the layered structure of R-ZrP is essentially retained with the layers being spread apart to accommodate the ammonium ions (the interlayer distance increased from 7.56 to 9.40 Å after ammonia intercalation).5a However, the local environment around Zr atom has undergone appreciable change during the intercalation. For example, the O-Zr-O bond angles are much more deviated

8578 J. Phys. Chem. C, Vol. 112, No. 23, 2008

Figure 2.

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Zr QCPMG NMR spectra of R-ZrP at different magnetic fields together with simulated spectra.

TABLE 1: Summary of Observed Parameters for 91Zr Chemical Shift and Quadrupole Coupling Tensors in Layered ZrP Materialsa sample

CQ/MHzb

ηQc

δiso/ppmd

Ω/ppme

κf

R

β

γ

R-ZrP γ-ZrP NH4-ZrP Na-ZrP, Zr1 Na-ZrP, Zr2 meso-lamellar ZrP

5.80(2) 9.20(5) 10.20(3) 7.81(12) 6.55(15) 12.00(3)

0.27(1) 0.13(2) 0.70(1) 0.97(2) 0.99(1) 0.41(3)

-385(5) -390(10) -405(10) -395(15) -400(20) -430(30)

10(2) 20(3) 80(40) 60(30) 30(20) 350(40)

0.2(1) 0.5(5) 0.4(6) 0.3(2) -0.5(5) 0.7(1)

80(10) 20(10) 0(5) 20(15) 10(10) 10(10)

50(5) 50(10) 10(15) 50(10) 30(10) 1(1)

5(5) 0(10) 0(5) 10(12) 10(30) 5(5)

a The CS tensor is described by three principal components ordered such that σ11 e σ22 e σ33. The EFG tensor is described by three principal components ordered such that |V11| e |V22| e |V33|. b CQ ) eQV33/h. c ηQ ) (V11 - V22)/V33. d δjj ) (σiso,ref - σjj)(106)/(1 - σiso,ref) ≈ σiso,ref - σjj, where jj ) 11, 22, or 33, δiso ) (δ11 + δ22 + δ33)/3. e Ω ) δ11 - δ33. f κ ) 3(δ22 - δiso)/Ω. The uncertainty in the last digits of each value is denoted in parentheses.

from ideal octahedral O-Zr-O angles with a maxima deviation of 9° compared with that of 1.14° in R-ZrP. Although a mean Zr-O bond distance of 2.055 Å was observed in NH4-ZrP, which is comparable to that of the parent compound (2.065 Å), the individual bond lengths are very different from each other with the minimum and maximum values being 1.99 and 2.15 Å, respectively. The largest deviation in Zr-O bond distance found in NH4-ZrP (0.16 Å) is almost 6 times greater in magnitude than that in R-ZrP (0.027 Å). Apparently, the more distorted octahedral Zr coordination environment in NH4-ZrP results in a much larger CQ value. The value of ηQ ()0.70) indicates that the EFG tensor is far from axial symmetrical, which is consistent with the fact that the pseudo-three-fold rotation axis in the parent R-ZrP mentioned earlier no longer exists after intercalation. To best fit the observed QCPMG spectra of the stationary samples at two fields requires the

inclusion of the CS tensor with Ω ) 80 ppm and κ ) 0.4. The small value of β suggests that V33 and σ33 are closely oriented (Table 1). γ-ZrP. The first zirconium phosphate with a layered structure of γ-type [γ-Zr(PO4)(H2PO4) · 2H2O] was prepared 4 years after that of the R-type.20 The structure of γ-ZrP (determined by powder X-ray diffraction) is more complex than that of the R-type.31 The layer consists of a rigid framework of ZrO6 octahedra placed in two different planes and joined to each other with PO4 tetrahedra inside and H2PO4 outside these planes (Figure 1C). The octahedral coordination of the Zr atom is completed by four oxygen atoms of the phosphate group (P1) and two oxygen atoms of the dihydrogenphosphate group (P2) (Figure 1C). The remaining two oxygens of the dihydrogenphosphate group bind to protons and project into the interlayer space. These hydroxyl groups are hydrogen-bonded to the water


Figure 3.

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J. Phys. Chem. C, Vol. 112, No. 23, 2008 8579

Zr QCPMG NMR spectra of (A) NH4-ZrP and (B) γ-ZrP at different magnetic fields together with simulated spectra.

molecules. There is only one type of Zr atom in the unit cell. This material exhibits interesting properties such as intercalation, ion exchange, and protonic conduction. Since γ-ZrP undergoes dehydration very easily during the sample spinning as evident from the 31P MAS NMR spectra (not shown), it is not possible to acquire a meaningful 91Zr QCPMG MAS spectrum. As a result, only static 91Zr QCPMG experiments were performed. To more accurately determine the quadrupolar and CS tensor parameters, static QCPMG spectra were acquired at two applied fields (9.4 and 14.1 T). As shown in Figure 3B the breadth of the CT spectra for γ-ZrP at both fields is appreciably broader than that of R-ZrP. The simulation reveals that the value of CQ is 9.20 MHz, compared with that of 5.80 MHz for R-ZrP. This larger CQ value for γ-ZrP can be rationalized by considering its structure. Since the octahedral Zr atoms in R- and γ-ZrP have identical atoms in the first (six oxygen atoms), second (six phosphorus atoms), and third (six oxygen atoms) coordination spheres, the difference in CQ results mainly from the distortions in the local ZrO6 octahedral environment. A close inspection of crystal structure reveals that the largest deviation in the O-Zr-O bond angle from an ideal octahedral angle increases from 1.14° in R-ZrP to 5.8° in γ-ZrP. The Zr-O bond lengths deviate up to 0.072 Å in γ-ZrP, compared with 0.027 Å in R-ZrP. Furthermore, the mean Zr-O bond lengths are 2.065 and 2.041 Å for R-ZrP and γ-ZrP, respectively. Although the difference is small, the slightly shorter average Zr-O bond length in γ-ZrP may also contribute to its larger CQ value since in a simple point charge model, the magnitude of V33 is proportional to 1/r3.32 The value of ηQ ()0.13) is closer to zero than the unity value, indicating that V33 is the pseudounique component of the EFG tensor. An examination of the crystal structure shows that there is a unique pseudo-C3 axis perpendicular to the two triangles defined by O1—O3—O5 and O2—O4—O6. It is likely that the V33 is approximately orientated along this pseudo-C3 axis. The skew of 0.5 suggests that the value of δ11 and δ22 are comparable (e.g., -381.7, -386.7, and -401.7 ppm were determined for δ11, δ22, and δ33, respectively). The Euler angles (Table 1) indicate that the largest component of the EFG tensor, V33, is not coincident with the least shielding direction of CS tensor, δ11. Na-ZrP. R-ZrP is also an insoluble ion exchanger with a number of interesting applications. The ion exchange behavior of R-ZrP has been studied extensively.5 Ion exchange occurs by replacement of the orthophosphate protons with a variety of alkali-metal, alkaline-earth-metal, and transition-metal cations. These cations then occupy positions between the layers accompanied by layer expansion to accommodate the ions and water molecules. Such structural changes should be manifested

in the corresponding 91Zr SS NMR spectra after ion exchange. Here, we present the 91Zr QCPMG NMR spectra of the fully sodium ion-exchanged phase of R-ZrP, Zr2(NaPO4)4 · 6H2O (Na-ZrP). The crystal structure of Na-ZrP was determined by powder X-ray diffraction,19 which belongs to the triclinic space group P1. Since the water content in the fully exchanged Na-ZrP depends on the relative humidity and temperature, spinning the sample even in a very short period results in dehydration as indicated by XRD pattern and 31P MAS NMR spectra (not shown), which leads to a significant broadening in the 91Zr signals. Consequently, only stationary spectra were obtained (Figure 4). The XRD data indicate that although the layers of the parent R-ZrP remain intact, there are now two crystallographically nonequivalent Zr sites in the unit cell (Figure 1D). The ideal approach to confirm the presence of multiple Zr sites is to acquire 91Zr MQMAS spectra at a high field. However, the dehydration caused by sample spinning precludes us from performing such experiments. Nonetheless, the three static spectra acquired at 9.4, 14.1 and 21.1 T can all be best fitted by including two Zr sites. The quadrupolar coupling parameters and chemical shift anisotropy are given in Table 1. The CQ values of two Zr sites differ by 19% and are larger than that of the parent material. To assign the 91Zr signals to the two crystallographically nonequivalent sites, we need to examine the relationship between the quadrupolar parameters and the distortion in ZrO6. A survey in literature revealed that for the metal centers in many octahedral compounds, the size of quadrupolar coupling constant can be correlated to the distortions in the octahedral site.13,33–35 Several parameters have been used for estimating the relative degree of distortion around an octahedral site. The most commonly used angular parameter is the shear strain (|ψ|)33b

|ψ| )

∑ i12 | tan(θi - θ0)|

(3)

where the sum runs over the actual O-Zr-O angles θi and θ0 is the ideal value (90°). Another simple angular parameter is the distortion index (DI):36

DI )

∑ i12 |θi - θ0|/12θ0

(4)

These parameters have been used to correlate with CQ values in 25MgO6,33c 67ZnO6,34 27AlO6,33b,d,35 and 49TiO633e,f octahedral sites. The above work in the literature shows that a more distorted site with a larger distortion parameter has a large CQ value and such a relation has been employed to assign the different 27Al sites in microporous materials35 as well as the multiple 27Al33b,d and 25Mg33c sites in minerals. The |ψ| and DI


Figure 4.

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Zr QCPMG NMR spectra of Na-ZrP at different magnetic fields together with simulated spectra.

TABLE 2: Structural Parameters of Layered ZrP Materials samples

R-ZrP

γ-ZrP

NH4-ZrP

quadrupole coupling constant CQ (MHz) shear strain parameter (|ψ|) distortion index (DI)

5.80(2)

9.20(5)

10.20(3)

0.198

0.802

1.249

0.0069

0.0296

0.0491

Na-ZrP 7.81(12) 6.55(15) 0.8614(Zr1) 0.544(Zr2) 0.0340(Zr1) 0.0202(Zr2)

values of the Zr sites in R-ZrP, γ-ZrP, NH4-ZrP, and Na-ZrP were calculated from their crystal structures (Table 2). For the first three compounds with only a single Zr site, their distortion parameters are compared to their corresponding CQ values in Table 2. The data follow the trend mentioned above; i.e., the less distorted site has a smaller quadrupolar coupling constant. On the basis of the discussion above, we tentatively assign the 91Zr signal with a smaller C value of 6.55 MHz to Zr(2), which Q is a less distorted site (Table 2). Correlation between 91Zr CQ and Structural Parameters. In SS NMR, the quadrupolar coupling constant is an important parameter which can be related to the bonding and structure. Numerous studies of solid materials involving quadrupolar nuclei have focused on the correlation of CQ with various structural parameters.13,33–35,37 One of the objectives of this work is to examine the possible relationship between 91Zr quadrupolar parameters extracted from the observed spectra and structure (e.g., the local distortion) in a series of structurally related layered zirconium phosphates. (a) Correlation between CQ and Angular Distortion Parameters. For the five points available, very good linear relationships between CQ and octahedral angular distortion parameters such as |ψ|, DI, and mean O-Zr-O angle (ϑ) were established (Figure 5A-C).

CQ (MHz) ) 4.32|ψ| + 4.76

(5)

CQ (MHz) ) 105.73DI + 4.97

(6)

CQ (MHz) ) -6.24ϑ + 565.81

(7)

In the literature, a linear relationship between CQ and ψ or DI has been reported for 25MgO6,33c 27AlO633b,35 and 49TiO633e,f sites. It is noted that in the present case of 91ZrO6 although

Figures 5B-C demonstrate an excellent linear relationship of CQ with both angular distortion parameters, the straight line does not pass through the origin in each case. This indicates that for this series of zirconium phosphates, the effects of the EFG are complex and cannot be completely described by the oversimplified relationship between CQ and angular distortion of the first coordination sphere alone. Other factors such as the geometrical arrangements of the P and O atoms in the second and third coordination sphere, the position, and the nature of the cations within the layer and lattice effects may also contribute to the EFG at the 91Zr nucleus. Nonetheless, our results show that angular distortion around Zr does contribute systemically toward CQ. A similar situation was also reported for 49TiO6.33e,f (b) Correlation between CQ and Bond Length Distortion Parameters. An attempt in correlating CQ with the Zr-O bond distances was also made. The plot of CQ vs mean Zr-O distance suggests that there appears no obvious correlation (Supporting Information Figure S2A). A different bond length distortion parameter, the longitudinal strain |R|,33b defined as

|R| )

∑ i12 | ln(li/l0)|

(8)

where 1i is the individual octahedral bond length and 10 is the “ideal” bond length (a perfect polyhedron with bond length having the same volume as the coordination polyhedron) is also used; the data are still rather scattered (Supporting Information Figure S2B). (c) Correlation between CQ and Geometry of Zr(OP)6. Since the P atoms in the second coordination sphere are also likely to affect the differences in the CQ value for the layered zirconium phosphates examined, the geometry involving Zr(OP)6 is also considered. A trend of CQ increasing with the mean Zr-O-P angle (ϑ) (Figure 5D) appears to exist:

CQ (MHz) ) 0.42ϑ - 57.32

(9)

It seems that a larger mean Zr-O-P angle corresponds to a larger CQ value. The possible correlation of CQ and mean Zr-P distances was also sought. Figure 5E suggests an approximate



Figure 5. Correlations between experimental quadrupolar coupling constant, CQ (91Zr) of layered ZrP materials and various structural parameters.

linear relationship between CQ and the mean Zr-P bond distance (in Å):

CQ (MHz) ) 37.41r(Zr-p) - 122.90

(10)

(d) Correlation between CQ and Isotropic Chemical Shift. Although the bond length distortion parameters are insensitive to CQ, there exits a good correlation between |R| and 91Zr isotropic chemical shift (δiso) (Figure 5F):

δiso (ppm) ) -226.23|R| - 377.89

(11)

Theoretical Calculations. Since the above-mentioned empirical relationships are only based on a limited number of data points, it is very important to verify the observed trends by theoretical calculations. A previous study by Hung and Schurko has shown that 91Zr electric field gradient tensors can be calculated reasonably well with an ab initio calculation method.16 In this work we have carried out quantum mechanical calcula-

tions to confirm the empirical correlations of a quadrupolar coupling constant with various structural parameters. Our approach is to construct a model cluster based on the geometry from crystal structure of R-ZrP and then systematically evaluate the effect of a specific structural parameter (such as ∠O-Zr-O, ∠P-O-Zr, and Zr-O distance) on EFG tensors by theoretical calculations. An octahedral cluster of Zr{O[P(OH)(-O)2]}6 was first cut off from the periodic structure of R-ZrP, where the Zr atom is at the center and the oxygen and phosphorus are then placed around it by using their crystallographic coordinates determined by single crystal diffraction.25 The cluster has two crystallographically nonequivalent P sites (P2 and P3) with equal occupancy. Each P site is bound to a hydroxyl group with the O-H distances being 0.92 and 0.61 Å for P2 and P3, respectively.25 For each P site, the two oxygen atoms attached to two other Zr atoms in the fourth coordination sphere in R-ZrP


Yan et al.

Figure 6. Structural representation of the Zr{O[P(OH)3]}64+ cluster investigated in the theoretical calculations and the EFG tensor orientation within the molecular frame.

TABLE 3: Theoretical Zirconium Quadrupolar Parameters in a Model Cluster Based on r-ZrP source

V11 (a.u.)

V22 (a.u.)

-

Experimental -

3-21G 6-31G**/5F(43333/433/43) 6-311G**/5F(43333/433/43) 6-31G**/3F(33333/333/33) 6-311G**/3F(33333/333/33)

0.041296 0.012998 0.01318 0.013127 0.007031

RHF 0.078987 0.048784 0.045330 0.058285 0.072901

3-21G 6-31G**/5F(43333/433/43) 6-311G**/5F(43333/433/43) 6-31G**/3F(33333/333/33) 6-311G**/3F(33333/333/33)

0.018909 0.031321 0.028651 0.017848 0.009384

DFT 0.067057 0.039993 0.040618 0.079704 0.103462

are terminated by hydrogen atoms placed along the O-Zr bond direction with an O-H distance equal to the O-H bond length of the hydroxyl group bound to the same P determined experimentally. The final structure of the cluster, Zr{O[P(OH)3]}64+, is shown in Figure 6A. The EFG tensors of this cluster were then calculated by RHF and hybrid DFT with the B3LYP functional using several basis sets, and the results are presented in Table 3. A good agreement between experimental and theoretical values (for both CQ and ηQ) was obtained when the RHF/3-21G method was employed. However, all other ab initio and hybrid DFT calculations underestimate the magnitude of the quadrupolar coupling constant. For the DFT method, a better match between theoretical and experimental values of CQ can be reached by using the 6-311G**/3F basis set, though a large value of asymmetry parameter is obtained. A previous study also indicated that RHF calculations provide a better prediction of 91Zr EFG parameters.16 The experimentally determined ηQ value of 0.27 suggests a nearly axial 91Zr EFG tensor in R-ZrP, implying that V11 and V22 are similar in magnitude. On the basis of the crystal structure, we suggested earlier that the V33 direction is likely along a unique pseudo-three-fold axis perpendicular to the two opposing triangle faces defined by the P atoms bound to O5, O6, and O11 and O4, O8, and O9 (Figure 1F). This prediction agrees with the RHF/3-21G calculation (which results in the best agreement between theoretical and experimental EFG tensor parameters). As shown in Figure 6B, the V33 component indeed orients approximately perpendicular to the O5-O6-O11 plane (the angle between the normal of the plane defined by the three P atoms bound to O5, O6, and O11 and the V33 direction is 15°). Consequently, V11 and V22 lie approximately parallel to

|CQ| (MHz)

ηQ

-

5.80(2)

0.27(1)

-0.120283 -0.061782 -0.058510 -0.071413 -0.079932

5.92 3.04 2.88 3.52 3.94

0.31 0.58 0.55 0.63 0.82

-0.085966 -0.071314 -0.069269 -0.097552 -0.112846

4.23 3.51 3.41 4.8 5.56

0.56 0.12 0.17 0.63 0.83

V33 (a.u.)

the O5-O6-O11 plane (Figure 6B). The tensor component corresponding to the least magnitude, V11, lies in the plane of O11-Zr-V33 (i.e., the angles of V11-Zr-O11 and V33-Zr-O11 are 31.55° and 58.55°, respectively). The V22 component almost lies in the O11-Zr-O4 plane (the bond angle of O11-Zr-O4 is 178.84°) leading to the bond angles of V22-Zr-O11 and V22-Zr-O4 of 92.65° and 86.89°, respectively. To better understand the empirical relationships between CQ and structural parameters presented earlier, we further conducted a series of theoretical calculations. We first investigated the effect of the O-Zr-O bond angle on the 91Zr CQ value. This was done by systematically changing the value of a single O-Zr-O angle of the cluster (i.e., the ∠O9-Zr-O11 angle), and the results of calculations using the DFT and RHF methods with different basis sets are given in Figure 7A and Supporting Information Table S2. Figure 7A shows clearly that regardless of the calculation methods and basis sets used, the same trend was observed for all the calculations; i.e., the calculated CQ value has a minimum value at 89.54° which is the original O9-Zr-O11 bond angle in R-ZrP. Either increasing or decreasing the O9-Zr-O11 angle results in an increase in the value of CQ. For each given O9-Zr-O11 angle, the shear strain, |ψ|, of the cluster was calculated. Figure 7B and Supporting Information Figure S3 illustrate the dependence of CQ as a function of |ψ|. It is evident that the calculated CQ values from each calculation can be classified into two groups: one corresponds to the O9-Zr-O11 bond angles smaller than 90° and the other corresponds to bond angles greater than 90°. Both sets of CQ data exhibit approximately a linear correlation with the |ψ|, although the slopes are different. These trends are parallel to the experimental results shown in Figure 5C.



Figure 7. Correlations between the calculated CQ (91Zr) value of the model cluster and various structural parameters.

We also conducted calculations to probe how the position of P atoms in the second coordination sphere affects the CQ value by systematically changing one of the Zr-O-P bond angles (i.e., Zr-O4-P2). Supporting Information Table S3 shows the calculation results obtained by using RHF/3-21G and DFT/321G methods. Figure 7C illustrates the calculated CQ values as a function of mean Zr-O-P bond angle calculated for each given Zr-O4-P2 bond angle. Again, a minimum value of CQ was found at the original Zr-O4-P2 angle of 160.63°, giving the mean Zr-O-P angle of 150.81°. Either increasing or decreasing the Zr-O4-P2 angle leads to an increase in the magnitude of CQ. Establishing a theoretical relationship between CQ and the mean Zr-O-P angle supports the empirical correlation shown in Figure 5D. The changes in the principal components of the 91Zr EFG tensors were also studied as a function of Zr-O bond distance. First, only one Zr-O bond distance (i.e., Zr-O11) was systematically varied. The results are shown in Figure 7D. There

is also a minimum CQ value at the original Zr-O bond distance of R-ZrP, indicating that CQ increases with increasing the degree of distortion in ZrO6 octahedron caused by unevenly changing the Zr-O bond length. We also performed the calculations by changing all the Zr-O bond lengths to the same extent simultaneously. In this way, the overall geometry of the cluster is retained. The results are shown in Supporting Information Figure S4 and Table S4. As expected, the magnitude of V33 monotonically decreases with increasing Zr-O bond lengths in a linear fashion. The value of ηQ does not change for a given method with a specific basis set. The above calculation results agree with observed empirical correlations of CQ value with various structural parameters. They also suggest that the Zr in R-ZrP appears to have an optimized local environment, yielding a small CQ value. Any distortion introduced during structural modification such as intercalation and ion exchange will inevitably lead to a larger CQ value as seen in NH4-ZrP and Na-ZrP.


Yan et al.

Figure 8. Meso-lamellar ZrP: (A) Powder XRD pattern; (B) 31P MAS NMR (* indicates spinning sidebands); and (C) static 91Zr QCPMG spectra at different magnetic fields together with simulated spectra.

Meso-lamellar-ZrP. The relationships between CQ and various structural parameters established both experimentally and theoretically can yield valuable structural information in situations where X-ray crystal structures are not available, as is often the case for the materials under investigation. To illustrate this point, we examined a novel meso-lamellar zirconium phosphate-based material. Recently, mesostructured zirconium phosphate-based materials with hexagonal and lamellar structures have received considerable attention due to their practically important applications.38 We have recently successfully prepared a mesostructured ZrP with a lamellar structure through surfactantassisted synthesis. The powder XRD pattern (Figure 8A) indicates a highly ordered meso-lamellar phase (d100 ) 32.3 Å), and the 31P MAS spectrum shows only one sharp resonance at -20.4 ppm, indicating only one P site (Figure 8B). To characterize the local Zr environment, we acquired 91Zr static QCPMG NMR spectra at two fields. The spectra at 9.4 and 21.1 T can be well-simulated by one set of quadrupolar and CS parameters, suggesting that there is only a single Zr site (Figure 8C). If there are more than one magnetically nonequivalent Zr nuclei, their environments must be very similar. The large breadth of 91Zr spectra clearly suggests a more distorted local environment around the metal center in mesostructured phase. Fitting the powder CT pattern yielded a relatively large quadrupole coupling constant (CQ ) 12.0 MHz) and CSA value (Ω ) 350 ppm). With the use of eqs 5 and 11, the shear and longitudinal strain were calculated to be 1.68 and 0.23. The large values of |ψ| and |R| indicate that the ZrO6 octahedra in the meso-lamellar phase experience the largest distortion in both bond length and angle compared with the layered ZrPs examined earlier. The average O-Zr-O angle is 88.75°. The mean Zr-P distance and Zr-O-P angle were estimated to be 3.601 Å and 165.1°, respectively. Conclusion In this work, we have successfully acquired the 91Zr NMR spectra of several representative layered zirconium phosphates

at several field strengths by using recently developed sensitivity enhancement techniques for low-γ quadrupolar nuclei such as QCPMG and related DFS-QCPMG. The observed spectra allow the 91Zr NMR interaction tensors to be extracted via spectral simulations. In this series of related layered ZrP-based materials, the quadrupolar parameters, in particular the quadrupolar coupling constants, are shown to be sensitive not only to the distortion in the ZrO6 polyhedron but also the spatial arrangement of the P atoms in the second coordination sphere (i.e., the configuration of Zr(OP)6 unit). The EFG tensors at the Zr site correlate well with the structural parameters reflecting the distortion in octahedral bond angles including mean O-Zr-O angle, distortion index, and shear strain. CQ seems less sensitive to the bond length parameters such as longitudinal strain (|R|) and mean Zr-O distance. But a good correlation between 91Zr isotropic chemical shift and |R| does exist. When the atoms in the second coordination sphere are considered, approximate relationships between CQ and the parameters defining the geometry of Zr(OP)6 unit such as the mean Zr-P distances and Zr-O-P angles also exist. Zirconium EFG tensors of a model cluster based on the structure of R-ZrP were calculated using RHF and DFT (B3LYP) methods with the best agreement between experiment and theory given by RHF/3-21G calculation. The effects of distortion of the Zr environment in the cluster on the CQ value were also evaluated by RHF and DFT calculations via individually varying a single structural parameter (Zr-O distance, ∠O-Zr-O, ∠Zr-O-P) in a systematic fashion. The results of the theoretical calculations regardless of the methods and basis sets used clearly support the empirical correlations. The empirical relationships between NMR parameters and various structural properties which were verified theoretically are important since these correlations can be used for spectral assignments for a ZrP with multiple sites and for obtaining partial structural information if the structure is unknown. As mentioned earlier, the structure of many cation-exchanged and intercalated/pillared layered zirconium phosphates are either unknown or poorly described. The 91Zr NMR properties

Layered Zirconium Phosphates presented here and the fact that 91Zr QCPMG NMR spectra of this type of material can be acquired at standard field strengths (9.4-14.1 T) suggests that 91Zr NMR is an attractive approach for characterizing this type of material. It allows one to directly probe the metal centers in the layers and provide information complementary to that obtained from powder XRD and 31P NMR as demonstrated in this work via a meso-structured lamellar ZrP. Acknowledgment. Y.H. thanks the Natural Science and Engineering Research Council of Canada for a research grant and the Canada Foundation for Innovation for an equipment grant. Funding from the Canada Research Chair and Premier’s Research Excellence Award programs is also gratefully acknowledged. The authors wish to thank Professor R. W. Schurko (University of Windsor) for providing QCPMG and DFS/ QCPMG pulse sequences and helpful discussion. Access to the 900 MHz NMR spectrometer was provided by the National Ultrahigh Field NMR Facility for Solids (Ottawa, Canada), a national research facility funded by the Canada Foundation for Innovation, the Ontario Innovation Trust, Recherche Que´bec, the National Research Council Canada, and Bruker BioSpin and managed by the University of Ottawa (http://www.nmr900.ca). We thank Dr. Victor Terskikh for technical assistance. Supporting Information Available: Additional experimental results (five figures and four tables). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Kumar, C. V.; Bhambhani, A.; Hnatiuk, N. In Handbook of Layered Materials; Auerbach, S. M., Carado, K. A., Dutta, P. K., Eds.; Marcel Dekker, Inc., New York, 2004; pp 313-372. (b) Marek. I. New Aspects of Zirconium Containing Organic Compounds; Springer GmbH: Berlin, 2005. (c) Marek, I. Titanium and Zirconium in Organic Synthesis; WILEY-VCH Verlag GmbH: Weinheim, Germany, 2002. (d) Di Monte, R.; Kaspar, J. Catal. Today 2005, 100, 27–35. (e) Reddy, B. M.; Sreekanth, P. M.; Reddy, V. R. J. Mol. Catal. A: Chem. 2005, 225, 71–78. (2) (a) Basu, B. Int. Mater. ReV. 2005, 50, 239–256. (b) BocanegraBernal, M. H.; de la Torre, S. D. J. Mater. Sci. 2002, 37, 4947–4971. (3) (a) Thompson, D. P. Br. Ceram. Trans. 2003, 102, 185–192. (b) Kim, S. W.; Abdel-Razek Khalil, K. J. Am. Ceram. Soc. 2006, 89, 1280– 1285. (c) Chen, Q. Z.; Boccaccini, A. R.; Zhang, H. B.; Wang, D. Z.; Edirisinghe, M. J. J. Am. Ceram. Soc. 2006, 89, 1534–1539. (4) (a) Kerrs, J. A. Fuel Cell 2005, 5, 230–247. (b) Mamak, M.; Coombs, N.; Ozin, G. J. Am. Chem. Soc. 2000, 122, 8932–8939. (c) Thampan, T. M.; Jalani, N. H.; Choi, P.; Datta, R. J. Electrochem. Soc. 2005, 152, A316–A325. (d) Jiang, S. P.; Chan, S. H. Mater. Sci. Technol. 2004, 20, 1109–1118. (5) (a) Clearfield, A.; Costantino, U. In ComprehensiVe Supramolecular Chemistry; Alberti, G., Bein, T., Eds.; Elsevier: Oxford, U.K., 1996; Vol. 7, pp 107-149. (b) Clearfield, A. Comments Inorg. Chem. 1990, 10, 89– 128. (c) Oliverra-Pastor, P.; Maireles-Torres, P.; Rodriguez-Castellon, E.; Jimenez-Lopez, A.; Cassagneau, T.; Jones, D. J.; Roziere, J. Chem. Mater. 1996, 8, 1758–1769. (d) Clearfield, A. Chem. ReV. 1988, 88, 125–148. (e) Clearfield, A. SolVent Extr. Ion Exch. 2000, 18, 655–678. (6) Alberti, G.; Mascar´os, S. M.; Vivani, R. Mater. Sci. Forum 1994, 152 (153), 87–98. (7) (a) Curini, M.; Rosati, O.; Costantina, U. Curr. Org. Chem. 2004, 8, 591–606. (b) Alberti, G.; Casciola, M.; Costantino, U.; Vivani, R. AdV. Mater. 1996, 8, 291–303. (c) Ye, G.; Janzen, N.; Goward, G. R. Macromolecules 2006, 39, 3283–3290. (d) Haddix, G. W.; Narayana, M. Chem. Ind. 1994, 55, 311–360. (8) (a) Vermeulen, L. A.; Thompson, M. E. Nature 1992, 358, 656. (b) Dutta, P. Nature 1992, 358, 621. (c) Vermeulen, L. A.; Thompson, M. E. Chem. Mater. 1994, 6, 77–81. (9) (a) Clearfield, A. Curr. Opin. Solid State Mater. Sci. 1996, 1, 268– 278. (b) Clearfield, A. Chem. Mater. 1998, 10, 2801–2810. (c) Zhang, B.; Clearfield, A. J. Am. Chem. Soc. 1997, 119, 2751–2752. (d) Clearfield, A.; Sharma, C. V. K.; Zhang, B. Chem. Mater. 2001, 13, 3099–3112. (10) (a) Clayden, N. J. J. Chem. Soc., Dalton Trans, 1987, 1877–1881. (b) MacLachlan, D. J.; Morgan, K. R. J. Phys. Chem. 1990, 94, 7656– 7661. (c) Segawa, K.; Nakajima, Y.; Nakata, S.; Asaoka, S.; Takahashi, H. J. Catal. 1986, 101, 81–89.

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