1H Magic Angle Spinning Research Articles

Investigation of Ti-doped NaAlH4 by solid-state NMR

ABSTRACTIn recent years, the development of Ti-doped NaAlH4 as a hydrogen storage material has gained attention because of its large weight percentage of hydrogen (∼5 %) compared to traditional interstitial hydrides. The addition of transition-metal dopants, in the form of Ti-halides, such as TiCl3, dramatically improves the kinetics of the absorption and desorption of hydrogen from NaAlH4. However, the role that Ti plays in enhancing the absorption and desorption of H2 is still unknown. In the present study, 27Al, 23Na, and 1H MAS (Magic Angle Spinning) NMR (Nuclear Magnetic Resonance) has been performed to understand the titanium speciation in Ti-doped NaAlH4. All experiments were performed on a sample of crushed single crystals exposed to Ti during growth, a sample of solvent-mixed 4TiCl3 + 112NaAlH4, a reacted sample of solvent-mixed TiCl3 + 3NaAlH4 with THF, and a reacted sample of ball-milled TiCl3 + 3NaAlH4. The 27Al MAS NMR has shown differences in compound formation between solvent-mixed TiCl3 + 3NaAlH4 with THF and the ball-milled TiCl3 + 3NaAlH4. 27Al MAS NMR of the ball-milled mixture of fully-reacted TiCl3 + 3NaAlH4 showed spectral signatures of TiAl3 while, the solvent-mixed 4TiCl3 + 112NaAlH4, which is totally reacted, does not show the presence of TiAl3, but suggests the existence of Al2O3.

1H detected 1H, 15N correlation spectroscopy in rotating solids

We describe new correlation experiments suitable for determining long-range 1H– 1H distances in 2H, 15N-labeled peptides and proteins. The approach uses perdeuteration together with back substitution of exchangeable protons during sample preparation as a means of attenuating the strong 1H– 1H dipolar couplings that broaden 1H magic angle spinning (MAS) spectra of solids. In the approach described here, we retain 100% of the 1H sensitivity by labeling and detecting all exchangeable sites. This is in contrast to homonuclear multiple pulse decoupling sequences that are applied during detection and that compromise sensitivity because of the requirement of sampling between pulses. As a result 1H detection provides a gain in sensitivity of >5 compared to the 15N detected version of the experiment (at a MAS frequency of 13.5 kHz). The pulse schemes make use of the favorable dispersion of the amide 15Ns resonances in the protein backbone. The experiments are demonstrated on a sample of the uniformly 2H, 15N-labeled dipeptide N-Ac-Val-Leu-OH and are analogous to the solution-state suite of HSQC-NOESY experiments. In this compound the 1H amide linewidths at 750 MHz vary from ∼0.67 ppm at ω r/2π∼5 kHz to ∼0.20 ppm at ω r/2π∼30 kHz, indicating that useful resolution is available in the 1H spectrum via this approach. Since the experiments circumvent the problem of dipolar truncation in the 1H– 1H spin system, they should make it possible to measure long-range distances in a uniformly labeled environment. Thus, we expect the experiments to be useful in constraining the global fold of a protein.

Cellular environment of metabolites and a metabonomic study of tamoxifen in endometrial cells using gradient high resolution magic angle spinning 1H NMR spectroscopy

High resolution magic angle spinning (HRMAS) 1H NMR spectroscopy was used to metabolically characterise Ishikawa cells, a human cell line derived from endometrial adenocarcinoma. The spectra obtained had well-resolved resonances from the nucleotide derivatives of uridine and adenosine. Using a combination of diffusion- and relaxation-weighted spectroscopy, the cellular environment of key metabolites previously identified as related to cell growth was also investigated. As Ishikawa cells are hormone-responsive, the metabolic action of tamoxifen, a selective estrogen receptor modulator (SERM), was also investigated. Cells were exposed to 5, 1 and 0.1 μM tamoxifen. Using the statistical regression technique of prediction to latent structures by partial least squares, a predictive model was built modelling the metabolic profile of the cells against exposure to tamoxifen. These spectral changes were characterised by increased resonance intensities from ethanolamine (3.26 ppm), glucose (3.34–3.94 ppm), glutamate (2.14, 2.32 ppm), tyrosine (7.24 ppm), uridine (7.85 ppm) and adenosine (8.20 ppm), and a relative decrease in contributions from myo-inositol resonances (3.30, 3.62, 3.55 ppm). The nucleotide changes suggest that tamoxifen affects RNA transcription, while the changes in ethanolamine and myo-inositol concentrations are indicative of cell membrane turnover.

Solid state 1H NMR study, humidity sensitivity and protonic conduction of gel derived phosphosilicate glasses

1H Magic Angle Spinning (MAS) NMR and 1H T1 and T1ρ relaxation times measurements were used to investigate structural aspects of gel derived glasses of composition 10P2O5·90SiO2 (10P) and 30P2O5·70SiO2 (30P), with the aim of understanding the different humidity sensitive behaviour observed for films of the same composition. The experimental evidence suggests that 10P and 30P gel samples heat treated at 100 °C have a similar POH environment while further heating causes a different structural evolution of POH units. In particular, the 30P sample heat treated at 400 °C shows a single 1H resonance while a number of resonances are seen in the 1H spectrum of the 10P sample heat treated at 400 °C. Further evidence of the different structure of 10P and 30P samples was given by the NMR relaxation times measurements, the gels were found to be heterogeneous on a nanometric scale. Domains, about 60 nm in size, related to 1H resonance in POH were evenly distributed in the matrix of both gels, only in the 10P gel also larger domains (about 700 nm) were found that can be related to 1H resonance in SiOH groups. Finally, MAS in combination with variable temperature static NMR spectra provided detailed information on the network structural features present in the 30P sample heat treated at 300 °C as well as in the 10P sample heat treated at 400 °C and allowed investigation of the dynamic processes taking place in these samples.

Solid-State Double Nuclear Magnetic Resonance Study of the Local Structure of Calcium Phosphate Nanoparticles Synthesized by a Wet-Mechanochemical Reaction

Calcium phosphate synthesized by a wet-mechanochemical reaction between Ca(OH)2 and H3PO4 is identified by X-ray diffractometry as an apparently single phase of hydroxyapatite (HAp). We examined more detail on the local structure of the product by high-resolution 1H and 31P solid-state double nuclear magnetic resonance (NMR) spectroscopy. Adsorbed and structural waters, as well as two kinds of OH groups in HAp, were observed in the inversion recovery 1H magic angle spinning (MAS) spectra. HAp and other calcium phosphate were observed in 1H→31P cross polarization (CP)/MAS spectra. The 31P differential cross polarization (DCP)/MAS and 1H inversion recovery → 31P CP/MAS measurements revealed different chemical interactions between 1H and 31P and, hence, could distinguish between ordered and disordered HAp. This micro-inhomogenity results from interiors and near-surface of nanoparticles as well as Ca2+-deficiency.

1H MAS NMR spectra of kaolinite/formamide intercalation compound

The high spinning speed 1H magic angle spinning nuclear magnetic resonance (1H MAS NMR) technique was employed to distinguish the two groups of surface hydroxyls of kaolinite and investigate the intercalation mechanism of kaolinite/formamide compound. The proton chemical shifts of the inner hydroxyl and inner surface hydroxyl of kaolinte are in the range of δ−1.3–−0.9 and δ 2.4–3.0 respectively. After formamide intercalation three proton peaks were detected. The proton peak of the inner surface hydroxyls of the intercalation compound shifts to high-field with δ 2.3–2.7, which is assigned to the formation of the hydrogen bond between the inner surface hydroxyl and formamide carbonyl group. Whereas, the proton peak of the inner hydroxyl shifts to δ −0.3 toward low-field, that is attributed to van der Waal’s effect between the inner hydroxyl proton and the amino group proton of the formamide which may be keyed into the ditrigonal hole of the kaolinite. The third peak, additional proton peak, is in the range of δ5.4–5.6, that is ascribed to the hydrogen bond formation between the amino group proton of formimide and SiO4 tetrahedral oxygen of the kaolinite.

Structural implications of water dissolution in haplogranitic glasses from NMR spectroscopy: influence of total water content and mixed alkali effect

To study the effects of total water content and alkali substitution on the structure of aluminosilicate glasses, two series of glasses belonging to the ternary system Quartz (Qz)-Albite (Ab)-Orthoclase (Or) were synthesized and investigated with nuclear magnetic resonance (NMR) spectroscopy. Series I consisted of seven glasses with normative composition Ab39Or32Qz29 (AOQ) and water contents ranging from 0 to 6 wt%. Series II consisted of dry and hydrous glasses (∼2.0 wt% H2O) with five compositions along the join Qz37Ab63-Qz34Or66 (AQ-OQ) varying the alkali content (Na/K) at constant Si/Al ratio. All glasses were investigated with 1H, 23Na, 27Al and 29Si magic angle spinning (MAS) NMR.29Si MAS spectra of AOQ glasses showed no change upon hydration, suggesting little variation of the Si environments although the large linewidth of the 29Si signal may hide the presence of some Si Q3-OH. The isotropic chemical shift (δiso) of 27Al showed no change upon hydration, regardless of the amount of dissolved water. The 27Al mean quadrupolar coupling constant (Cq) decreased with increasing water content, indicating a general increase of symmetry of the charge distribution around Al, which suggests the absence of significant amounts of Al Q3-OH. Nonetheless, the evolution of Cq upon hydration suggests a correlation with OH concentration in the quenched glass. The evolution of 23Na isotropic chemical shifts upon hydration appears to be correlated with total water content or with the concentration of dissolved H2O molecules. In general, the NMR data are consistent with the water solubility model of Kohn et al. (1989), involving the exchange of charge balancing cations by protons. However, in addition to the presence of molecular water, 1H-NMR results showed at least two types of OH groups of which one may be related to Al-OH. Although the small intensity of this signal indicates that only a minor fraction of OH groups is present in this species, it demonstrates that water dissolution in aluminosilicate glasses might involve several mechanisms of hydroxyl formation.NMR data for Series II showed only a significant mixed alkali effect (nonlinear behaviour) on NMR parameters for 23Na but not for 29Si or 27Al. Therefore, these data suggest that the mixed alkali effect is related to the charge balancing cation rather than a modified aluminosilicate network.

1H-1H MAS correlation spectroscopy and distance measurements in a deuterated peptide.

In this Communication, we demonstrate the use of deuteration together with back substitution of exchangeable protons as a means of attenuating the strong 1H-1H couplings that broaden 1H magic angle spinning (MAS) spectra of solids. The approach facilitates 15N-1H correlation experiments as well as experiments for the measurement of 1H-1H distances. The distance measurement relies on the excellent resolution in the 1H MAS spectrum and homonuclear double quantum recoupling techniques. The 1H-1H dipolar recoupling can be analyzed in an analytical fashion by fitting the data to a 2- or 3-spin system. The experiments are performed on a sample of the dipeptide N-Ac-Val-Leu-OH, which was synthesized from uniformly [2H, 15N] labeled materials and back-exchanged in H2O.

Epitope Diversity of N-Glycans from Bovine Peripheral Myelin Glycoprotein P0 Revealed by Mass Spectrometry and Nano Probe Magic Angle Spinning 1H NMR Spectroscopy

The carbohydrate structures present on the glycoproteins in the central and peripheral nerve systems are essential in many cell adhesion processes. The P0 glycoprotein, expressed by myelinating Schwann cells, plays an important role during the formation and maintenance of myelin, and it is the most abundant constituent of myelin. Using monoclonal antibodies, the homophilic binding of the P0 glycoprotein was shown to be mediated via the human natural keller cell (HNK)-1 epitope (3-O-SO(3)H-GlcUA(beta1-3)Gal(beta1-4)GlcNAc) present on the N-glycans. We recently described the structure of the N-glycan carrying the HNK-1 epitope, present on bovine peripheral myelin P0 (Voshol, H., van Zuylen, C. W. E. M., Orberger, G., Vliegenthart, J. F. G., and Schachner, M. (1996) J. Biol. Chem. 271, 22957-22960). In this study, we report on the structural characterization of the detectable glycoforms, present on the single N-glycosylation site, using state-of-the-art NMR and mass spectrometry techniques. Even though all structures belong to the hybrid- or biantennary complex-type structures, the variety of epitopes is remarkable. In addition to the 3-O-sulfate present on the HNK-1-carrying structures, most of the glycans contain a 6-O-sulfated N-acetylglucosamine residue. This indicates the activity of a 6-O-sulfo-GlcNAc-transferase, which has not been described before in peripheral nervous tissue. The presence of the disialo-, galactosyl-, and 6-O-sulfosialyl-Lewis X epitopes provides evidence for glycosyltransferase activities not detected until now. The finding of such an epitope diversity triggers questions related to their function and whether events, previously attributed merely to the HNK-1 epitope, could be mediated by the structures described here.

High-Resolution Solid State NMR Investigation of the Filler-Rubber Interaction: Part III. Investigation on the Structure and Formation Mechanism of Carbon Gel in the Carbon Black-Filled Styrene—Butadiene Rubber

Abstract This investigation describes the elastomer—filler interaction and its formation mechanism using solid state high-resolution, high-speed 1H magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy. Pulsed NMR measurements were carried out on pure styrene-butadiene rubber (SBR), and solvent extracted carbon gels from freshly prepared and storage matured master batches. The effects of filler loading, storage maturation, severity of extraction and experimental temperature on the elastomer-filler interaction were examined and discussed. High resolution in NMR was achieved by higher temperature/low spinning rate, room temperature/high spinning and combined rotation and multiple-pulse spectroscopy (CRAMPS) techniques. High-speed magic-angle spinning (MAS) was found to be the most suitable method to achieve high resolution. Proton spin—spin relaxation time, T2, was measured successfully for each of the principal resonance species present in the samples. These measurements reveal an insight into the site-specific nature of the polymer—filler interaction. The relative immobilization of the dynamics of different protons, due to the presence of carbon black filler, and their temperature dependence evidence that the main chain vinyl proton is the most significantly immobilized one compared to the aromatic and methylene species. The effects of storage maturation and severity of extraction on the dynamics of the conformational jump have also been discussed. This work clearly demonstrates the importance of unsaturation on the bound rubber formation. It also provides first direct physical evidence confirming the hypothesis that bound rubber variation during the storage maturation is due to slow progressive replacement of short rubber chains by larger ones.

Dynamics of Membrane Penetration of the Fluorescent 7-Nitrobenz-2-Oxa-1,3-Diazol-4-yl (NBD) Group Attached to an Acyl Chain of Phosphatidylcholine

Location and dynamic reorientation of the fluorophore 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) covalently attached to a short (C6) or a long (C12) sn2 acyl chain of a phosphatidylcholine molecule was investigated by fluorescence and solid-state NMR spectroscopy. 2H NMR lipid chain order parameters indicate a perturbation of the phospholipid packing density in the presence of NBD. Specifically, a decrease of molecular order was found for acyl chain segments of the lower, more hydrophobic region. Molecular collision probabilities determined by 1H magic angle spinning nuclear Overhauser enhancement spectroscopy indicate a highly dynamic reorientation of the probe in the membrane due to thermal fluctuations. A broad distribution of the fluorophore in the lipid bilayer is observed with a preferential location in the upper acyl chain/glycerol region. The distribution of the NBD group in the membrane is quite similar for both the long- and the short-chain analog. However, a slight preference of the NBD group for the lipid-water interface is found for C12-NBD-PC in comparison with C6-NBD-PC. Indeed, as shown by dithionite fluorescence assay, the long-chain analog reacts more favorably with dithionite, indicating a better accessibility of the probe by dithionite present in the aqueous phase. Forces determining the location of the fluorophore in the lipid water interface are discussed.

High-resolution magic angle spinning 1H-NMR spectroscopy studies on the renal biochemistry in the bank vole (Clethrionomys glareolus) and the effects of arsenic (As3+) toxicity

1. High-resolution magic angle spinning (MAS) 1H-NMR spectroscopy was used to study renal metabolism and the toxicity of As3+, a common environmental contaminant, in the bank vole (Clethrionomys glareolus), a wild species of rodent. 2. Following a 14-day exposure to an environmentally relevant dose of As2O3 (28 mg kg -1 feed), voles displayed tissue damage at autopsy. MAS 1H spectra indicated abnormal lipid profiles in these samples. 3. Tissue necrosis was also evident from measurements of the apparent diffusion coefficient of water in the intact tissue using MAS 1H diffusion-weighted spectroscopy, its first application to toxicology. 4. Comparison of renal tissue from the wood mouse (Apodemus sylvaticus) exposed to identical exposure levels of As3+ suggested that the bank vole is particularly vulnerable to As3+ toxicity.

Investigation of water environments in a C18 bonded silica phase using 1H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy.

High resolution 1H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra have been obtained on typical C18 bonded silicas used in chromatographic solid-phase extraction separations. It has been shown for the first time that water molecules distributed in distinct physico-chemical environments within the chromatographic system can be detected directly using a simple 1H MAS NMR measurement. The resonances assigned to water protons in differing physico-chemical environments have distinct chemical shifts, line widths, relaxation times (T1 and T2) and also exhibit temperature dependent coalescence behaviour. This novel MAS approach may lead to a better understanding of the environments of other analytes in mixtures during such separations.

The biochemical profile of rat testicular tissue as measured by magic angle spinning 1H NMR spectroscopy

The testis is the principal organ of male fertility, responsible for the production of spermatozoa and their maturation into sperm. However, the underlying biochemistry of the testis is relatively understudied. The fluidic and homogeneous nature of the testis makes it an ideal organ for high resolution magic angle spinning (MAS) 1H NMR spectroscopy. In this study we have catalogued the low molecular weight metabolites. The testis contains large amounts of creatine, of which a substantial proportion was shown to be extracellular using bipolar gradients to measure apparent diffusion coefficients. The tissue also contained relatively high amounts of uridine.

Characterization of a New Hexasodium Diphosphopentamolybdate Hydrate, Na6[P2Mo5O23]·7H2O, by 23Na MQMAS NMR Spectroscopy and X-ray Powder Diffraction

A novel hexasodium disphosphopentamolybdate hydrate, Na6[P2Mo5O23]x7H2O, has been identified using X-ray powder diffraction, 1H, 23Na, and 31P magic-angle spinning (MAS) NMR, and 23Na multiple-quantum (MQ) MAS NMR. Powder XRD reveals that the hydrate belongs to the triclinic spacegroup P1 with cell dimensions a = 10.090(3) A, b = 15.448(5) A, c = 8.460(4) A, alpha = 101.45(6) degrees, beta = 104.09(2) degrees, gamma = 90.71(5) degrees, and Z = 2. The number of water molecules of crystallization has been determined on the basis of a quantitative evaluation of the 1H MAS NMR spectrum, the crystallographic unit cell volume, and a hydrogen content analysis. The 23Na MQMAS NMR spectra of Na6[P2Mo5O23]x7H2O, obtained at three different magnetic fields, clearly resolve resonances from six different sodium sites and allow a determination of the second-order quadrupolar effect parameters and isotropic chemical shifts for the individual resonances. These data are used to determine the quadrupole coupling parameters (CQ and eta Q) from simulations of the complex line shapes of the central transitions, observed in 23Na MAS NMR spectra at the three magnetic fields. This analysis illustrates the advantages of combining MQMAS and MAS NMR at moderate and high magnetic fields for a precise determination of quadrupole coupling parameters and isotropic chemical shifts for multiple sodium sites in inorganic systems. 31P MAS NMR demonstrates the presence of two distinct P sites in the asymmetric unit of Na6[P2Mo5O23].7H2O while the 31P chemical shielding anisotropy parameters, determined for this hydrate and for Na6[P2Mo5O23]x13H2O, show that these two hydrates can easily be distinguished using 31P MAS NMR.

States and Dynamic Behavior of Protons and Water Molecules in H3PW12O40 Pseudoliquid Phase Analyzed by Solid-State MAS NMR

The states and dynamic behavior of acidic protons and water molecules in solid H3PW12O40·nH2O (0 < n < 6), which would be closely related to its pseudoliquid phase catalysis, were quantitatively elucidated by the comprehensive application of 31P, 1H, and 17O magic-angle spinning (MAS) NMR. 1H and 17O MAS NMR were sensitive to the local environment in the pseudoliquid phase (e.g., hydrogen bonding), and 31P MAS NMR was effective especially to quantify the states of the protons. At 173 K, several peaks appeared in the 31P MAS NMR spectra that were reasonably assigned to phosphorus atoms in polyanions (PW12O403-) having different numbers of acidic proton(s) directly attached to them. Hence, the acidic protons reside either directly on the polyanions as “isolated acidic protons” or as H3O+ or H5O2+, and the amounts of these species were determined as a function of water content. The relative intensity of the 31P MAS NMR peaks obeyed binomial distribution for all the range of 0 < n < 6, which shows that the isolated acidic protons and protonated water molecules (H3O+ and H5O2+) are distributed uniformly (i.e., randomly) in the solid. The distribution of these species was well explained by the random removal of water from the solid bulk. At 298 K, the 31P MAS NMR peaks coalesced, revealing that the isolated acidic protons, which are the origin of the strong acidity in the pseudoliquid phase, migrate between the neighboring polyanions much faster than catalytic reactions. This is the first quantitative observation of the protons in hydrated heteropolyacids by spectroscopic methods.

High-Resolution Magic Angle Spinning 1H NMR Spectroscopy of Intact Liver and Kidney: Optimization of Sample Preparation Procedures and Biochemical Stability of Tissue during Spectral Acquisition

High-resolution magic angle spinning (MAS) 1H NMR spectroscopy has been used to investigate the biochemical composition of whole rat renal cortex and liver tissue samples. The effects of a number of sample preparation procedures and experimental variables have been investigated systematically in order to optimize spectral quality and maximize information recovery. These variables include the effects of changing the sample volume in the MAS rotor, snap-freezing the samples, and the effect of organ perfusion with deuterated saline solution prior to MAS NMR analysis. Also, the overall biochemical stability of liver and kidney tissue MAS NMR spectra was investigated under different temperature conditions. We demonstrate improved resolution and line shape of MAS NMR spectra obtained from small spherical tissue volume (12 μl) rotor inserts compared to 65 μl cylindrical samples directly inserted into the MAS rotors. D2O saline perfusion of the in situ afferent vascular tree of the tissue immediately postmortem also improves line shape in MAS NMR spectra. Snap-freezing resulted in increased signal intensities from α-amino acids (e.g., valine) in tissue together with decreases in renal osmolytes, such as myo-inositol. A decrease in triglyceride levels was observed in renal cortex following stasis on ice and in the MAS rotor (303 K for 4 h). This work indicates that different tissues have differential metabolic stabilities in 1H MAS NMR experiments and that careful attention to sample preparation is required to minimize artifacts and maintain spectral quality.

Determining the Structure of Trimethylphosphine Bound to the Brønsted Acid Site in Zeolite HY: Double-Resonance NMR and ab Initio Studies

Solid-state NMR methods and ab initio calculations have been employed to investigate the structure of the trimethylphosphine (TMP)−Brønsted acid site complex in zeolite HY. 27Al/31P and 27Al/1H rotational echo double-resonance NMR experiments performed at −150 °C were utilized to measure Al−P and Al−HB distances for the acid site−TMP complex of 3.95 (±0.05) and 2.8−3.1 Å, respectively, where HB is the Brønsted acid site proton. A more accurate measurement of the Al−HB distance was not possible since models that assume the presence of isolated Al−H spin pairs are not valid in this case. A P−HB distance of 1.40 (±0.02) Å was obtained by fitting the spinning sidebands in the 1H magic angle spinning (MAS) NMR spectrum. These internuclear distances are within the range of the Al−P, Al−HB, and P−HB distances obtained from ab initio calculations for the ion pair (IP) TMPH+−zeolite complex that is formed by transferring a Brønsted acid proton to TMP. In contrast to the 31P MAS NMR spectra, which indicated that the only stable species was TMPH+, ab initio calculations on small cluster models predicted that the neutral complex should be more stable than the IP complex. However, use of a larger zeolite fragment in the calculations enhanced the relative stability of the IP structure.

A new method for determining the P-V-T properties of high-density H 2O using NMR: results at 1.4–4.0 gpa and 700–1100°c

A new method for determining the density of H 2O at high pressures and temperatures has been developed using 1H magic angle spinning NMR. Powdered γ-Al 2O 3 was recrystallised in the piston–cylinder apparatus under hydrothermal conditions to form corundum containing submicroscopic fluid inclusions. After quenching, 1H NMR spectra of the included water were measured at room temperature with the result that the shift relative to H 2O at 25°C and 1 bar was found to be dependent on the predicted density of the trapped fluid. The shifts appear to be related to changes in the bulk magnetic susceptibility of water that increases linearly with density. The linear relationship between density and measured shift was calibrated using two measurements of the depression of freezing point in high-density water and enabled us to determine densities in the range 1.0–1.25 g cm −3. Therefore, the technique is complementary to those based on homogenisation of two phase (liquid–vapour) fluid inclusions; the latter techniques are only applicable where water density is <1 g cm −3. Because of the importance of high-density fluids under subduction zone conditions the density of water was determined using the new methodology at 1.4–4.0 GPa and 700–1100°C to test the available equations of state. We find that for the equations of state of Brodholt and Wood, Saul and Wagner, and Pitzer and Sterner the average difference between the calculated and measured values of density is about 1%, less than our estimate of the experimental error. Other widely used equations of state predict densities that are not consistent with our data. For example, those of Saxena and Fei and Holland and Powell predict densities, which are too high by an average of 7.6% and 3.8%, respectively, over the pressure and temperature range studied here.

Different water solubility mechanisms in hydrous glasses along the Qz-Ab join:: Evidence from NMR spectroscopy

The compositional dependence of water incorporation mechanisms in melts and glasses belonging to the quartz (Qz)-albite (Ab) join was studied with nuclear magnetic resonance (NMR) spectroscopy. Dry and hydrous glasses (containing 3.8 ± 0.1 wt% water) ranging in composition from Qz 90Ab 10 to Qz 28Ab 72 (in wt%) were synthesised and studied with 1H, 23Na, 27Al and 29Si magic angle spinning (MAS) NMR and 1H- 29Si cross polarisation (CP) MAS NMR at magnetic fields of 8.45 and 14.1 T. The results show that both molecular H 2O and OH groups are present in the hydrous glasses, represented by a broad and a narrow component in the static 1H NMR spectra. The changes in the 23Na MAS NMR spectra of the hydrated glasses indicate that sodium associates with water in all compositions studied. In contrast to the Ab-rich glasses, the 29Si MAS spectra of Qz-rich glasses (Qz 73Ab 27 - Qz 90Ab 10) change upon water incorporation, indicating the presence of Si-OH groups at least in the Qz-richest sample. The 27Al MAS data demonstrate that Al is only present in tetrahedral coordination for all glasses studied and that unless δ iso for Al Q 4(4Si) and Al Q 3(3Si)-OH are identical, Al-OH groups cannot be present in significant concentrations. Thus, in hydrous Ab-rich glasses there is probably no significant depolymerisation as suggested by Kohn et al. (1989a). However, more 27Al data for appropriate model compounds or reliable calculations of 27Al shifts and quadrupolar coupling constants are required before this conclusion can be considered to be definitive. For Qz-rich glass compositions a second solubility mechanism involving the formation of Si-OH and depolymerisation of the silicate network is inferred. The data suggest that only in Qz-rich glasses do both mechanisms coexist.