Applied Magnetic Resonance Special Issue: Applications of Solid-State NMR to Characterizing Materials

Issue edited by John V. Hanna and Mark E. Smith:

Calcium Phosphates and Hydroxyapatite: Solid-State NMR Experiments and First-Principles Calculations
F. Pourpoint, C. Gervais , L. Bonhomme-Coury, T. Azaïs, C. Coelho, F. Mauri, B. Alonso, F. Babonneau and C. Bonhomme
Various calcium phosphates and hydroxyapatite (HAp) have been fully characterized by one- and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments and first principles calculations of NMR parameters, such as chemical shift anisotropy (CSA) and electric field gradient tensors for all nuclei. Such compounds act as useful biocompatible materials. The projector augmented wave (PAW) and gauge including PAW methods allowed the complete assignment of spectra, including 1H magic-angle spinning (MAS) spectra for which ultimate resolution is not attained experimentally. 1H CSA tensors and orientation of the principal axes systems have been also discussed. 17O parameters have been calculated for a large variety of oxo-bridges and terminal oxygen atoms, including P–O–Si fragments characteristic for silicophosphate phases. The (δiso, C Q) sets of values allowed the clear distinction between the various oxygen atoms in a calculated 17O 3-quantum MAS experiment. Such an approach should be of great help for the description of interfaces in complex materials, in terms of structure and chemical composition.

Solid-State NMR Studies on Ionic closo -Dodecaborates
I. Tiritiris, T. Schleid and K. Müller
Alkali metal dodecahydro-closo-dodecaborates M2[B12H12] (M = K, Rb, Cs, NH4, N(CH3)4) and the perhalogenated cesium salts Cs2[B12X12] (X = Cl, Br, I) are studied by solid-state 11B nuclear magnetic resonance (NMR) spectroscopy as well as X-ray diffraction (XRD) and differential scanning calorimetry. The present work addresses the molecular dynamics of the anionic [B12X12]2− icosahedra which is examined by variable-temperature 11B NMR line shape studies between 120 and 370 K. Characteristic line shape effects are observed which strongly depend on the actual substituent X and the counterion M+. All alkali metal dodecahydro-closo-dodecaborates M2 [B12H12] exhibit at elevated temperatures 11B NMR spectra with a single isotropic line which proves the presence of an efficient molecular process, resulting in dynamic (rotational) disorder along with vanishing dipolar and quadrupolar interactions. The positional order of the boron clusters, however, remains unaffected, as shown by the XRD data. At lower temperatures, the underlying motions are frozen on the NMR timescale resulting in characteristic 11B NMR spectra with a dominant homonuclear 11B–11B dipolar splitting. The per-halogenated cesium salts Cs2[B12X12] behave differently. Hence, from the experimental 11B NMR spectra at room temperature a substantial mobility is only seen for the [B12Cl12]2− anion. Obviously, the degree of anion mobility depends on the size of the substituent X in the [B12X12]2− clusters (X = H, Cl, Br, I). A quantitative analysis of the experimental 11B NMR spectra of the alkali metal dodecahydro-closo-dodecaborates M2 [B12H12] is achieved by line shape simulations, considering [B12H12]2− ions undergoing reorientational jumps between icosahedral sites. From the motional correlation times the activation energies are derived. It is found that a correlation exists between the activation energies, the motional correlation times and the lattice constant. Hence, the activation energies and correlation times strongly increase with decreasing size of the cation M+, which reflects an increasing sterical hindrance due to a decreasing crystallo-graphic lattice constant in the same direction.

NMR Insights into Wasteforms for the Vitrification of High-Level Nuclear Waste
D. Holland , B. G. Parkinson, M. M. Islam, A. Duddridge, J. M. Roderick, A. P. Howes and C. R. Scales
Magic-angle spinning nuclear magnetic resonance of 11B, 29Si and 27Al has been used to study the distribution of nonbridging oxygen atoms (NBO) in an alkali borosilicate glass to which surrogate oxides for high-level radioactive waste have been added. The properties of such glasses are shown to depend on the fraction N 4 of four-coordinated boron atoms (B4) and on the fraction of silicate tetrahedra possessing one NBO, Q3. The aqueous corrosion rate increases with Q3 content, as does weight loss due to evaporation from the melt. The activation energy for direct current conduction scales with N 4. Values of N 4 obtained for these glasses deviate from those predicted by the currently accepted model and are strongly affected by the modifier or intermediate nature of the surrogate oxide and also by its effect on the distribution of NBO between the silicate and borate polyhedra.

MAS NMR Strategies for the Characterization of Supported Molybdenum Catalysts
J.-B. d'Espinose de Lacaillerie and Z. Gan
After recalling a few advances made by 1H, 29Si, and 27Al on the understanding of hydro-desulfuration (HDS) molybdenum-based catalysts supported on amorphous oxides, we critically evaluate the potential of 95Mo magic-angle spinning nuclear magnetic resonance (MAS NMR) for gaining further insight into the structure of uncalcined precursors and sulfided HDS catalysts. It is shown that when performed at a very high field (19.6 T), it is indeed sensitive to the nature of the molybdenum–support interaction. In particular, a wide distribution of the molybdenum environment present on the surface was evidenced. However, the possibility to characterize sulfided catalysts by 95Mo MAS NMR appeared still an unmet challenge.

Solid-State MAS NMR Studies of Sulfonic Acid-Functionalized SBA-15
R. Kanthasamy, I. K. Mbaraka, B. H. Shanks and S. C. Larsen
Solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) was used to characterize propylsulfonic acid-functionalized SBA-15 materials prepared by the cocondensation method. The propylsulfonic acid-group concentration and distribution was systematically varied from 2.5 to 10%. As a way of coupling two acid sites and controlling the acid site spatial distribution, tethered propylsulfonic acid groups were incorporated into SBA-15. 13C, 29Si and 1H MAS NMR were used to determine the extent of functionalization. Probe molecules, such as triethylphosphine oxide and diphenyldiphosphines were used to evaluate acid site strength and acid site distributions, respectively.

Multinuclear MAS NMR Investigation of Sol-Gel and Ball-Milled Nanocrystalline Ga2O3
L. A. O'Dell, S. L. P. Savin, A. V. Chadwick and M. E. Smith
71Ga magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to characterize the structural evolution of nanocrystalline Ga2O3 samples prepared by sol-gel and ball-milling techniques. 29Si and 27Al MAS NMR have also been used to characterize silica and alumina Zener pinning phases. 71Ga NMR parameters are reported for the α- and β-Ga2O3 phases, and more tentatively for the δ-Ga2O3 phase. By simulating the octahedrally coordinated gallium NMR line of β-Ga2O3 using Gaussian distributions in χQ, the extent of disorder in the Ga2O3 crystallites has been quantified. The ball-milled samples contain much more inherent disorder than the sol-gel samples in the nano-phase, which was observed from simulations of the 71Ga MAS NMR spectra. The silica pinning phase produced highly crystalline and densely aggregated nanocrystalline Ga2O3, as well as the smallest nanocrystal sizes.

Covalency Measurements via NMR in Lithium Metal Phosphates
S. L. Wilcke, Y.-J. Lee, E. J. Cairns and J. A. Reimer
31P nuclear magnetic resonance (NMR) shifts on the order of thousands of parts per million are observed for olivine LiMPO4 (M = Mn, Fe, Co, Ni) samples, a promising class of Li ion rechargeable battery electrode materials. Variable-temperature 31P NMR measurements of shift are used to determine that the supertransferred hyperfine interaction is the dominant mechanism giving rise to these unusually large observed 31P shifts. Various models for predicting 31P and 7Li shifts in LiMPO4 (M = Mn, Fe, Co, Ni) were investigated. Alloys of LiFe1−x Mn x PO4, where x varies from 0 to 1, were also investigated by 7Li NMR. Covalency constants, calculated from variable-temperature NMR shifts and magnetic susceptibility data, are determined for the P–O–M bonds in LiMPO4 (M = Mn, Fe, Co, Ni) and compared to the covalency constants of the Li–O–M bond. The sign and relative magnitude of the covalency constants are discussed in terms of positive and negative spin densities at the nuclei of interest. The covalency constants for the Li–O–M and P–O–M bonds were measured for Li1.8Na0.2FeMn2(PO4)3 and compared to the covalency constants measured in the olivine LiMPO4 (M = Mn, Fe, Co, Ni) samples. The Li1.8Na0.2FeMn2(PO4)3 structure has a volume per transition metal atom and Li–O–M bond distances that are similar to those of the olivine LiMPO4 (M = Mn, Fe, Co, Ni) samples.

The Challenge of Paramagnetism in Two-Dimensional 6,7Li Exchange NMR
L. S. Cahill, R. P. Chapman, C. W. Kirby and G. R. Goward
6,7Li fast magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy is used to study LiMn2O4 and Li3V2(PO4)3. The presence of paramagnetic transition metal centers in these materials has a profound effect on the resulting NMR spectra. Lithium ion mobility has been studied by two-dimensional (2-D) exchange spectroscopy (EXSY) in Li3V2(PO4)3 but an absence of lithium ion exchange was observed for LiMn2O4. Several differences between the two materials are explored to explain these results. LiMn2O4 experiences a greater donation of electron spin density to the Li nucleus via the Fermi-contact interaction when compared with Li3V2(PO4)3. This contributes to a greater hyperfine chemical shift and a larger dependence of chemical shift on temperature. The delocalized electrons in LiMn2O4 cause temperature-independent T 1 relaxation rates and shorter relative T 2 values. The relative rates of ionic conductivity and spin–lattice or spin–spin relaxation in LiMn2O4 and Li3V2(PO4)3 are contrasted to illustrate the constraints on the use of 2-D EXSY to characterize ion dynamics in paramagnetic materials.

Shift Anisotropy Tensors in Amorphous Natural-Abundance Solids: High-Resolution 29Si Chemical Shift Anisotropy Distributions under Very Slow Sample Rotation
D. Sakellariou and T. Charpentier
The magic-angle turning technique is applied to amorphous natural-abundance silicate materials and high-resolution silicon-29 correlations between the isotropic and anisotropic chemical shifts are obtained. Very narrow tilted spinning sideband patterns are resolved in the two-dimensional spectra whose line width corresponds to the natural line width of the silicon nuclei. Various numerical approaches are implemented to extract the distribution of the chemical shift anisotropy tensors in these materials and their results are compared.

NMR Studies of Heat-Induced Transitions in Structure and Cation Binding Environments of a Strontium-Saturated Swelling Mica
G. M. Bowers, M. C. Davis, R. Ravella, S. Komarneni and K. T. Mueller
In this work, we combine 27Al, 29Si, 19F, and 23Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) to characterize the structure and interlayer cation environments in a strontium-saturated member of the swelling mica family before and after a heat-induced collapse of the interlayer space. The 27Al and 29Si MAS NMR demonstrate that the sample consists mainly of swelling mica, though the composition does not match the ideal structural formula. Aluminum NMR also shows that a portion of the aluminum shifts from a tetrahedral to an octahedral coordination environment upon heating. Changes in the 29Si and 19F NMR after heating are consistent with a structural rearrangement of the tetrahedral sheet to permit the binding of larger cations in the ditrigonal cavity. The 23Na MAS NMR results indicate the presence of three unique sodium environments before and after heating. The heat-invariant resonance is consistent with the presence of sodium carbonate. The other two resonances are associated with interlayer sodium and reflect a migration of sodium to a dominantly anhydrous ditrigonal binding structure with heating. Quantitative elemental analysis and NMR data presented here suggest strontium is bound deep within the ditrigonal cavity of the collapsed micas.

NMR Study of a Rare-Earth Aluminoborosilicate Glass with Varying CaO-to-Na2O Ratio
A. Quintas, T. Charpentier, O. Majérus, D. Caurant, J.-L. Dussossoy and P. Vermaut
The effect of substituting two Na+ by one Ca2+ in a rare-earth aluminoborosilicate glass is investigated by multinuclear magic-angle spinning (MAS) and multiple-quantum (MQ)MAS nuclear magnetic resonance (NMR) spectroscopy. Quantitative analysis of the 23Na and 27Al MAS/MQMAS data along with the 11B MAS NMR data provides complementary information enabling to cast light on different structural key points. A strong decrease of the N 4 = BO4/(BO3 + BO4) ratio is observed consecutively to this substitution, indicating that sodium is more favorable than calcium to the formation of BO4 units. The experimental N 4 ratio is compared to the Dell and Bray model prediction and it is shown that several adjustments, due to the presence in our glass of Nd and Zr, are necessary to obtain acceptable agreement with experimental data. 29Si MAS NMR data also put in evidence an effect of the substitution on the polymerization degree. Glass in glass phase separation is clearly detected when the ratio of CaO to Na2O is greater than 1 and a different evolution of NMR parameters is observed for the ratio of CaO to Na2O being less than or equal to 1. Concerning aluminum charge compensation, it is demonstrated that, as long as no phase separation is detected, the negative charge of AlO4 − entities is almost exclusively balanced by sodium cations. Finally, changes of the sodium ions organization within the glass network are also evidenced by spin–lattice relaxation and spin echo decay measurements.

Laser-Heated High-Temperature NMR: A Time-Resolution Study
R. Winter, A. Jones, R. Shaw-West, M. Wolff, P. Florian and D. Massiot
The time resolution achievable in in situ high-temperature nuclear magnetic resonance experiments is investigated using laser heating of refractory materials. Three case studies using 27Al in alumina nanoparticles, 29Si in silicon carbide and 23Na in a glass-forming mixture of sodium carbonate and quartz have been conducted to distinguish the cases of (a) a fast-relaxing, high natural abundance nucleus, (b) a probe nucleus with low abundance and low spin–lattice relaxation rate, and (c) a complex and changing system of industrial relevance. The most suitable nucleus for in situ high-temperature studies is one with high abundance but slow relaxation because the differential relaxation time between hot and cold parts of the sample effectively removes the signal from the cold material. There is no "in situ penalty" from the diminishing Boltzmann polarization at high temperature since this effect is balanced by a corresponding increase of the spin–lattice relaxation rate.

Crystalline Aluminium Borates with the Mullite Structure: A 11B and 27Al Solid-State NMR Study
K. J. D. MacKenzie, M. E. Smith, T. F. Kemp and D. Voll
27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were acquired at 8.45, 14.1 and 16.45 T for a series of aluminium borates with the mullite structure (Al6−x B x O9, where x has nominal values of 1 to 4) augmented with 27Al multiple-quantum MAS NMR spectra at 8.45 T. Even though the 27Al NMR spectra are complex, simulation of the combined set of data produced a relatively well-defined set of parameters (e.g., quadrupolar interaction, isotropic chemical shift, etc.) for each site. The 11B MAS NMR spectra of the same compounds were also acquired at 14.1 T. Linear changes in the X-ray a-, b- and c-cell parameters with composition suggest that these compounds constitute a continuous series. Based on a Rietveld structural refinement of the compound synthesized as Al4B2O9, the resulting site occupancies and relative site distortions allow the identification of particular sites with specific NM resonances. Changes in the 27Al and 11B MAS NMR spectra of the related compounds with x = 1–4 show at the lowest Al contents a greater degree of asymmetry in the Al sites of the octahedral chains. A fairly distorted cross-linking tetrahedral site, which persists throughout the composition range, is accompanied in the lower Al compositions by two 5-fold coordinated Al–O units which are replaced by two more-regular tetrahedral Al–O sites as the Al content increases. In the compounds of lowest Al composition (i.e., highest B content) both the tetrahedral and trigonal cross-linking sites are distinguishable, but as the Al content increases, the BO4 units progressively disappear.

29Si, 27Al, 1H and 23Na MAS NMR Study of the Bonding Character in Aluminosilicate Inorganic Polymers
M. R. Rowles, J. V. Hanna , K. J. Pike, M. E. Smith and B. H. O'Connor
29Si, 27Al, 1H and 23Na solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to relate nominal composition, bonding character and compressive strength properties in aluminosilicate inorganic polymers (AIPs). The 29Si chemical shift varies systematically with Si-to-Al ratio, indicating that the immediate structural environment of Si is altering with nominal composition. Fast 1H MAS and 29Si T SiH/T 1ρ relaxation measurements demonstrated that occluded pore H2O mobility within the disordered cavities is slow in comparison with H2O mobility characteristics observed within the ordered channel structures of zeolites. The 27Al MAS NMR data show that the Al coordination remains predominantly 4-coordinate. In comparison with the 29Si MAS data, the corresponding 27Al MAS line shapes are relatively narrow, suggesting that the AlO4 tetrahedral geometry is largely unperturbed and the dominant source of structural disorder is propagated by large distributions of Si–O bond angles and bond lengths. Corresponding 23Na MAS and multiple-quantum MAS NMR data indicate that Na speciation is dominated by distributions of hydration states; however, more highly resolved 23Na resonances observed in some preparations supported the existence of short-range order. New structural elements are proposed to account for the existence of these Na resonances and an improved model for the structure of AIPs has also been proposed.

A Theoretical Study of 51V Electric Field Gradient Tensors in Pyrovanadates and Metavanadates
A. Y. H. Lo, J. V. Hanna and R. W. Schurko
A computational study of the 51V electric field gradient (EFG) tensors in pyrovanadates, α-Zn2V2O7, Cd2V2O7, β-Mg2V2O7 and BaCaV2O7, and the metavanadates, LiVO3, α-NaVO3, KVO3, ZnV2O6 and MgV2O6, is presented. Restricted Hartree–Fock and hybrid density functional theory calculations have been used to investigate the effects of the size of vanadium-oxygen clusters, basis set size, proton-termination and embedded cluster techniques on the accuracy of the calculated EFG tensors. Good agreement between theory and experiment is obtained for most of the vanadates. A sound methodology is suggested for calculating the EFG tensor in pyrovanadates which contain isolated V2O7 4− clusters. For metavanadates, the charges of the bridging oxygen atoms can be differentiated from those of terminal oxygen atoms by terminating the former with hydrogen atoms, and embedded cluster molecular orbital calculations are useful in accounting for the long-range electrostatic interactions which influence the EFG tensor components. EFG tensor orientations vary for different pyrovanadate structural types, and individual components are confined by symmetry elements in the metavanadates. A preliminary comparison is made between 51V EFG tensors calculated with ab initio and plane wave methods. Theoretical EFG tensor components and orientations, in combination with experimental 51V solid-state nuclear magnetic resonance data, are demonstrated to be useful tools for prediction of molecular structure.