A. P. Howes

A spectrometer designed for 6.7 and 14.1 T DNP-enhanced solid-state MAS NMR using quasi-optical microwave transmission

A Dynamic Nuclear Polarisation (DNP) enhanced solid-state Magic Angle Spinning (MAS) NMR spectrometer operating at 6.7 T is described and demonstrated. The 187 GHz TE(13) fundamental mode of the FU CW VII gyrotron is used as the microwave source for this magnetic field strength and 284 MHz (1)H DNP-NMR. The spectrometer is designed for use with microwave frequencies up to 395 GHz (the TE(16) second-harmonic mode of the gyrotron) for DNP at 14.1T (600 MHz (1)H NMR). The pulsed microwave output from the gyrotron is converted to a quasi-optical Gaussian beam using a Vlasov antenna and transmitted to the NMR probe via an optical bench, with beam splitters for monitoring and adjusting the microwave power, a ferrite rotator to isolate the gyrotron from the reflected power and a Martin-Puplett interferometer for adjusting the polarisation. The Gaussian beam is reflected by curved mirrors inside the DNP-MAS-NMR probe to be incident at the sample along the MAS rotation axis. The beam is focussed to a ~1 mm waist at the top of the rotor and then gradually diverges to give much more efficient coupling throughout the sample than designs using direct waveguide irradiation. The probe can be used in triple channel HXY mode for 600 MHz (1)H and double channel HX mode for 284 MHz (1)H, with MAS sample temperatures ≥85 K. Initial data at 6.7 T and ~1 W pulsed microwave power are presented with (13)C enhancements of 60 for a frozen urea solution ((1)H-(13)C CP), 16 for bacteriorhodopsin in purple membrane ((1)H-(13)C CP) and 22 for (15)N in a frozen glycine solution ((1)H-(15)N CP) being obtained. In comparison with designs which irradiate perpendicular to the rotation axis the approach used here provides a highly efficient use of the incident microwave beam and an NMR-optimised coil design.

NATURAL ABUNDANCE SOLID STATE 43CA NMR

Abstract 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.

Determination of titanium NMR parameters of ATiO3 compounds: correlations with structural distortion.

Solid state 47,49Ti NMR spectra have been obtained for a number of perovskite and ilmenite ATiO3 compounds. The 49Ti quadrupole coupling constant varies from 2.75 MHz (CaTiO3) to 15.5 MHz (MgTiO3) and the electric field gradient at the titanium site was found to correlate well with the shear strain, independent of structure. The chemical shift in the perovskite structures varies by approximately 160 ppm and correlates well with the mean Ti-O distance. The 25Mg and 113Cd NMR parameters are also reported for the relevant compounds.

Structural properties of multi-component silicon oxycarbide glasses derived from metal alkoxide precursors

Abstract Existing sol-gel polymerisation/pyrolysis routes employed to form simple silicon oxycarbide (SiOC) glass compositions have been adapted to form more complex, multi-component glasses based on borosilicon oxycarbide (BSiOC) and aluminosilicon oxycarbide (AlSiOC) systems. Each system has been characterised at the gel, glass and glass ceramic stage by X-ray powder diffraction (XRD), 29 Si, 27 Al and 11 B magic angle spinning nuclear magnetic resonance (MAS NMR) and transmission electron microscopy (TEM). The boron component of the BSiOC glass system exhibited predominantly BO 3 coordination. The Al component of the AlSiOC glass system exhibited combinations of tetrahedral, pentacoordinated and octahedral coordination over a range of pyrolysis temperatures. For both systems SiC bonds were formed and retained throughout the hydrolysis, polymerisation and inert atmosphere pyrolysis stages with evidence of β-SiC crystallisation at elevated temperatures (> 1300°C). Crystallisation of both spinel and mullite phases has also been observed at elevated temperatures (> 1300°C) in the AlSiOC glass system.

Ultra-high resolution 17O solid-state NMR spectroscopy of biomolecules: A comprehensive spectral analysis of monosodium L-glutamate·monohydrate

Monosodium L-glutamate monohydrate, a multiple oxygen site (eight) compound, is used to demonstrate that a combination of high-resolution solid-state NMR spectroscopic techniques opens up new possibilities for (17)O as a nuclear probe of biomolecules. Eight oxygen sites have been resolved by double rotation (DOR) and multiple quantum (MQ) NMR experiments, despite the (17)O chemical shifts lying within a narrow shift range of <50 ppm. (17)O DOR NMR not only provides high sensitivity and spectral resolution, but also allows a complete set of the NMR parameters (chemical shift anisotropy and electric-field gradient) to be determined from the DOR spinning-sideband manifold. These (17)O NMR parameters provide an important multi-parameter comparison with the results from the quantum chemical NMR calculations, and enable unambiguous oxygen-site assignment and allow the hydrogen positions to be refined in the crystal lattice. The difference in sensitivity between DOR and MQ NMR experiments of oxygen in bio/organic molecules is also discussed. The data presented here clearly illustrates that a high resolution (17)O solid-state NMR methodology is now available for the study of biomolecules, offering new opportunities for resolving structural information and hence new molecular insights.

Lone-pair effects and structural trends in x SnO?(1 ? x) P2O5 glasses deduced from?31P and?119Sn nuclear magnetic resonance

Tin phosphate glasses, of general formula xSnO ? (1 ? x)P2 O5 (0.3 < x < 0.8), have been prepared by conventional melt-quench techniques and their structures studied using ?31P and ?119Sn nuclear magnetic resonance. The distribution of [PO4] Qn species changes with composition in accordance with the simple binary model, and the changes in chemical shift can be explained by the redistribution of electron charge from the P = O double bond. Sn(II) is found to occupy a highly asymmetric site, typical of a sterically active lone pair of electrons. The 119Sn parameters of the chemical shift tensor change systematically with x, reflecting the change in local environment from one where the next nearest neighbours are predominantly Q2 phosphorus to one where they are predominantly Q0 phosphorus.

Density-structure relations in mixed-alkali borosilicate glasses by 29Si and 11B MAS-NMR

Abstract Densities and boron and silicate speciation in the mixed-alkali borosilicate glass system R (1/2Na 2 O+1/2Li 2 O)·B 2 O 3 · K SiO 2 have been measured over a wide range of R and K values. Previous models for the density of single-alkali borates and borosilicates and mixed-alkali borates have been used in an attempt to identify the roles of the two alkalies in this mixed-alkali borosilicate. Results are discussed in terms of the type and abundance of structural units present and the manner in which the alkali species apportion themselves between the borate and silicate units. Structural data obtained from 11 B and 29 Si magic angle spinning nuclear magnetic resonance (MAS–NMR) illustrate the similarities and differences between the mixed- and single-alkali systems. Below a critical value of R ( R max ), the current system exhibits a variation in N 4 , the fraction of tetrahedrally co-ordinated borons, similar to the Na-borosilicate system. Beyond this point, model N 4 predictions initially overestimate and then underestimate the experimental results seen in the current work. A semi-empirical formula, developed previously, has been employed to investigate the trends in density variation. Comparison, with previous reports, of the associated volumes of the silica Q 4 units shows a larger initial volume, V 6 , but a smaller value of d V 6 , the incremental change in volume of the [SiO 4 ] unit on introducing non-bridging oxygens (NBO). Some preliminary evidence is also presented for the existence of borosilicate units, below R max , not accounted for in most borosilicate density models.

Separation of isotropic chemical and second-order quadrupolar shifts by multiple-quantum double rotation NMR

Using a two-dimensional multiple-quantum (MQ) double rotation (DOR) experiment the contributions of the chemical shift and quadrupolar interaction to isotropic resonance shifts can be completely separated. Spectra were acquired using a three-pulse triple-quantum z-filtered pulse sequence and subsequently sheared along both the nu(1) and nu(2) dimensions. The application of this method is demonstrated for both crystalline (RbNO(3)) and amorphous samples (vitreous B(2)O(3)). The existence of the two rubidium isotopes ((85)Rb and (87)Rb) allows comparison of results for two nuclei with different spins (I=3/2 and 5/2), as well as different dipole and quadrupole moments in a single chemical compound. Being only limited by homogeneous line broadening and sample crystallinity, linewidths of approximately 0.1 and 0.2 ppm can be measured for (87)Rb in the quadrupolar and chemical shift dimensions, enabling highly accurate determination of the isotropic chemical shift and the quadrupolar product, P(Q). For vitreous B(2)O(3), the use of MQDOR allows the chemical shift and electric field gradient distributions to be directly determined-information that is difficult to obtain otherwise due to the presence of second-order quadrupolar broadening.

Boron environments in Pyrex® glass—a high resolution, Double-Rotation NMR and thermodynamic modelling study

It is shown, using the important technological glass Pyrex® as an example, that 1D and 2D (11)B Double-Rotation (DOR) NMR experiments, in combination with thermodynamic modelling, are able to provide unique structural information about complex glasses. (11)B DOR NMR has been applied to Pyrex® glass in order to remove both dipolar and quadrupolar broadening of the NMR lines, leading to high resolution spectra that allow unambiguous, accurate peak fitting to be carried out, of particular importance in the case of the 3-coordinated [BO(3)] (B3) trigonal planar environments. The data obtained are of sufficient quality that they can be used to test the distributions of borate and borosilicate superstructural units predicted by the thermodynamics-based Model of Associated Solutions. The model predicts the dominant boron-containing chemical groupings in Pyrex® glass to be those associated with B(2)O(3) and sodium tetraborate (with smaller amounts of sodium triborate, sodium diborate, sodium pentaborate, danburite and reedmergnerite). Excellent agreement is found between model and experiment provided the (11)B peaks with isotropic chemical shifts of -1.4 ppm and 0.5 ppm are assigned to B4 species from borosilicate units ([B(OSi)(4)] and [B(OSi)(3)(OB)]) and borate superstructural units (mainly triborate rings with some pentaborate and diborate) respectively. The peaks with isotropic shifts of 14 ppm and 18.1 ppm are then assigned to B3 in borate superstructural units (mainly triborate and pentaborate along with connecting B3) and boroxol rings respectively. The assignments of the DOR NMR peaks, are supported by the presence of cross-peaks in (11)B spin-diffusion DOR NMR spectra which can be used to develop a structural model in which B(2)O(3)-like regions are linked, via borate and borosilicate superstructural units, to the majority silica network. Pyrex® is thus shown to have a heterogeneous structure, with distinct molecular groupings that are far removed from a random distribution of network polyhedra with only short-range order.

A combined 14N/27Al nuclear magnetic resonance and powder X-ray diffraction study of impurity phases in β-sialon ceramics

Beta-sialons are ceramic phases occurring in the SiO(2)-Si(3)N(4)-AlN-Al(2)O(3) system. A series of samples with differing compositions has been investigated by magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy and powder X-ray diffraction (XRD). Although the constituent nitrogen nuclei occupy positions of low symmetry in the beta-sialon structure, 14N NMR spectra could be recorded for the samples examined. The origin of the 14N signal could be traced to the presence of an aluminium nitride (AlN) impurity phase with the help of 27Al NMR and XRD results. Similarly, the existence of Al(2)O(3) grains could be readily detected for a number of samples. Thus, the combination of 14N and 27Al NMR is shown to be an especially effective tool in identifying and characterizing impurity phases in sialon ceramics, complementing the results obtained from standard XRD analysis.