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Solid-state NMR spectroscopy and computational methods are used to probe the transformation of 10-hydroxy-10,9-boroxophenanthrene to its corresponding anhydride in the solid state. DFT calculations are used to assist the assignment of the NMR spectra of both these boron-containing heteroaromatic compounds, in solution and in the solid state. Solid-state NMR spectroscopy reveals that the dehydration of crystalline 10-hydroxy-10,9-boroxophenanthrene proceeds at relatively low temperatures—a transformation, that is, undetectable by X-ray diffraction and gravimetric analysis. Computational methods are used to elucidate a plausible reaction pathway for this transformation and to explain its detection only by NMR methods.

Molecular modeling has been used to assist in the design of a new structure directing agent (SDA) for the synthesis of the AlPO(4) form of STA-2, bis-cliazabicyclooctane-butane (BDAB). This is incorporated as a divalent cation within the large cages of STA-2, as determined via a combination of solid-state (13)C and (15)N MAS NMR, supported by (14)N and (1)H-(15)N HMQC solution NMR and density functional calculations. As prepared AlPO4 STA-2 containing cationic SDA molecules achieves neutrality by the inclusion of hydroxide ions bridging between 5-fold coordinated framework Al atoms. Synchrotron X-ray powder diffraction data of the dehydrated as-prepared form indicates triclinic symmetry (Al(12)P(12)O(48)(OH)(2)center dot BDAB, P1, a = 12.3821(2) angstrom, b = 12.3795(2) angstrom, c = 12.3797(3) angstrom, alpha = 63.3585(8)degrees, 63.4830(7)degrees, gamma = 63.4218(7)) with the distortion from rhombohedral R (3) over bar symmetry resulting from the partial order of hydroxide ions in bridging Al-OH-Al sites within cancrinite cages. Upon calcination in oxygen, the organic SDA is removed, leaving AlPO(4) STA-2 with a pore volume of 0.22 cm(3) g(-1) (R (3) over bar, Al(36)P(36)O(144), a = 12.9270(2) angstrom, c = 30.7976(4) angstrom). Dehydrated calcined AlPO(4) STA-2 has two crystallographically distinct P and Al sites: (31)P MAS NMR resolves the two distinct P sites, and although 27AI MAS NMR only partially resolves the two Al sites, they are separated by MQMAS. Furthermore, 2D (27)Al -> (31)P MQ-J-HETCOR correlation spectroscopy confirms that each framework Al is linked to the two different P sites via AI-O-P connections in a 3:1 ratio (and vice versa for P linked to different Al). The (27)Al and (31)P resonances are assigned to the crystallographic Al and P sites by calculation of the NMR parameters using the CASTEP DFT program for an energy-minimized AlPO(4)(SAT) framework.

The use of first-principles DFT calculations to interpret solid-state 89Y MAS NMR spectra of Y2Ti2-xSnxO7 pyrochlores, materials with applications for the encapsulation of actinide-bearing radioactive waste, is investigated. Although NMR is a sensitive probe of local structure, which does not rely on the presence of long-range order, spectra of disordered materials are often complex and difficult to interpret. We show how calculations can be used alongside experiment to confirm that Y occupies only the eight-coordinate pyrochlore A site in these materials and that the 89Y isotropic chemical shift is primarily affected by the number of Sn/Ti on the neighboring B sites. Small changes in local geometry and more distant B-site cation substitutions are shown to have a smaller effect on the chemical shift, and will result in broadening, shoulders, and small splittings in the NMR spectrum. In general, the results confirm the validity of the assumptions made in the previous spectral analysis, although in a very s...

We describe a detailed study of the aluminum phosphate AlPO-53 in both its as-made and calcined forms. In its as-made state, AlPO-53(A), the material is templated by methylammonium cations and contains occluded water molecules and also hydroxide ions that bridge pairs of aluminum atoms, increasing their coordination number to 5. Solid-state NMR experiments confirm the local environment of the aluminum and phosphorus atoms proposed in a previous structural model from powder X-ray diffraction. P-31 NMR shows the presence of four distinct resonances with an intensity ratio of 1: 1:2:2, consistent with the expected six crystallographic p Sites. Al-27 triple-quantum MAS NMR resolves five aluminum peaks, two with NMR parameters characteristic of four-coordinate Al and three of five-coordinate Al. One of these latter signals has a greater intensity than that of the others, consistent with the presence of two overlapping signals from two distinct crystallographic Al sites. First-principles calculations of NMR parameters provide a complete spectral assignment and confirm our interpretation of unresolved spectra. AlPO-53(A) is found to convert easily into a second crystalline phase on moderate heating (upon spinning in the NMR rotor for an extended period, for example), and variable-temperature powder X-ray experiments, together with TGA, suggest that this is a dehydration process yielding a second aluminophosphate, JDF-2. This is confirmed using both P-31 and Al-27 NMR, with the spectral assignment of JDF-2 supported by first-principles calculations. Calcination of AlPO-53(A) or of the dehydrated material, JDF-2, at 300 degrees C yields the microporous open-framework material AlPO-53(B), a tetrahedral network with three Al and three P sites, as confirmed by NMR and first-principles calculations. In addition to demonstrating the power of the combined use of NMR, first-principles calculations, and diffraction for detailed structural investigations, we show that the possibility of a reversible dehydration in as-made AlPO-53 and similar systems is an important consideration in structural studies and provides evidence that the published structural model for AlPO-53(A) may be incomplete.

A multi-technique investigation involving X-ray diffraction, solid-state NMR and first-principle calculations was carried out on the aluminophosphate material AlPO4-15. A synchrotron X-ray single-crystal diffraction study was carried out on the same sample as that used in solid-state NMR studies. The model from the single crystal study, together with a model from a literature high resolution study of the same material, was used as starting points for the first-principles calculations of the NMR parameters. This enabled the 31P and 27Al NMR spectra to be unambiguously assigned and all the NMR parameters calculated agreed well with the experimental spectra even without relaxing the X-ray derived structural models.

The hydrothermal reaction between silica glass and aqueous solutions of amines and sodium hydroxide has been investigated for the preparation of large crystals of siliceous zeolites. In the presence of n-butylamine at 160 °C needle-shaped crystals of ZSM-22 (TON-type) are formed. By adjusting the amount of n-butylamine used in crystallisation, crystals of up to 400 μm in length may be formed after 5 weeks of reaction. Calcination in air at 800 °C produces crystals free of template and scanning electron microscopy shows the samples to be free of cracks, with no evidence of loss of integrity of the specimens. The quality and indentity of the ZSM-22 crystals, both before and after calcination, is verified by using single crystal X-ray diffraction and 29Si MAS NMR and the refined structures compared to those previously reported for the same material. If the amine used in synthesis is 1,6-diaminohexane, usually used to prepare TON-type zeolites, then samples of ZSM-48, a disordered zeolite, are produced. These have very different crystal morphology, the samples being made up of intergrown plate-shaped crystallites agglomerated into spherical objects. The results we present illustrate the scope for the general use of the ‘bulk material dissolution’ method in the synthesis of siliceous zeolites as large single crystals.

First-principles calculations of O-17 quadrupolar and chemical shift NMR parameters were performed using CASTEP, a density functional theory (I)FT) code, to try and interpret high-resolution O-17 NMR spectra of the humite group minerals hydroxyl-chondrodite (2Mg(2)SiO(4)center dot Mg(OH)(2)) and hydroxyl-clinohumite (4Mg(2)SiO(4)center dot Mg(OH)(2)), which are models for the incorporation of water within the Earth's upper mantle. The structures of these humite minerals contain two possible crystallographically inequivalent H sites with 50% occupancy. Isotropic O-17 multiple-quantum magic angle spinning (MQMAS) spectra were therefore simulated using the calculated O-17 NMR parameters and assuming either a static or dynamic model for the positional disorder of the H atoms. Only the dynamic disorder model provided simulated spectra that agree with experimental O-17 MQMAS spectra of hydroxyl-chondrodite and hydroxyl-clinohumite. Previously published O-17 satellite-transition magic angle spinning (STMAS) spectra of these minerals showed significant dynamic line-broadenings in the isotropic frequency dimension. We were able to reproduce these line-broadenings with at least qualitative accuracy using a combination of the same dynamic model for the positional H disorder, calculated values for the change in O-17 quadrupolar NMR parameters upon H exchange, and a simple analytical model for dynamic line-broadening in MAS NMR experiments. Overall, this study shows that a combination of state-of-the-art NMR methodology and first-principles calculations of NMR parameters is able to provide information on dynamic processes in solids with atomic-scale resolution that is unobtainable by any other method.

Doping of low levels of sodium onto the cation sites of ceria-zirconia mixed oxides allows reduction of the materials to take place at lower temperatures than undoped materials, a desirable property for practical applications in supports for redox catalysis. Synthesis is achieved in a single step under mild hydrothermal conditions directly from aqueous mixtures of metals salts at high pH.

The structural effects of incorporating Cs into the monoclinic and tetragonal hollandites Ba1.2−xCsxMg1.2−x/2Ti6.8+x/2O16 and Ba1.2−xCsxAl2.4−xTi5.6+xO16 have been studied using powder neutron diffraction and 133Cs and 27Al MAS NMR. Addition of Cs to the monoclinic structure induces a ‘shear-type collapse’, in agreement with previously published results. NMR spectra show that the addition of Cs does not change the local structure around the Al cations within the tunnel walls. An algorithm is given that allows a prediction of unit cell parameters to be made for tetragonal hollandites containing barium.

The 29Si and 17O NMR parameters of six polymorphs of MgSiO3 were determined through a combination of high-resolution solid-state NMR and first-principles gauge including projector augmented wave (GIPAW) formalism calculations using periodic boundary conditions. MgSiO3 is an important component of the Earth's mantle that undergoes structural changes as a function of pressure and temperature. For the lower pressure polymorphs (ortho-, clino-, and protoenstatite), all oxygen species in the 17O high-resolution triple-quantum magic angle spinning (MAS) NMR spectra were resolved and assigned. These assignments differ from those tentatively suggested in previous work on the basis of empirical experimental correlations. The higher pressure polymorphs of MgSiO3 (majorite, akimotoite, and perovskite) are stabilized at pressures corresponding to the Earth's transition zone and lower mantle, with perovskite being the major constituent at depths660 km. We present the first 17O NMR data for these materials and confirm previous 29Si work in the literature. The use of high-resolution multiple-quantum MAS (MQMAS) and satellite-transition MAS (STMAS) experiments allows us to resolve distinct oxygen species, and full assignments are suggested. The six polymorphs exhibit a wide variety of structure types, providing an ideal opportunity to consider the variation of NMR parameters (both shielding and quadrupolar) with local structure, including changes in coordination number, local geometry (bond distances and angles), and bonding. For example, we find that, although there is a general correlation of increasing 17O chemical shift with increasing Si-O bond length, the shift observed also depends upon the exact coordination environment.

A short and convenient synthesis of fused 1,2,5-selenadiazoles from the corresponding 1,2,5-thiadiazoles and selenium dioxide has been developed.

The authors acknowledge financial support from the Russian Foundation for Basic Research (Project 13-03-00072), from the Presidium of the Russian Academy of Sciences (Programme No. 8, Project 8.14) and from the Leverhulme Trust (Project IN-2012-094). A short and convenient synthesis of fused 1,2,5-selenadiazoles from corresponding 1,2,5-thiadiazoles and selenium dioxide has been developed. Postprint

We describe a novel method, which we call STARTMAS, that allows acquisition of high-resolution or ‘isotropic’ spin I = 3/2 NMR spectra in solids in real time. Unlike the DOR technique of Samoson et al. [Mol. Phys. 65 (1988) 1013], STARTMAS is performed on a standard MAS probe and fast spinning rates are possible. The method consists of a multiple-pulse sequence interleaved with real-time data acquisition and exploits satellite (mI = ±3/2 ↔ ±1/2) and double-quantum (±3/2 ↔ ∓1/2) transitions. We show that STARTMAS experiments can be used to obtain one-dimensional DOR-like spectra or provide an ‘ultrafast’ route to high-resolution two-dimensional spectra.

29 Si MAS NMR spectroscopy on samples of 10-A phase synthesized from oxides (6.0 GPa/600 °C/400 h) and from partial transformation of talc (6.5 GPa/650 °C/12.5 h) reveals that this phase contains Q 2 -type Si sites in a ratio Q 3 :Q 2 of 5.33:1. It is proposed that the Q 2 arise from adjacent vacancies in the tetrahedral sheets for which charge balance is most likely achieved by hydroxylation via a hydrogarnet-like substitution involving the formation of Q 2 silanol groups. Variable-contact-time 29 Si { 1 H} CP/MAS NMR spectra of the talc/10-A phase product support the assignment of Q 2 Si to the proposed SiO 3 (OH) groups. Electron microprobe analysis, including oxygen, gives the following empirical formula normalized to three Mg apfu and inferring a hydrogarnet component Si → 4H associated with Si vacancies: Mg 3 Si 3.83(8) O 9.32 (OH) 2.68. 1.1(4)H 2 O. The observed Mg:Si indicates a significant Si deficiency relative to talc. Comparison of the 29 Si MAS NMR and microprobe data indicates that Si vacancies likely occur as single isolated entities, rather than as pairs or clusters, and that between 1 in 18 and 1 in 23 Si sites is vacant. The results suggest new and intriguing possibilities for the incorporation of excess H into the 10-A phase and, potentially, other phyllosilicates under upper-mantle conditions.

The sidebands observed in the magic angle spinning (MAS) NMR spectrum of a spin I = 1 2 H nucleus may be very strongly broadened if motion is present in the solid. The broadening arises from interference between the line-narrowing effects of MAS and the dynamics-driven reorientation of the 2 H quadrupole tensor. Here, we show that this motional broadening is absent or much reduced in the corresponding double-quantum MAS NMR spectrum. This observation suggests a new approach to studying dynamics in solids and examples are drawn from 2 H MAS NMR of oxalic acid dihydrate, sodium tetrathionate dihydrate, and the synthetic polymer PMMA.

Multiple-quantum magic angle spinning (MQMAS) and satellite-transition magic angle spinning (STMAS) are two well-known techniques for obtaining high-resolution, or "isotropic", NMR spectra of quadrupolar nuclei. It has recently been shown that dynamics-driven modulation of the quadrupolar interaction on the microsecond timescale results in linewidths in isotropic STMAS spectra that are strongly broadened, while, in contrast, the isotropic MQMAS linewidths remain narrow. Here, we use this novel methodology in an (27)AI (I = 5/2) NMR study of the calcined-dehydrated aluminophosphate AIPO-14 and two forms of as-synthesized AlPO-14, one prepared with isopropylamine (C3H7NH2) as the template molecule and one with piperidine (C5H10NH). For completeness, the P-31 and C-13 (both I=1/2) MAS NMR spectra are also presented. A comparison of the (27)AI MQMAS and STMAS NMR results show that, although calcined AIOPO-14 appears to have a rigid framework structure, the extent of motion in the two as-synthesized forms is significant, with clear evidence for dynamics on the microsecond timescale in the immediate environments of all four AI sites in each material. Variable-temperature (27)AI STMAS NMR studies of the two as-synthesized AIPO forms reveal the dynamics to be complex, with the motions of both the guest water molecules and organic template molecules shown to be contributing. The sensitivity of the STMAS NMR experiment to the presence of microsecond timescale dynamics is such that it seems likely that this methodology will prove useful in NMR studies of host-guest interactions in a wide variety of framework materials.

Defect-pyrochlores based on the formulation CsM {sub 0.5}W{sub 1.5}O{sub 6} (M=Ti, Ti/Zr, Zr and Hf) have been studied using neutron diffraction and magic-angle spinning nuclear magnetic resonance (MAS NMR). The results show that structural changes are linearly linked to the change in ionic-radius for the B-site, e.g. the unit cell changes from 10.2763 A for CsTi{sub 0.5}W{sub 1.5}O{sub 6} to 10.3820 A for CsZr{sub 0.4}W{sub 1.6}O{sub 6}. Changes in the NMR chemical shift correlate with the change in electronegativity on the B-site, and show the presence of only one Cs crystal site.

Solid-state NMR has become the method of choice for determining details of molecular-level structure in heterogeneous systems. Though spin-1/2 nuclei still form the core of most such studies, quadrupolar nuclei are increasingly being used. This review assesses what is currently possible, from achieving high-resolution spectra for quadrupolar nuclei (a prerequisite for most structure determination work), to forming correlation spectra which give qualitative details of spatial proximity of nuclei and the determination of internuclear distances, between quadrupolar spins and quadrupolar and spin-1/2 nuclei. Examples are given of each method discussed, and the advantages and disadvantages of the various experiments for different possible applications are assessed. © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part A 28A: 183–248, 2006.

Structural characteristics of the (Y1−xLax)2Ti2O7 system have been determined using time-of-flight (TOF) neutron and constant wavelength powder X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe micro-analysis (EPMA) and 89Y (I = ½) magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). Lattice parameters obtained from neutron data suggest that a pyrochlore (Fdm, Z = 8) solid solution exists from x = 0 to x = 0.134, whereupon a monoclinic La2Ti2O7-type phase (P21, Z = 4) exsolves. Both phases coexist for 0.134 ≤ x ≤ 0.807 and a single-phase solid solution with the monoclinic structure is stable for x > 0.807. Unit cell volumes from X-ray diffraction of samples synthesised within the pyrochlore regime show excellent agreement with the expected linear increase in unit cell volume across a solid solution, increasing from 1028.46 A3 for Y2Ti2O7 to 1037.49 A3 for (Y0.90825La0.09175)2Ti2O7. The limit of the monoclinic solid solution as observed by XRD is higher than that found from neutron diffraction (0.88 compared to 0.807), but this difference is attributed to variation in the sintering temperatures employed. Spectral analysis of NMR spectra for Y-rich compositions suggests that increasing proportions of La are incorporated onto the pyrochlore A-site randomly, up to the limit of solid solution. The NMR spectra of the monoclinic phase show four resonances – two broad and two sharp. These have been attributed to the four crystallographically distinct ‘A’-type sites within the structure. At lower Y concentrations, the sites corresponding to the broad NMR resonances are preferentially occupied. We have tentatively assigned these broad resonances to the ‘slab-edge’ sites in the monoclinic perovskite structure, based on their relatively small size and lower average coordination numbers compared to the perovskite-like sites within the centre of the slabs.

The authors thank Johnson Matthey and the EPSRC for an Industrial CASE award to L.M. We gratefully acknowledge the IAESTE UK for a scholarship to B.E. They also thank the Leverhulme Trust (F/00 268/BJ), EPSRC (EP/J501542/1), and the EaStCHEM Research Computing Facility. The trivalent metal cations Al3+, Cr3+, and Fe3+ were each introduced, together with Sc3+, into MIL-100(Sc,M) solid solutions (M=Al, Cr, Fe) by direct synthesis. The substitution has been confirmed by powder X-ray diffraction (PXRD) and solid-state NMR, UV/Vis, and X-ray absorption (XAS) spectroscopy. Mixed Sc/Fe MIL-100 samples were prepared in which part of the Fe is present as α-Fe2O3 nanoparticles within the mesoporous cages of the MOF, as shown by XAS, TGA, and PXRD. The catalytic activity of the mixed-metal catalysts in Lewis acid catalysed Friedel–Crafts additions increases with the amount of Sc present, with the attenuating effect of the second metal decreasing in the order Al>Fe>Cr. Mixed-metal Sc,Fe materials give acceptable activity: 40 % Fe incorporation only results in a 20 % decrease in activity over the same reaction time and pure product can still be obtained and filtered off after extended reaction times. Supported α-Fe2O3 nanoparticles were also active Lewis acid species, although less active than Sc3+ in trimer sites. The incorporation of Fe3+ into MIL-100(Sc) imparts activity for oxidation catalysis and tandem catalytic processes (Lewis acid+oxidation) that make use of both catalytically active framework Sc3+ and Fe3+. A procedure for using these mixed-metal heterogeneous catalysts has been developed for making ketones from (hetero)aromatics and a hemiacetal. Postprint

REM is a Royal Society Industry Fellow and thanks the Royal Society for the provision of the Brian Mercer Award for Innovation, and thanks Scottish Enterprise for support. REM also thanks the British Heart Foundation for a New Horizons Award (NH/11/8/29253). REM and TD thank the EPSRC for funding (EP/K025112/1 and EP/K005499/1). The highly porous nature of metal-organic frameworks (MOFs) offers great potential for the delivery of therapeutic agents. Here, we show that highly porous metal-organic frameworks can be used to deliver multiple therapeutic agents—a biologically active gas, an antibiotic drug molecule, and an active metal ion—simultaneously but at different rates. The possibilities offered by delivery of multiple agents with different mechanisms of action and, in particular, variable timescales may allow new therapy approaches. Here, we show that the loaded MOFs are highly active against various strains of bacteria. Publisher PDF

The authors acknowledge the EPSRC for the award of a studentship to S.S. Accepted Oct 8th 2014 Solid-state nuclear magnetic resonance (NMR) spectroscopy has long been established as offering unique atomic-scale and element-specific insight into the structure, disorder, and dynamics of materials. NMR spectra of quadrupolar nuclei (I > (1)/2) are often perceived as being challenging to acquire and to interpret because of the presence of anisotropic broadening arising from the interaction of the electric field gradient and the nuclear electric quadrupole moment, which broadens the spectral lines, often over several megahertz. Despite the vast amount of information contained in the spectral line shapes, the problems with sensitivity and resolution have, until very recently, limited the application of NMR spectroscopy of quadrupolar nuclei in the solid state. In this Perspective, we provide a brief overview of the quadrupolar interaction, describe some of the basic experimental approaches used for acquiring high-resolution NMR spectra, and discuss the information that these spectra can provide. We then describe some interesting recent examples to showcase some of the more exciting and challenging new applications of NMR spectra of quadrupolar nuclei in the fields of energy materials, microporous materials, Earth sciences, and biomaterials. Finally, we consider the possible directions that this highly informative technique may take in the future. Postprint

The very broad resonances of quadrupolar (spin I1/2) 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.

The high sensitivity of the satellite-transition (ST) MAS NMR technique was exploited to obtain high-resolution 17O MAS NMR spectra of the three polymorphs of Mg2SiO4: forsterite ({alpha}-Mg2SiO4), wadsleyite (s-Mg2SiO4), and ringwoodite ({gamma}-Mg2SiO4). High NMR sensitivity was important in this application because 17O-enriched, Fe-free materials are required for 17O NMR and high-pressure syntheses of the dense s and {gamma} polymorphs result in a only a few milligrams of these solids. In all, eight distinct O species were identified and assigned: three in forsterite, four in wadsleyite, and one in ringwoodite, in agreement with the number of O sites in their crystal structures. The isotropic chemical shifts extracted are in excellent agreement with a previously published correlation with Si-O bond length. However, unexpectedly large quadrupolar coupling constants were found for the non-bridging O species in the dense polymorphs wadsleyite and ringwoodite.

Pollucite, CsAlSi2O6, a Cs polymorph of leucite (KAlSi2O6), has been proposed for ceramic immobilization of 135Cs and 137Cs fission products. 133Cs NMR of both low- (tetragonal) and high-temperature (cubic) forms of pollucite exhibit a considerable distribution of local Cs environments. 29Si and 27Al NMR data from directly prepared pollucite show greater Al/Si disorder than either leucite, or pollucite produced by ion exchange. Little evidence for Cs motion is observed in tetragonal or cubic pollucite, and only at high temperatures (∼850°C) is any substantial dynamic behavior detected. Dynamic NMR lineshape calculations allow a determination of the frequency of Cs motion and diffusivity.

Protocols are presented for obtaining natural abundance 17 O magic angle spinning and static NMR spectra in the solid state. Rotor-assisted population transfer (RAPT), Carr–Purcell–Meiboom–Gill (CPMG) echo trains and cross-polarisation (CP) are all used to obtain spectra of sites with large as well as small electric field gradients in proton and non-proton containing inorganic materials. Spectra are of sufficient quality to obtain the typical NMR parameters by standard fitting of the spectra. The protocol is then applied to identifying the changes that accompany radioactive decay in zircon (ZrSiO 4 ) where enrichment is impossible. The 17 O NMR spectra of a partially metamict zircon sample clearly show evidence of bridging oxygens being produced as a consequence of radiation damage. The spectra have been acquired at moderate magnetic fields over periods typically of 60 h (1 weekend) and it is concluded that a ‘routine’ overnight 17 O experiment of 15 h at high field (e.g. 21 T) may well be possible.

Satellite-transition magic angle spinning (STMAS) is a recently introduced technique for recording high-resolution NMR spectra of quadrupolar nuclei in solids. We present numerical calculations of STMAS signal intensity as a function of radio frequency field strength (v(1)) and spinning rate (v(R)) and show that the sensitivity advantage of STMAS over the older multiple-quantum technique (MQMAS) is greatest for low v(1) field strengths and high v(R) rates, making STMAS particularly suitable for the study of low-gamma nuclei. The practical implementation of STMAS in NMR of low-y nuclei is discussed and several experimental examples of high-resolution K-39 (I = 3/2) and Mg-25 (I = 5/2) NMR spectra are presented, including Mg-25 spectra of brucite (Mg(OH)(2)), diopside (CaMgSi2O6), and talc (Mg3Si4O10(OH)(2)) recorded at the natural Mg-25 abundance of 10%.

Spin-locking of half-integer quadrupolar nuclei, such as 23Na (I=3/2) and 27Al (I=5/2), is of renewed interest owing to the development of variants of the multiple-quantum and satellite-transition magic angle spinning (MAS) nuclear magnetic resonance experiments that either utilize spin-locking directly or offer the possibility that spin-locked states may arise. However, the large magnitude and, under MAS, the time dependence of the quadrupolar interaction often result in complex spin-locking phenomena that are not widely understood. Here we show that, following the application of a spin-locking pulse, a variety of coherence transfer processes occur on a time scale of approximately 1/omegaQ before the spin system settles down into a spin-locked state which may itself be time dependent if MAS is performed. We show theoretically for both spin I=3/2 and 5/2 nuclei that the spin-locked state created by this initial rapid dephasing typically consists of a variety of single- and multiple-quantum coherences and nonequilibrium population states and we discuss the subsequent evolution of these under MAS. In contrast to previous work, we consider spin-locking using a wide range of radio frequency field strengths, i.e., a range that covers both the "strong-field" (omega1 >> omegaQPAS and "weak-field" (omega1 << omegaQPAS limits. Single- and multiple-quantum filtered spin-locking experiments on NaNO2, NaNO3, and Al(acac)3, under both static and MAS conditions, are used to illustrate and confirm the results of the theoretical discussion.

We investigate the nature of higher-order effects arising in solid-state nuclear magnetic resonance (NMR) when quadrupolar nuclei are subject to significant chemical shift anisotropies. It is shown that the quadrupole interaction can give rise to shielding-derived terms that are not entirely averaged away by conventional magic-angle spinning (MAS). These terms are proportional to the square of the z component of the spin angular momentum and therefore leave unaffected both the central and other −mI↔+mI symmetric multiple-quantum transitions, yet lead to noticeable effects when monitoring other nonsymmetric transitions within the spin manifold. The recently-developed satellite-transition (ST) MAS NMR method for the simultaneous averaging of the first- and second-order quadrupole effects makes such quadrupole-shielding cross terms observable. Although this may present a resolution limitation to this averaging scheme, it opens up new possibilities for determining the coupling parameters of the quadrupolar nu...

Multiple-quantum (MQ) and satellite-transition (ST) magic-angle spinning (MAS) are two very similar techniques used to obtain high-resolution or ‘isotropic’ NMR spectra of half-integer quadrupolar nuclei. In a variety of materials it is observed that some STMAS peaks are very broad compared with the corresponding MQMAS peaks, sometimes so broad that they are unobservable. We present 17 O ( I =5/2) NMR spectra of two materials, chondrodite (2Mg 2 SiO 4 · Mg(OH) 2 ) and clinohumite (4Mg 2 SiO 4 ·Mg(OH) 2 ), exhibiting this phenomenon and show that the cause is motional broadening arising from the combined effects of molecular reorientation, the quadrupolar interaction and MAS.

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.

Amorphous sodium gallium silicate materials are isolated after short periods of heating a gel of composition Ga2O3 ∶ SiO2 ∶ 4NaOH ∶ 80H2O at 100 °C. On extended heating (periods in excess of 24 hours), the crystalline zeolite gallium hydroxosodalite is produced from the reaction mixture. We have studied the materials using 23Na (I = 3/2) MAS and multiple-quantum MAS (MQMAS) NMR spectroscopy in order to extract information about the local environment of the sodium in the amorphous solids, and thereby gain information about the role of sodium during the crystallisation of the zeolite. The average 23Na isotropic chemical shift, δiso, and the quadrupolar parameter, PQ = CQ(1 + η2/3)½, were evaluated by analysis of the two-dimensional MQMAS spectra. In addition to the sodium gallium silicate materials, a number of crystalline sodium silicates, aluminates and gallates were also studied and their NMR parameters compared with literature values and related to crystallographic data describing the local environment of the sodium atoms. Although no simple correlation is known between 23Na NMR parameters and the coordination number or average sodium–oxygen interatomic distance, a general trend of increasing δiso with decreasing coordination number and interatomic distance is apparent. The data from the amorphous sodium gallium silicates strongly suggest that even after very short periods of heating the local environment of sodium is very similar to that found in the crystalline zeolite product. Additional 71Ga (I = 3/2) MAS NMR spectra were also recorded from the amorphous samples to provide information on the local gallium environment and these show the presence of tetrahedral gallium, as in the crystalline zeolite, even in the samples prepared after the shortest period of heating.

Two-dimensional triple-quantum 1 7 O (1 = 5/2) MAS NMR powder spectra were obtained for orthoenstatite, clinoenstatite, and protoenstatite (MgSiO 3 polymorphs) and diopside (CaMgSi 2 O 6 ). The spectra were analyzed to yield the 1 7 O isotropic chemical shifts (δ C S ) and quadrupolar coupling parameters (P Q = C Q (1 + η 2 /3) 1 / 2 ) of the oxygen sites in each sample. High-resolution isotropic projections exhibit resolved 1 7 O peaks equal in number to the number of crystallographically inequivalent oxygen sites. Bridging and nonbridging oxygens were distinguished by their characteristic P Q values. Further assignments were possible by comparing the isotropic 1 7 O NMR spectra for this series of similar compounds with that reported previously for forsterite (Mg 2 SiO 4 ). Full assignments of the three oxygen sites in protoenstatite and diopside and of the six sites in orthoenstatite and clinoenstatite are suggested.

S.V. is supported by an EPSRC Doctoral Training Grant; B.E.B. acknowledges an EaStCHEM Hirst Academic Fellowship by the School of Chemistry, St Andrews and funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (REA 334496) The synthesis of rigid symmetric polyradical model systems with inter-spin distances between 1.4 and 4 nm and their room temperature continuous wave (CW) EPR spectra are reported. Conditions for attachment of the spin-label via esterification have been optimized on the direct synthesis of polyradicals from commercially available polyphenols and the carboxylic acid functionalized nitroxide TPC. A common synthetic protocol utilizing 4-hydroxy-4′-iodobiphenyl as a key building block has been used to synthesize an equilateral biradical and a triradical in only two steps from commercially available starting materials. The first synthesis of a tetraradical based upon an adamantane core bearing six equivalent nitroxide–nitroxide distances is also reported. These systems are very promising candidates for studying multi-spin effects in pulsed EPR distance measurements. Publisher PDF

The authors thank the Leverhulme Trust (F/00 268/BJ), EPSRC (EP/E041825/1, EP/J501542/1, and EP/F018096/1) and the European Commission FP6 Marie Curie Research Training Network “INDENS” (MRTN-CT-2004–005503), and the Leverhulme Trust (F/00 268/BJ) for support. Accepted 23rd October 2014. The substitution of the divalent cations Mg2+ and Zn2+ into the aluminophosphate (AlPO) framework of STA-2 has been studied using an “NMR crystallographic” approach, combining multinuclear solid-state NMR spectroscopy, X-ray diffraction and first-principles calculations. Although the AlPO framework itself is inherently neutral, the positive charge of the organocation template in an as-made material is usually balanced either by the coordination to the framework of anions from the synthesis solution, such as OH− or F−, and/or by the substitution of aliovalent cations. However, the exact position and distribution of the substituted cations can be difficult to determine, but can have a significant impact upon the catalytic properties a material exhibits once calcined. For as-made Mg substituted STA-2, the positive charge of the organocation template is balanced by the substitution of Mg2+ for Al3+ and, where required, by hydroxide anions coordinated to the framework. [27] Al MAS NMR spectra show that Al is present in both tetrahedral and five-fold coordination, with the latter dependent on the amount of substituted cations, and confirms the bridging nature of the hydroxyl groups, while high-resolution MQMAS spectra are able to show that Mg appears to preferentially substitute on the Al1 site. This conclusion is also supported by first-principles calculations. The calculations also show that 31P chemical shifts depend not only on the topologically-distinct site in the SAT framework, but also on the number of next-nearest-neighbour Mg species, and the exact nature of the coordinated hydroxyls (whether the P atom forms part of a six-membered ring, P(OAl)2OH, where OH bridges between two Al atoms). The calculations demonstrate a strong correlation between the 31P isotropic chemical shift and the average bond angle. In contrast, for Zn substituted STA-2, both X-ray diffraction and NMR spectroscopy show less preference for substitution onto Al1 or Al2, with both appearing to be present, although that into Al1 appears slightly more favoured. Postprint

Funding: Engineering and Physical Sciences Research Council (EPSRC) Four related sterically restricted pen-substituted acenaphthenes have been prepared containing mixed tin phosphorus moieties in the proximal 5,6-positions (Acenap[SnR3][(PPr2)-Pr-i]; Acenap = acenaphthene-5,6-diyl; R-3 = Ph-3 (1), Ph2Cl (2), Me2Cl (3), Bu2Cl (4)). The degree of intramolecular P-Sn bonding within the series was investigated by X-ray crystallography, solution and solid-state NMR spectroscopy, and density functional theory (DFT/B3LYP/SBKJC/PCM) calculations. All members of the series adopt a conformation such that the phosphorus lone pair is located directly opposite the tin center, promoting an intramolecular donor acceptor P -> Sn type interaction. The extent of covalent bonding between Sn and P is found to be much greater in triorganotin chlorides 2-4 in comparison with the triphenyl derivative 1. Coordination of a highly electronegative chlorine atom naturally increases the Lewis acidity of the tin center, enhancing the Ip(P)-sigma*(Sn-Y) donor acceptor 3c-4e type interaction, as indicated by conspicuously short Sn-P peri distances and significant (1)J(P-31,Sn-119) spin spin coupling constants (SSCCs) in the range 740-754 Hz. Evidence supporting the presence of this interaction was also found in solid-state NMR spectra of some of the compounds which exhibit an indirect spin spin coupling on the same order of magnitude as observed in solution. DFT calculations confirm the increased covalent bonding between P and Sn in 2-4, with notable WBIs of ca. 0.35 obtained, in comparison to 0.1 in 1. Postprint

Periodic density functional theory (DFT) calculations have recently emerged as a popular tool for assigning solid-state nuclear magnetic resonance (NMR) spectra. However, in order for the calculations to yield accurate results, accurate structural models are also required. In many cases the structural model (often derived from crystallographic diffraction) must be optimised (i.e., to an energy minimum) using DFT prior to the calculation of NMR parameters. However, DFT does not reproduce weak long-range "dispersion'' interactions well, and optimisation using some functionals can expand the crystallographic unit cell, particularly when dispersion interactions are important in defining the structure. Recently, dispersion-corrected DFT (DFT-D) has been extended to periodic calculations, to compensate for these missing interactions. Here, we investigate whether dispersion corrections are important for aluminophosphate zeolites (AlPOs) by comparing the structures optimised by DFT and DFT-D (using the PBE functional). For as-made AlPOs (containing cationic structure-directing agents (SDAs) and framework-bound anions) dispersion interactions appear to be important, with significant changes between the DFT and DFT-D unit cells. However, for calcined AlPOs, where the SDA-anion pairs are removed, dispersion interactions appear much less important, and the DFT and DFT-D unit cells are similar. We show that, while the different optimisation strategies yield similar calculated NMR parameters (providing that the atomic positions are optimised), the DFT-D optimisations provide structures in better agreement with the experimental diffraction measurements. Therefore, it appears that DFT-D calculations can, and should, be used for the optimisation of calcined and as-made AlPOs, in order to provide the closest agreement with all experimental measurements. Postprint

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.

A new approach to combining cross-polarization with multiple-quantum MAS in a two-dimensional NMR experiment is demonstrated, involving cross-polarization from 1 H to the single-quantum coherences of a quadrupolar nucleus. These coherences are then converted directly to multiple-quantum coherences rather than via a population state. In two separate methods, pure-absorption lineshapes are obtained using a z-filter and a shifted echo, respectively, with the latter, a `reversed split-t1' method, demonstrated to be particularly effective. The efficiency of both sequences may be improved by incorporating `fast-amplitude-modulation' (FAM) pulses for both the conversion and excitation of triple-quantum coherences.

Two-dimensional 27Al multiple-quantum magic angle spinning (MQMAS) NMR spectroscopy is used to extract the isotropic chemical shifts and quadrupolar parameters of five amorphous aluminosilicates, all of approximately mullite composition (3Al2O3·2SiO2) but of widely differing synthetic origin. Three principal types of Al site are apparent in each sample: two of these are conventionally assigned to 4- and 6-coordinate Al, while the nature of the third site, observed at a shift of δ≈30 ppm , remains a subject of debate. In some of the more anhydrous samples, two 6-coordinate Al sites are observed. Significant distributions of isotropic chemical shifts and quadrupolar parameters are evident in each of the Al sites resolved in the two-dimensional spectra and lineshape fitting is used to estimate the means and widths of these. Additional data are obtained from 27Al{1H} CPMAS NMR experiments and suggest that the protons in the samples are most closely associated with particular 6-coordinate Al sites. The NMR results from the five samples are compiled and compared with those reported for other amorphous and crystalline aluminosilicates and the possible nature of the δ≈30 ppm site is discussed.

MHH thanks the Ministry of Education, Brunei for award of a scholarship. EPSRC are thanked for a DTG studentship to MDP. Perovskites of nominal composition NaCe1−xLaxTi2O6 (0≤x≤1) crystallise directly under hydrothermal conditions at 240 °C. Raman spectroscopy shows distortion from the ideal cubic structure and Rietveld analysis of powder X-ray and neutron diffraction reveals that the materials represent a continuous series in rhombohedral space group R3̄c. Ce LIII-edge X-ray absorption near edge structure spectroscopy shows that while the majority of cerium is present as Ce3+ there is evidence for Ce4+. The paramagnetic Ce3+ affects the chemical shift and line width of 23Na MAS NMR spectra, which also show with no evidence for A-site ordering. 2H MAS NMR of samples prepared in D2O shows the inclusion of deuterium, which IR spectroscopy shows is most likely to be as D2O. The deuterium content is highest for the cerium-rich materials, consistent with oxidation of some cerium to Ce4+ to provide charge balance of A-site water. Publisher PDF

Mixtures of the clay mineral kaolinite with gibbsite, when heated, form mullite, a high-temperature ceramic. The 27Al multiple-quantum magic angle spinning (MQMAS) NMR technique is used to study the local Al environment in mixtures of kaolinite and gibbsite that have been ground together for varying amounts of time. This mechanical treatment forms new chemical species that are amorphous in nature and, therefore, unsuited to analysis by X-ray diffraction. Both novel and established cross-polarization techniques are employed to characterize the starting materials and mixtures, confirm the spatial proximity of 1H and 27Al nuclei, and “edit” the two-dimensional 27Al MQMAS NMR spectra, thereby allowing overlapping peaks corresponding to octahedral Al sites to be studied.

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.

The metastable ilmenite polymorph of NaNbO(3), instead of the expected perovskite polymorph, may be prepared directly in one step under mild hydrothermal conditions by lowering pH and using close to stoichiometric amounts of metal precursors; in situ energy-dispersive X-ray diffraction shows that crystallisation occurs rapidly via a sequence of intermediate crystalline phases.

Three- and five-quantum 17 O MAS NMR experiments are used to resolve fully the three crystallographically distinct oxygen species in forsterite (Mg 2 SiO 4 ). The chemical shift and quadrupolar parameters extracted from these spectra are compared with the literature values obtained using conventional 17 O MAS and dynamic-angle-spinning (DAS) NMR.

The 27Al multiple-quantum magic angle spinning (MQMAS) NMR technique is used to investigate the thermal transformation between AlMePO-β and AlMePO-α. The required spectral resolution is achieved using a novel split-t1 version of the basic five-quantum MAS experiment. The five-quantum split-t1 27Al MAS NMR spectrum of a sample where the thermal transformation has been interrupted before complete conversion is compared with that of a simple physical mixture of the α- and β-polymorphs. The two spectra are shown to be different, a result that does not appear to support the proposition by Carter et al. (J. Mater. Chem. 1997, 7, 2287) of a topotactic reconstructive transformation. On the basis of our 27Al MQMAS results, we outline a possible alternative (and rather more commonplace) mechanism for the thermal transformation.

Using 27 Al ( I =5/2) NMR of aluminium acetylacetonate, we show that it is possible to cross-polarize from a spin I =1/2 nucleus ( 1 H ) directly to the central triple-quantum transition of a half-integer quadrupolar nucleus ( 27 Al ) in a powdered sample under MAS conditions. The optimum conditions for this multiple-quantum cross-polarization (MQCP) are investigated experimentally and compared with existing theoretical results. The new technique is applied to the recently introduced two-dimensional MQMAS experiment for recording high-resolution NMR spectra of half-integer quadrupolar nuclei.