Pure-phase two-dimensional niobium-93 nutation spectroscopic study of lead metaniobate and the piezoelectric lead magnesium niobate

An NMR technique to measure pure-phase two-dimensional nutation NMR spectra, that yields higher resolution than traditional nutation experiments is reported. Using this technique 93Nb nutation NMR spectra of PbNb2O6 and the technologically important Pb(Mg1/3Nb2/3)O3 (PMN) have been measured and the quadrupolar coupling constant of the niobium site in PbNb2O6 (C(Q) = 19 +/- 2 MHz) determined. Estimates of the quadrupolar coupling constants for three different resonances associated with different niobium(V) sites in PMN (C(Q) < 1.2 MHz, approximately 17 MHz, and > 62 MHz) are also reported.     

Strategies for extracting NMR parameters from 23Na MAS, DOR and MQMAS spectra. A case study for Na4P2O7

The 23Na magic-angle spinning (MAS), double rotation (DOR) and multiple-quantum magic-angle spinning (MQMAS) NMR spectra of anhydrous sodium pyrophosphate, Na4P2O7, measured at five different Larmor frequencies (nuL) ranging from 105.8 MHz (corresponding to 400 MHz 1H frequency) to 211.6 MHz (800 MHz) are analysed and the complete set of NMR parameters (C(qcc), etaQ and delta(iso)) of the four crystallographically inequivalent sodium sites were determined with high accuracy. Different approaches of spectra evaluation are discussed and their results are compared. The most reliable results are obtained from a combined evaluation of five DOR and three MQMAS spectra but also from two DOR and one MAS spectra or even from a single MQMAS spectrum all data can be derived. It is shown that Na4P2O7 may serve as a useful reference material for experimental set-up and reliability tests of the various NMR experiments.     

Satellite-transition MAS NMR of spin I = 3/2, 5/2, 7/2, and 9/2 nuclei: sensitivity,resolution, and practical implementation

The satellite-transition MAS (STMAS) experiment offers an alternative approach to established methods such as dynamic angle spinning (DAS), double rotation (DOR), and multiple-quantum MAS (MQMAS) for obtaining high-resolution NMR spectra of half-integer quadrupolar nuclei. Unlike the multiple-quantum experiment, STMAS involves two-dimensional correlation of purely single-quantum coherences; satellite transitions in t(1) (or F(1)) and the central transition in t(2) (or F(2)). To date, STMAS has primarily been demonstrated for nuclei with spin quantum numbers I = 3/2 and, to a lesser extent, I > 5/2. However, many chemically relevant nuclei possess I > 3/2, such as (17)O and (27)Al (both I = 5/2), (59)Co (I = 7/2), and (93)Nb (I = 9/2). Here, we discuss the application of STMAS to nuclei with spin quantum numbers from I = 3/2 to 9/2. First, we consider the practical implementation of the STMAS experiment using (87)Rb (I = 3/2) NMR as an example. We then extend the discussion to include nuclei with higher spin quantum numbers, demonstrating (27)Al, (45)Sc (I = 7/2), (59)Co, and (93)Nb STMAS experiments on both crystalline and amorphous samples. We also consider the possibility of experiments involving satellite transitions other than m(I) = +/- 1/2 <--> +/- 3/2 and, using (93)Nb NMR, demonstrate the correlation of all single-quantum satellite transitions up to and including m(I) = +/- 7/2 <--> +/- 9/2. The absolute chemical shift scaling factors in these experiments are discussed, as are the implications for isotropic resolution.     

Solid-state single and triple-quantum 93Nb MAS NMR studies of ferroelectric Pb(Mg1/3Nb2/3)O3 and a related pyrochlore

Pb(Mg1/3Nb2/3)O3 (PMN), a well-known relaxor ferroelectric material, and a related pyrochlore phase have been studied by single- and triple-quantum 93Nb MAS NMR spectroscopy. The assignment of the NMR resonances has been attempted.     

Local structure in perovskite relaxor ferroelectrics: high-resolution 93Nb 3QMAS NMR

Solid solutions of (1'-x)Pb(Mg(1/3)Nb(2/3))O3xPb(Sc(1/2)Nb(1/2))O3 (PMN/PSN) have been investigated using high-resolution 93Nb 3-quantum magic-angle spinning nuclear magnetic resonance experiments (3QMAS NMR). In previous MAS NMR investigations, the local B-cation ordering in these relaxor ferroelectric solid solutions was quantitatively determined. However, in conventional one-dimensional MAS spectra the effects of chemical shifts and quadrupole interaction are convoluted; this, in addition to the insufficient resolution, precludes reliable extraction of the values of isotropic chemical shift and quadrupole coupling product. In the current 3QMAS investigation, 93Nb spectra are presented for concentrations x=0, 0.1, 0.2, 0.6, 0.7, and 0.9 at high magnetic field (19.6 T) and fast sample spinning speed (35.7 kHz). Seven narrow peaks and two broad components are observed. The unique high-resolution of the two-dimensional 3QMAS spectra enables unambiguous and consistent assignments of spectral intensities to the specific 28 nearest B-site neighbor (nBn) configurations, (NMg, NSc, NNb) where each number ranges from 0 to 6 and their sum is 6. It is now possible to isolate the isotropic chemical shift and quadrupole coupling product and separately determine their values for most of the 28 nBn configurations. The isotropic chemical shift depends linearly on the number of Mg2+ cations in the configuration; delta iso CS=(13.7 +/- 0.1)NMg-970 +/- 0.4 ppm, regardless of the ratio NSc/NNb. For the seven Nb5+-deficient configurations (NMg, 6-NMg, 0) and the pure niobium configuration (0, 0, 6), the quadrupole coupling products (and hence the electric field gradients) are small (PQ approximately 6-12 MHz) and for the remaining configurations containing small, ferroelectric active Nb5+ ions, the quadrupole coupling products are significantly larger (PQ approximately 40 MHz), indicating larger electric field gradients.     

Local structure in perovskite relaxor ferroelectrics: high-resolution Nb 3QMAS NMR

Solid solutions of (1'-x)Pb(Mg(1/3)Nb(2/3))O3xPb(Sc(1/2)Nb(1/2))O3 (PMN/PSN) have been investigated using high-resolution 93Nb 3-quantum magic-angle spinning nuclear magnetic resonance experiments (3QMAS NMR). In previous MAS NMR investigations, the local B-cation ordering in these relaxor ferroelectric solid solutions was quantitatively determined. However, in conventional one-dimensional MAS spectra the effects of chemical shifts and quadrupole interaction are convoluted; this, in addition to the insufficient resolution, precludes reliable extraction of the values of isotropic chemical shift and quadrupole coupling product. In the current 3QMAS investigation, 93Nb spectra are presented for concentrations x=0, 0.1, 0.2, 0.6, 0.7, and 0.9 at high magnetic field (19.6 T) and fast sample spinning speed (35.7 kHz). Seven narrow peaks and two broad components are observed. The unique high-resolution of the two-dimensional 3QMAS spectra enables unambiguous and consistent assignments of spectral intensities to the specific 28 nearest B-site neighbor (nBn) configurations, (NMg, NSc, NNb) where each number ranges from 0 to 6 and their sum is 6. It is now possible to isolate the isotropic chemical shift and quadrupole coupling product and separately determine their values for most of the 28 nBn configurations. The isotropic chemical shift depends linearly on the number of Mg2+ cations in the configuration; delta iso CS=(13.7 +/- 0.1)NMg-970 +/- 0.4 ppm, regardless of the ratio NSc/NNb. For the seven Nb5+-deficient configurations (NMg, 6-NMg, 0) and the pure niobium configuration (0, 0, 6), the quadrupole coupling products (and hence the electric field gradients) are small (PQ approximately 6-12 MHz) and for the remaining configurations containing small, ferroelectric active Nb5+ ions, the quadrupole coupling products are significantly larger (PQ approximately 40 MHz), indicating larger electric field gradients.     

27Al multiple-quantum MAS NMR of mechanically treated bayerite (alpha-Al(OH)3) and silica mixtures

Two-dimensional 27Al multiple-quantum magic angle spinning (MQMAS) NMR experiments are used to study mixtures of bayerite (alpha-Al(OH)3) with either silicic acid (SiO2.nH2O) or silica gel (SiO2) that have been ground together for varying lengths of time. This mechanical treatment produces changes in the 27Al MAS and MQMAS NMR spectra that correspond to the formation of new Al species. Mean values of the quadrupolar interaction (PQ) and isotropic chemical shift (deltacs) are extracted from the two-dimensional 27Al NMR spectra for each of these species. The presence of significant distributions of both 27Al quadrupolar and chemical shift parameters is demonstrated and the effect of grinding duration on the magnitudes of these distributions is discussed.     

Single- and multiple-quantum cross-polarization in NMR of quadrupolar nuclei in static samples

Cross-polarization from a spin I=1/2 nucleus (e.g., ¹H) to a spin S = 3/2 nucleus (e.g., ²³Na) or a spin S = 5/2 nucleus (e.g., ²⁷A1 or ⁿO) in static powder samples is investigated. The results of conventional (single-quantum), three-quantum, and five-quantum cross-polarization experiments are presented and discussed. Based on a generalization of an existing theory of cross-polarization to quadrupolar nuclei, computer simulations are used to model the intensity and lineshape variations observed in cross-polarized NMR spectra as a function of the radio-frequency field strengths of the two simultaneous spin-locking pulses. These intensity and lineshape variations can also be understood in terms of the spin S = 3/2 or 5/2 nutation rates determined from experimental quadrupolar nutation spectra. The results of this study are intended as a preliminary step towards understanding single- and multiple-quantum cross-polarization to quadrupolar nuclei under MAS conditions and the application of these techniques to the MQMAS NMR experiment.     

Combined 17O NMR and 11B-31P double resonance NMR studies of sodium borophosphate glasses

17O enriched sodium borophosphate glasses were prepared from isotopically enriched NaPO3 and H3BO3. These glasses have been studied by 17O, 11B and 31P NMR including 17O and 11B multiple quantum magic angle sample spinning (MQMAS), 11B-31P heteronuclear correlation (HETCOR) NMR and 11B{31P} rotational echo double resonance (REDOR). For comparison, the crystalline borophosphates BPO4 and Na5B2P3O13 were included in the investigations. The latter compound shows three sharp 31P resonances at -0.2, -2 and -8 ppm and two BO4 sites that can only be resolved by MQMAS. The 17O NMR spectra were recorded using both the static echo method at medium magnetic field (9.4 T) as well as MAS and MQMAS methods at high field (17.6 T). In total, five oxygen sites were identified in these borophosphate glasses: P-O-P, Na...O-P, P-O-B, B-O-B, Na...O-B. However, these five sites are not present simultaneously in any of the glasses. The 17O MQMAS spectra prove that P-O-B links play a major role in borophosphate glasses. These results are confirmed by the complementary 11B MAS spectra that show the presence of asymmetric and symmetric trigonal groups BO3a and BO3s and two tetrahedral BO4 units. 11B{31P} REDOR NMR is used to give independent information to assign the 11B lines to structural units present in the glasses. These REDOR measurements reveal that B-O-P bonds are present for each borate unit, including the BO3 groups. Particularly, a structural proposal for the two different BO4 resonances is given in terms of a different number of bonded phosphate tetrahedra. The 31P MAS spectra are usually broad and not well resolved. It is shown by 11B-31P HETCOR NMR that a possible structural assignment of a 31P signal at about -20 ppm to Q2 units as in binary sodium phosphate glasses is wrong and that the phosphate tetrahedron belonging to this resonance must be connected to borate groups.     

Triple quantum MQMAS spectroscopy of (I=7/2) in Na3Co(NO2)6 and trans-Co[(en2)(NO2)2]NO3 interplay between the quadrupole coupling and anisotropic shielding tensors

The purpose of this paper is to investigate the interplay between the chemical shielding anisotropy and quadrupole interaction in MQMAS spectra. in the compounds Na₃Co(NO₂)₆ and trans-Co[(en₂)(NO₂)₂]NO₃ provides model systems for such an investigation. Furthermore, only few results have been reported on the application of the MQMAS method to a spin I=7/2. The possibilities of the MQMAS spectroscopy for determining the relative orientation of the two tensors and its advantage over previous techniques are discussed. Reported experimental spectra at different spinning speeds of Na₃Co(NO₂)₆ are accurately reproduced by our theoretical simulations. The calculations are based on a recent approach, summarized in the present paper, which allows one to perform efficient simulations of MQMAS spectra including all interactions and their time-dependence throughout the experiment. This is necessary for calculating accurate MQMAS spectra including the spinning sideband pattern. In the case of trans-Co[(en₂)(NO₂)₂]NO₃ where the quadrupolar interaction and chemical shielding are stronger and their axes are non-coincident, the MQMAS spectrum is strongly distorted due to the unsufficient spinning speed and RF power. In this case, MAS at different spinning speeds is shown to provide valuable information.     

Multiple-quantum MAS NMR of quadrupolar nuclei. Do five-, seven- and nine-quantum experiments yield higher resolution than the three-quantum experiment?

The question of whether or not higher-order (five-, seven- and nine-quantum) multiple-quantum magic angle spinning (MQMAS) experiments yield isotropic NMR spectra of half-integer quadrupolar nuclei with higher resolution than the basic three-quantum MAS experiment is examined. The frequency dispersion is shown theoretically to be greatly increased in higher-order MQMAS spectra, but it is argued that whether or not this translates into an increase in resolution depends upon the ratio of the homogeneous to inhomogeneous contributions to the isotropic linewidth. Experimentally, it is demonstrated using three-, five- and seven-quantum 45Sc MAS NMR and three- and five-quantum 27Al MAS NMR of crystalline samples that higher-order MQMAS experiments can yield a real and useful increase in resolution but that, owing to the presence of inhomogeneous broadening in the isotropic spectra, this increase is less than the theoretically predicted value. A number of practical issues relating to resolution in MQMAS NMR are also pointed out.     

Multiple-quantum cross-polarization and two-dimensional MQMAS NMR of quadrupolar nuclei

Cross-polarization from (1)H to the multiple-quantum coherences of a quadrupolar nucleus is used in combination with the two-dimensional multiple-quantum magic angle spinning (MQMAS) NMR experiment in order to extract high-resolution CPMAS NMR spectra. The technique is demonstrated on (23)Na (S = 3/2), (17)O, (27)Al (both S = 5/2), and (45)Sc (S = 7/2) nuclei, showing the applicability of multiple-quantum cross-polarization to systems with differing spin quantum number, gyromagnetic ratio, and relative nuclide abundance. The utility of this two-dimensional MAS NMR experiment for spectral editing and site-specific measurement of cross-polarization intensities is demonstrated. The possibility of direct cross-polarization to higher order multiple-quantum coherences is also considered and three-, five-, and seven-quantum cross-polarized (45)Sc MAS NMR spectra are presented.     

Both experimental and theoretical investigations of solid-state 17O NMR for L-valine and L-isoleucine

We have presented an experimental investigation of the carboxyl oxygen NMR parameters for four distinct sites in l-valine and l-isoleucine. The carboxyl (17)O quadrupolar coupling constant, C(Q), and isotropic chemical shift, delta(iso), for these compounds are obtained by analyzing two-dimensional (17)O multiple-quantum magic-angle spinning (MQMAS) and/or 1D MAS spectra. The values of C(Q) and delta(iso) found to be in the range of 7.00-7.85 MHz, and 264-314 ppm, respectively. Extensive quantum chemical calculations at the density functional levels have been performed for a full cluster of l-valine molecules and a few theoretical models. The calculated results indicated that there was a correlation between the (17)O NMR parameters and C-O bond lengths, which was helpful for the spectral assignment. They also demonstrated that the torsion angle of l-valine plays an important role in determining the magnitudes of (17)O NMR parameters.     

A Al MAS, MQMAS and off-resonance nutation NMR study of aluminium containing silica-based sol-gel materials

Aluminium containing hybrid materials were prepared via the sol-gel method using aluminium sec-butoxide complexed with ethylacetoacetate (Al(OBu^s)₂EAA or Al(OBu^s)₃/EAA mixtures). As silanes, phenyltrimethoxysilane (PhTMS) or phenyltriethoxysilane (PhTES), 3-glycidoxypropyl trimethoxysilane (Glymo) and tetraethylorthosilicate (TEOS) were used. After room temperature drying of the samples the ²⁷Al single pulse excitation (SPE) magic angle spinning (MAS) NMR shows that octahedral (5 ppm) and tetrahedral (55 ppm) coordinated aluminium species are present in the materials. The relative amount of these two species depends on the preparation method. However, the Al(IV)/Al(VI) ratio is lower than 3 (typically 2.3) in all materials, indicating the presence of a small amount of an aluminate phase. Annealing of the samples at 100, 150 and 200 °C results in the formation of an extra signal at 30 ppm (peak maximum measured at 11.7 T). Based on the resonance frequency this signal is generally assigned to a pentahedrally coordinated aluminium species. Hydration/dehydration processes of annealed samples were studied with ²⁷Al SPE MAS NMR, multiple-quantum MAS NMR (MQMAS) and off-resonance nutation NMR. Upon hydration of the annealed sample the signal intensity around 30 ppm decreases in intensity and at the same time the intensity of the signal around 55 ppm increases by the same amount (tetrahedrally coordinated aluminium). The MQMAS spectra reveal that the signal around 30 ppm is not caused by a fivefold-coordinated aluminium species but mainly by tetrahedrally coordinated aluminium species in a distorted environment, experiencing large quadrupole induced shifts and small chemical shifts due to conformational changes in the polymeric network. From the MQMAS NMR spectra it can be concluded that the linebroadening observed in the ²⁷A1 MAS NMR spectra is due to both a distribution in isotropic chemical shifts and a distribution in quadrupole coupling constants (C_qcc = e²qQ/h). Hydration of the sample results in a decrease of the average C_qcc for the tetrahedrally coordinated aluminium from 6 to 4 MHz, whereas the average C_qcc of the octahedrally coordinated aluminium is hardly influenced (4 MHz). These MQMAS results are confirmed by off-resonance nutation experiments.     

Al Multiple-Quantum MAS NMR of Mechanically Treated Bayerite (α-Al(OH)3) and Silica Mixtures

Two-dimensional ²⁷Al multiple-quantum magic angle spinning (MQMAS) NMR experiments are used to study mixtures of bayerite (α-Al(OH)₃) with either silicic acid (SiO₂.nH₂O) or silica gel (SiO₂) that have been ground together for varying lengths of time. This mechanical treatment produces changes in the ²⁷Al MAS and MQMAS NMR spectra that correspond to the formation of new Al species. Mean values of the quadrupolar interaction (P_Q) and isotropic chemical shift (δ_CS) are extracted from the two-dimensional ²⁷Al NMR spectra for each of these species. The presence of significant distributions of both ²⁷Al quadrupolar and chemical shift parameters is demonstrated and the effect of grinding duration on the magnitudes of these distributions is discussed.     

Site-selective QPASS for the isolation of large quadrupolar coupling environments

Spectral editing of high spinning rate quadrupolar powder patterns observed using the QPASS experiment was achieved through the coupling of QPASS with the selective pi/2-RAPT enhancement sequence. The resulting pi/2-RAPT-QPASS sequence yields spectra that are dominated by the powder patterns form sites with large quadrupolar couplings thus reducing the overlap of patterns from multiple sites of different symmetry in a material. The 93Nb isotropic chemical shifts and quadrupolar coupling parameters were determined for the two niobium crystallographic sites in the layered KCa2Nb3O10. The asymmetric surface site in the structure was selectively enhanced and easily fit to second-order quadrupolar powder pattern with this method.     

Rotor-synchronized acquisition of quadrupolar satellite-transition NMR spectra: practical aspects and double-quantum filtration

The very broad resonances of quadrupolar (spin I > 1/2) nuclei are resolved by magic angle spinning (MAS) into a large number of spinning sidebands, each of which often remains anisotropically broadened. The quadrupolar interaction can be removed to a first-order approximation if the MAS NMR spectrum is acquired in a rotor-synchronized fashion, aliasing the spinning sidebands onto a centreband and thereby increasing the signal-to-noise ratio in the resulting, possibly second-order broadened, spectrum. We discuss the practical aspects of this rotor-synchronization in the direct (t(2)) time domain, demonstrating that the audiofrequency filters in the receiver section of the spectrometer have a significant impact on the precise timings needed in the experiment. We also introduce a novel double-quantum filtered rotor-synchronized experiment for half-integer spin quadrupolar (spin I = 3/2, 5/2, etc.) nuclei that makes use of central-transition-selective inversion pulses to both excite and reconvert double-quantum coherences and yields a simplified spectrum containing only the ST(1) (m(I) = +/-1/2 <--> +/-3/2) satellite-transition lineshapes. For spin I = 5/2 nuclei, such as (17)O and (27)Al, this spectrum may exhibit a significant resolution increase over the conventional central-transition spectrum.     

Oxygen sites in the zeolite stilbite: a comparison of static, MAS, VAS, DAS and triple quantum MAS NMR techniques

Static, magic angle spinning (MAS), variable angle spinning (VAS), dynamic angle spinning (DAS) and triple quantum magic angle spinning (3QMAS) NMR techniques were applied to separate and quantify oxygen signals from Al–O–Si and Si–O–Si sites of ¹⁷O-enriched samples of the mineral stilbite, a natural zeolite. DAS experiments showed that there was a distribution of quadrupolar coupling constants, asymmetry parameters and isotropic chemical shifts. Two methods were used to study the quantification problem of DAS and 3QMAS. Our results showed that DAS was quantitative. In 3QMAS, signal intensity from sites with larger quadrupolar coupling constants was reduced because of less efficient excitation. All techniques have shown a clear difference in rates of exchange between the two types of sites with interchannel H₂O molecules.     

Solid-state S MAS NMR of inorganic sulfates

Solid-state (33)S MAS NMR spectra of a variety of inorganic sulfates have been obtained at magnetic field strengths of 4.7, 14.1, 17.6, and 18.8 T. Some of the difficulties associated with obtaining natural abundance (33)S NMR spectra have been overcome by using a high magnetic field strength and magic angle spinning (MAS). Multiple factors were considered when analyzing the spectral linewidths, including magnetic field inhomogeneity, dipolar coupling, chemical shift anisotropy, chemical shift dispersion, and quadrupolar coupling. In most of these sulfate samples, quadrupolar coupling was the dominant line broadening mechanism. Nuclear electric quadrupolar coupling constants (C(q)) as large as 2.05 MHz were calculated using spectral simulation software. Spectral information from these new data are compared with X-ray measurements and GAUSSIAN 98W calculations. A general correlation was observed between the magnitude of the C(q) and the increasing difference between S-O bond distances within the sulfate groups. Solid-state (33)S spin-lattice (T(1)) relaxation times were measured and show a significant reduction in T(1) for the hydrated sulfates. This is most likely the result of the modulation of the time-dependent electric field gradient at the nuclear site by motion of water molecules. This information will be useful in future efforts to use (33)S NMR in the compositional and structural analysis of sulfur containing materials.     

Solid-state 33S MAS NMR of inorganic sulfates

Solid-state (33)S MAS NMR spectra of a variety of inorganic sulfates have been obtained at magnetic field strengths of 4.7, 14.1, 17.6, and 18.8 T. Some of the difficulties associated with obtaining natural abundance (33)S NMR spectra have been overcome by using a high magnetic field strength and magic angle spinning (MAS). Multiple factors were considered when analyzing the spectral linewidths, including magnetic field inhomogeneity, dipolar coupling, chemical shift anisotropy, chemical shift dispersion, and quadrupolar coupling. In most of these sulfate samples, quadrupolar coupling was the dominant line broadening mechanism. Nuclear electric quadrupolar coupling constants (C(q)) as large as 2.05 MHz were calculated using spectral simulation software. Spectral information from these new data are compared with X-ray measurements and GAUSSIAN 98W calculations. A general correlation was observed between the magnitude of the C(q) and the increasing difference between S-O bond distances within the sulfate groups. Solid-state (33)S spin-lattice (T(1)) relaxation times were measured and show a significant reduction in T(1) for the hydrated sulfates. This is most likely the result of the modulation of the time-dependent electric field gradient at the nuclear site by motion of water molecules. This information will be useful in future efforts to use (33)S NMR in the compositional and structural analysis of sulfur containing materials.