Quadrupolar Line Shapes Research Articles

95Mo Magic Angle Spinning NMR at High Field: Improved Measurements and Structural Analysis of the Quadrupole Interaction in Monomolybdates and Isopolymolybdates

In this study, 95Mo quadrupole couplings in various molydbates were measured easily and accurately with magic angle spinning (MAS) NMR under a directing field of 19.6 T. The resonance frequency of 54 MHz was sufficiently high to remove acoustic ringing artifacts, and the spectra could be analyzed in the usual terms of chemical shift and quadrupolar line shapes. For monomolybdates and molybdite, the quadrupole coupling dominated the NMR response, and the quadrupole parameters could be measured with better accuracy than in previous lower field studies. Moreover, despite the low symmetry of the molybdenum coordination, the usefulness of such measurements to probe molybdenum environments was established by ab initio density functional theory (DFT) calculations of the electric field gradient from known structures. The experimental NMR data correlated perfectly with the refined structures. In isopolymolybdates, the resonances were shapeless and DFT calculations were impossible because of the large and low symmetry unit cells. Nevertheless, empirical but clear NMR signatures were obtained from the spinning sidebands analysis or the MQMAS spectra. This was possible for the first time thanks to the improved baseline and sensitivity at high fields. With the generalization of NMR spectrometers operating above 17 T, it was predicted that 95Mo MAS NMR could evolve as a routine characterization tool for ill-defined structures such as supported molybdates in catalysis.

Formation of {Cu6[S2P(OC2H5)2]6} on Cu2S Surfaces from Aqueous Solutions of the KS2P(OC2H5)2Collector: Scanning Electron Microscopy and Solid-State31P Cross-Polarization/Magic Angle Spinning and Static65Cu NMR Studies

The interactions of synthetic chalcocite surfaces with diethyldithiophosphate, potassium salt, K[S2P(OC2H5)2], were studied by means of 31P cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopy and scanning electron microscopy (SEM). To identify the species formed on the Cu2S surfaces, a polycrystalline {CuI6[S2P(OC2H5)2]6} cluster was synthesized and analyzed by SEM, powder X-ray diffraction techniques and solid-state 31P CP/MAS NMR and static 65Cu NMR spectroscopy. 31P chemical shift anisotropy (CSA) parameters, delta(cs) and eta(cs), were estimated and used for assigning the bridging type of diethyldithiophosphate ligands in the {CuI6[S2P(OC2H5)2]6} cluster. The latter data were compared to 31P CSA parameters estimated from the spinning sideband patterns in 31P NMR spectra of the collector-treated mineral surfaces: formation of polycrystalline {CuI6[S2P(OC2H5)2]6} on the Cu2S surfaces is suggested. The second-order quadrupolar line shape of 65Cu was simulated, and the NMR interaction parameters, CQ and etaQ, for the copper(I) diethyldithiophosphate cluster were obtained.

Backbone dynamics in collagen

Peptide backbone motions of collagen have been extensively studied in the past. The experimental results were interpreted using a model of a collagen rod librating about its helix axis. Considering the size of the collagen molecule and the presence of cross-linked molecules, motional amplitudes derived for the helix axis libration were unusually high. Using solid-state NMR 13C chemical shift anisotropy and 2H quadrupolar lineshape analysis for five different isotope labelled collagens we show that motional averaging of the NMR interactions occurs primarily via small-angle librations about internal bond directions. This type of dynamics is compatible with both the presence of cross-links in collagen and the X-ray data, as well as dynamic models used for other proteins.

Solid-State 17O NMR of Amino Acids

17O solid-state NMR from 14 amino acids is reported here, greatly increasing the number investigated. In most cases well-separated resonances from carbonyl and hydroxyl oxygens with distinct second-order quadrupolar line shapes are observed using a 600 MHz spectrometer with fast magic angle spinning (MAS). This is in contrast to the motionally averaged resonances usually seen from amino acids in solution. For amino acids double-angle rotation (DOR) produces a decrease in the line width by more than a factor of 40, providing very high resolution, ∼1 ppm, spectra. The oxygen lines in alanine and the carbonyl oxygens in l-glutamic acid hydrochloride are assigned using 1H-decoupled DOR. The NMR interaction parameters for amino acids show a wide variation of χQ, from 6.4 to 8.6 MHz, η, from 0.0 to 0.9, and δiso, from 83 to 353 ppm. The high quality of the MAS NMR line shapes obtained at 14.1 T means that even small changes in parameters can be very accurately deduced, offering the possibility of 17O NMR as a sensitive probe of structural changes in these and related compounds. The d- and l-forms of glutamic acid hydrochloride are shown to have the same NMR parameters to within error, which are very different from those reported in the literature for the d,l-form. A strong correlation (∼−1200 ppm/Å) is found between δiso and the C−O bond length of the carbonyl oxygens. On the basis of these data, enriching specific amino acids in more complex polypeptides and proteins could provide site-selective information about the bonding and functionality of different sites in biomolecules. An estimate is made of the possible detection limit for such species.

27Al MQ-MAS NMR as a Tool for Structure Determination in Nanocomposite Materials: The Nature of Al Pillars in “Al13−heidi” Pillared Clays

27Al 3QMAS 2D NMR was used to study the molecular-level organization of a clay pillared with the “Al13(heidi)63+” polycation, whose unusual structure is stabilized by hydroxyethyliminodiacetate (heidi) ligands. The quadrupolar line shape and the isotropic chemical shifts were first unambiguously determined on a solid salt of the polycation. This compound was then used to identify the fingerprint of the Al13(heidi)x3+ polycation in a sample of saponite clay that had been intercalated with a solution containing Al(III) and heidi ligands, showing that the basic structure of the Al polymer had been preserved upon intercalation. A more detailed analysis of 3QMAS 2D NMR line shapes indicated, however, that a selective partial hydrolysis reaction had occurred, most probably leading to Al13(heidi)33+ polycations (loss of three heidi ligands). Thus, 3QMAS NMR allowed to choose one structural model for these nanocomposites from the several compatible with previously published characterizations.

59Co Chemical Shift Anisotropy and Quadrupole Coupling for K3Co(CN)6 from MQMAS and MAS NMR Spectroscopy

59Co triple-quantum (3Q) MAS and single-pulse MAS NMR spectra of K3Co(CN)6 have been obtained at 14.1 T and used in a comparison of these methods for determination of small chemical shift anisotropies for spin I=7/2 nuclei. From the 3QMAS NMR spectrum a spinning sideband manifold in the isotropic dimension with high resolution is reconstructed from the intensities of all spinning sidebands in the 3QMAS spectrum. The chemical shift anisotropy (CSA) parameters determined from this spectrum are compared with those obtained from MAS NMR spectra of (i) the complete manifold of spinning sidebands for the central and satellite transitions and of (ii) the second-order quadrupolar lineshapes for the centerband and spinning sidebands from the central transition. A good agreement between the three data sets, all of high precision, is obtained for the shift anisotropy (δσ=δiso−δzz) whereas minor deviations are observed for the CSA asymmetry parameter (ησ). The temperature dependence of the isotropic 59Co chemical shift has been studied over a temperature range from −28 to +76°C. A linear and positive temperature dependence of 0.97 ppm/°C is observed.

NMR and X-ray Spectroscopy of Sodium−Silicon Clathrates

The 23Na and 29Si NMR spectra of the sodium−silicon clathrate materials Na8Si46 and NaxSi136 (1 < x < 24) and the parent Zintl phase NaSi have been studied in detail with a view to ironing out a number of ambiguities in the published literature and to determining properties of these potential thermoelectric materials. Sharp spectra are obtained only when the clathrate cages are close to fully occupied by Na, so that crystallographic symmetry is achieved. Signals from Na in the small and large cages of both structures have been unequivocally assigned. The pseudospherical cages give isotropic 23Na lines, whereas the other cages produce first-order quadrupolar line shapes. Electric field gradients derived from these spectra, and ab initio calculations are in remarkable agreement. The large Knight shifts of both types of nuclei have rather unusual temperature dependences, which reflect the changing distributions of unpaired electron density in the conduction bands. As the Na content of NaxSi136 is reduced there is drastic broadening of the 23Na and 29Si spectra due to a random distribution of vacant cages and hence of environments, and the Si spectra shift to lower frequencies, indicating a reduction in conduction s-electron density on the Si. Intensity data suggest a preferential loss of Na from the large cages. XPS results show a protective coating of silica on the surface of the clathrates. NMR, XPS, and XANES results all indicate transfer of electrons from Na to the Si framework, as predicted by calculations. XANES shows a lowering of the absorption edge, and hence of the conduction band, of both clathrates relative to crystalline Si.

The system Ga2O3(Al2O3)–GeO2(SiO2) studied by NMR, XRD, IR and DTA

The formation of germanate phases with the mullite structure [Ga6Ge2O13, Al6Ge2O13 and (Ga,Al)6Ge2O13] from oxide mixtures and hybrid sol–gel precursors was studied by thermal analysis, XRD, IR spectroscopy and multinuclear MAS NMR. The course of the thermal reactions in all the Ga-containing oxide mixtures, irrespective of the starting composition, is dominated by the intermediate formation below 1150 °C of α- and β-Ga4GeO8 and by the volatilization of GeO2 at higher temperatures. In oxide mixtures containing alumina, Ga incorporation in the mullite-type phase occurs only above 1150 °C. IR spectroscopy confirms the incorporation of both Al and Ga into the mullite-type structure at higher temperature. No reaction with Ga2O3 occurred where the GeO2 was replaced by SiO2. Thermal treatment of gel precursors produces intermediate amorphous phases in which the Al is in typical sites with a 27Al MAS NMR resonance at 33 ppm, in addition to tetrahedral and octahedral sites. Crystallization of the amorphous intermediates below 1300 °C produces mixtures of the mullite phase and Ga4GeO8. Many of the 71Ga MAS NMR spectra are dominated by the quadrupolar lineshape of β-Ga2O3 from the structural units of α-Ga4GeO8 or unreacted starting material. The 71Ga MAS NMR spectrum of Ga6Ge2O13 is broad and featureless, as are the 69Ga MAS NMR spectra of these systems. The 17O NMR spectra of an isotopically enriched gel and its reaction products are also reported.

Prospects for 137Ba MAS NMR spectroscopy of ceramics and related inorganic materials

The 11.7T 137Ba MAS NMR spectra are presented for 10 ceramic materials and related inorganic compounds. The spectra of compounds in which Ba occurs in highly symmetric sites (BaZrO 3, BaTiO 3, BaO) show sharp resonances, with a well-resolved quadrupolar lineshape in the case of BaTiO 3. In other Ba-containing ceramics and ceramic precursor compounds (e.g. BaAl 2Si 2O 8, BaAl 2O 4, Ba 2SiO 4, BaCO 3), the Ba typically occurs in irregular polyhedral sites, giving spectra which are considerably broader. Some of the present spectra show evidence of an expected negative trend in the resonance positions with increasing Ba coordination number. The results suggest that 137Ba MAS NMR spectroscopy has potential value for studying some types of Ba-containing ceramics and ceramic precursors. ©

Effects of Finite Rf Pulses and Sample Spinning Speed in Multiple-Quantum Magic-Angle Spinning (MQ-MAS) and Multiple-Quantum Quadrupolar Carr−Purcell−Meiboom−Gill Magic-Angle Spinning (MQ-QCPMG-MAS) Nuclear Magnetic Resonance of Half-Integer Quadrupolar Nuclei

The multiple-quantum magic-angle spinning (MQ-MAS) solid-state nuclear magnetic resonance (NMR) experiment and a sensitivity-enhanced variant detecting the second-order quadrupolar powder pattern through a train of quadrupolar Carr−Purcell−Meiboom−Gill refocusing pulses (MQ-QCPMG-MAS) are analyzed with respect to the effects of finite radio frequency (rf) pulse irradiation and the MAS frequency. Taking these effects explicitly into account, it is possible to accurately determine optimum conditions for excitation of MQ coherences and reconversion of these into detectable single-quantum coherences as well as simulate the second-order quadrupolar lineshape necessary to extract accurate parameters for quadrupolar coupling interactions and isotropic chemical shifts. This accurate determination is of great importance for the exploitation of MQ-MAS and MQ-QCPMG-MAS NMR experiments for quantitative determination of site populations. The various effects are described analytically and demonstrated by numerical simulations and by 87Rb MQ-MAS and MQ-QCPMG-MAS experiments on RbNO3.

Sites and dynamics of hydrogen and deuterium in V-H-D alloys studied by1Hand2HNMR

Vanadium-hydrogen-deuterium (V-H-D) alloys with $([\mathrm{H}]+[\mathrm{D}])/[\mathrm{V}]=0.8$ have been studied by means of x-ray diffraction, thermal analysis, and ${}^{1}\mathrm{H}$ and ${}^{2}\mathrm{H}$ NMR. The crystal structures of ${\mathrm{VH}}_{0.82}$ and ${\mathrm{VH}}_{0.6}{\mathrm{D}}_{0.2}$ are body-centered-tetragonal (bct), while those of ${\mathrm{VH}}_{0.2}{\mathrm{D}}_{0.6}$ and ${\mathrm{VD}}_{0.81}$ are body-centered-cubic (bcc). ${\mathrm{VH}}_{0.4}{\mathrm{D}}_{0.4}$ consists of bct and bcc, demonstrating the existence of an immiscibility region. The phase transition from the ${\ensuremath{\delta}}_{\mathrm{D}}$ phase to the ${\ensuremath{\alpha}}_{\mathrm{D}}$ phase is observed in ${\mathrm{VD}}_{0.81}$ between 200 and 240 K. The temperature range of the similar phase transition becomes broadened and shifted toward the lower temperature as the [H]/[D] ratio increases. In ${\mathrm{VH}}_{0.82}$ no phase transition is observed in the temperature range between 120 and 295 K. In ${\mathrm{VH}}_{0.81},$ H atoms occupy octahedral (O) sites, and H diffusion takes place between the ordered O sites. In ${\mathrm{VH}}_{0.6}{\mathrm{D}}_{0.2},$ H and D atoms occupy both the O and tetrahedral (T) sites, and the H and D atoms in the T sites diffuse faster than those in the O sites do. The diffusion between the ordered O sites is confirmed by the residual ${}^{1}\mathrm{H}$ dipolar broadening and the ${}^{2}\mathrm{H}$ quadrupolar line shape. In ${\mathrm{VH}}_{0.2}{\mathrm{D}}_{0.6}$ and ${\mathrm{VD}}_{0.81},$ H and D atoms occupy the T sites. In the ${\ensuremath{\delta}}_{\mathrm{D}}$ phase H and D atoms diffuse among the ordered T sites, and consequently the ${}^{2}\mathrm{H}$ quadrupolar line shape is preserved and the ${}^{2}\mathrm{H}$ relaxation is caused dominantly by the dipolar interaction. Around the phase transition temperature range, the diffusion changes drastically. In the ${\ensuremath{\alpha}}_{\mathrm{D}}$ phase the diffusion takes place between the disordered T sites, and another motional mode is additionally present. Temperature and frequency dependences of ${}^{1}\mathrm{H}$ and ${}^{2}\mathrm{H}$ spin-lattice relaxation times can be described by modified Bloembergen-Purcell-Pound equations, suggesting that H and D atoms undergo some kind of correlated motions. Activation energies and mean residence times of H and D atoms have been estimated for each site.

Triple quantum cross-polarization NMR of [formula omitted] and [formula omitted] spin systems in solids

Changes in the triple quantum cross-polarization (TQCP) matching profiles and 27 Al/ 23 Na second-order quadrupolar lineshapes, are observed as a function of the I=1/2 rf field strength for both static and MAS conditions. These were interpreted by considering the orientation dependence of the nutation frequency associated with the TQ transition. Only a fraction of the spins in the powder, with specific orientations of the EFG tensor, can match the Hartmann–Hahn condition at the same time, for a fixed I=1/2 rf field strength. Good fits between 19 F→ 23 Na experimental and calculated TQCP lineshapes were observed for moderate to high 19 F rf field strengths.

Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors

A novel approach to quadrupolar-echo (QE) NMR of half-integer quadrupolar nuclei in static powders is analyzed. By acquisition of the QE spectrum during a Carr−Purcell−Meiboom−Gill (CPMG) train of selective π pulses, the second-order quadrupolar line shape for the central transition is split into a comb of sidebands leading to a considerable increase in the sensitivity compared to a conventional QE spectrum. The applicability of the method for determination of magnitudes and relative orientation of chemical shielding and quadrupolar coupling tensors is examined. Through numerical simulation and iterative fitting of experimental 87Rb (RbClO4 and RbVO3) and 59Co spectra (Co(NH3)5 Cl3), it is demonstrated that the quadrupolar CPMG experiment represents a useful method for studying half-integer quadrupolar nuclei exhibiting large quadrupolar coupling combined with anisotropic chemical shielding interactions. Sensitivity enhancements by a factor of up to about 30 are observed for the samples studied.

Natural abundance solid state 43Ca NMR

43Ca MAS NMR spectra have been collected for 12 solid phases including silicates, carbonates and sulphates. In all cases 43Ca was present at natural abundance, the first time such experiments have been reported. In general the lines are broad and featureless, though in the case of calcite a second order quadrupolar lineshape was observed. Within each class of compounds (silicates, carbonates etc.), the isotropic chemical shift appears to be linearly correlated with the mean CaO bond length of the first coordination shell of calcium, with a similar gradient for all of the classes of compounds studied.

MAS NMR study of pentacoordinated magnesium in grandidierite

The 11.7 T 25 Mg, 27 Al, 29 Si, and 11 B MAS NMR spectra are reported for well-characterized grandidierite, (Mg,Fe)Al 3SiBO9, which contains both Al and Mg in fivefold coordination with oxygen. The 25 Mg spectrum is the first to be reported for 5 Mg, and exhibits a quadrupolar lineshape from which the nuclear quadrupolar coupling constant (3.8 6 0.1 MHz), the asymmetry parameter (0.6 6 0.05), and the isotropic chemical shift (55 6 2 ppm) were derived by spectral simulation. These spectroscopic parameters are discussed in terms of the crystallographic geometry of the fivefold-coordinated site.

Detection of phase and antisite structure of Ti–Al alloys by27Al solid state nuclear magnetic resonance

Abstract Static room-temperature 27Al nuclear magnetic resonance (NMR) spectra for Ti1−xAlx alloys in the range 0·25 ≤ × ≤ 0.55 are reported for the first time. The main resonances associated with the α2 and γ phases are clearly identified. The spectra follow the phase distribution in that, for samples annealed at 1473 K for 6h in the range 0·43 ≤ × ≤ 0·48, resonances from both α2 and γ are observed. Careful quantitative analysis of the 27Al NMR spectra indicates that from single-phase samples all aluminium nuclei from the central transition are recorded, whereas for two-phase samples there is a loss of signal that is a consequence of the two-phase alloy microstructure. For the γ-phase the NMR centre band shows a distinctive, largely second-order quadrupolar line shape together with associated satellite transitions which yield a quadrupolar coupling constant CQ = 8·15 MHz and an axial Knight shift anisotropy of 130ppm. For the α2 phase the centre-band simulation requires only an axial Knight shift anisotropy of 270ppm and no quadrupolar interaction. An additional resonance associated with the γ-phase is also detected and quantified, and this is attributed to aluminium on antisites.

Quadrupolar Echo Spectra of the Tunneling CD3Group

Abstract Deuteron NMR spectra of both single crystal and powder samples of acetylsalicylic acid–CD3were measured using the quadrupolar-echo technique. The experiments were done in the temperature range 17–100 K, with a special emphasis on the range 20– 30 K, in which the observable tunneling frequency decreases rapidly from its low-temperature value of 2.7 down to 1.2 MHz. In the tunneling regime, modulations of the line intensities and phases as a function of the echo time τ are observed in the single-crystal spectra. The modulation frequency is equal to the orientation-dependent displacement of the inner satellite pairs (α lines) from the Larmor frequency. These effects were confirmed in numerical simulations and fully explain the phase-modulation effects observed previously in quadrupolar-echo spectra of methyl-deuterated methanol andpara-xylene guest molecules in some inclusion compounds. By measuring the temperature and orientation dependence of the quadrupolar lineshapes, it was found that the echo spectra are more sensitive to the value of the tunneling frequency than the spectra obtained from the free induction decay. It is pointed out that, because of the modulation effects, special care must be taken when structural parameters are to be extracted from quadrupolar-echo spectra, in particular from spectra of powder samples.

Observation of 91Zr NMR in zirconium-based metals and oxides

The characterisation by 91Zr NMR at room temperature is reported for Zr metal (hexagonal close-packed), ZrH 2 (tetragonal), ZrC (cubic), ZrCo (cubic), ZrO 2 (monoclinic), ZrO 2 (3 mol% Y 2O 3, tetragonal) and ZrO 2 (9.3 mol% MgO). The density of states deduced from the Knight shift for zirconium metal agrees with band structure calculations. It is shown for the first time that the quadrupolar lineshape distinguishes between the various zirconia-based phases.

Aluminum-27 NMR study of AIP0 4-21 and andalusite. Advantages of high-field and very fast MAS

High-field (11.7 T) 21Al NMR spectra of the aluminophosphate AIP0 4-21 and the mineral andalusite obtained with magic-angle spinning (MAS) at up to 15 kHz are reported. Such rapid spinning compared to conventional MAS at <5 kHz can result in considerable spectral simplification by moving the spinning sidebands outside the region containing the centerbands. For AIP0 4-21, which contains appreciably distorted A10 4 and A10 5 groups, high-speed spinning provides a spectrum with well-defined centerbands that allow the quadrupole coupling constant e 2 gQ/ h, the asymmetry parameter η and the isotropic chemical shift δ iso to be estimated for the 27Al nucleus in each group. For andalusite, MAS at 15 kHz is still inadequate for obtaining an unambiguous spectrum of the highly distorted A10 6 species due to its very large e 2 gQ/ h (15.6 MHz). In contrast, the less highly distorted A10 5 species (ezgQ/h = 5.9 MHz) gives a strong centerband exhibiting the expected second-order quadrupolar lineshape.

23Na-NMR of solid state ionomers: lineshape, shift, and hydration effects

23Na-NMR spectra at 52.9 MHz have been obtained for a series of sodium sulfonate and carboxylate model compounds and for solid poly(styrene-co-styrene sulfonate) and poly(ethylene-co-methacrylic acid). Spectra of the model compounds show spin-rate independent features resembling pure quadrupolar lineshapes, but with anomalies. 23Na-NMR spectra of the dried ionomers are featureless and much broader than those of the models and are shifted 30–40 ppm upfield from NaCl. The sulfonated ionomer spectrum changes to a narrow line at 0 ppm upon exposure to 45% humidity at 20°C for 12 hours. The carboxylic ionomer shows no change on room temperature air exposure but 12 hours at 95°C in watersaturated air changes the spectrum markedly with some evidence for exchange among water and carboxyl groups. This type of experiment with a range of acid and Na content under rigidly controlled conditions should help elucidate the aggregation of salt groups in various ionomers.