Oxygen-17 Nuclear Magnetic Resonance Research Articles

Transesterification catalyst strength as observed by solution-based oxygen-17 nuclear magnetic resonance spectroscopy

Natural abundance oxygen-17 nuclear magnetic resonance spectroscopy (17O-NMR) was applied to study homogeneous transesterification catalysts for fatty acid methyl ester production. Alkoxide catalysts were made with selected permutations of lithium, sodium, magnesium, and potassium hydroxides with ethanol, methanol, isopropanol, and glycerol. Pilot scale transesterification reactions of Canola oil were completed and the 17O-NMR spectra were collected from the reaction products. The alkoxide solutions showed the downfield shifts relative to the reagent alcohol, in a linear relationship with the concentration of catalyst ion. The electron deshielding of the oxygen of the alkoxide catalyst is observed directly by 17O-NMR, and the fastest catalyst has the largest downfield (high frequency) shift. This effect is most pronounced in potassium methoxide and less pronounced for less polar solvents such as isopropanol.

Phenol, the simplest aromatic monohydroxy alcohol, displays a faint Debye-like process when mixed with a nonassociating liquid.

Solvated in propylene carbonate, viscous phenol is studied using dielectric spectroscopy and shear rheology. In addition, several oxygen-17 and deuteron nuclear magnetic resonance (NMR) techniques are applied to specifically isotope labeled equimolar mixtures. Quantum chemical calculations are used to check the electrical field gradient at phenol's oxygen site. The chosen combination of NMR methods facilitates the selective examination of potentially hydrogen-bond related contributions as well as those dominated by the structural relaxation. Taken together the present results for phenol in equimolar mixtures with the van der Waals liquid propylene carbonate provide evidence for the existence of a very weak Debye-like process that originates from ringlike supramolecular associates.

Racing toward Fast and Effective 17O Isotopic Labeling and Nuclear Magnetic Resonance Spectroscopy of N-Formyl-MLF-OH and Associated Building Blocks.

Solid-state 1H, 13C, and 15N nuclear magnetic resonance (NMR) spectroscopy has been an essential analytical method in studying complex molecules and biomolecules for decades. While oxygen-17 (17O) NMR is an ideal and robust candidate to study hydrogen bonding within secondary and tertiary protein structures for example, it continues to elude many. We discuss an improved multiple-turnover labeling procedure to develop a fast and cost-effective method to 17O label fluoroenylmethyloxycarbonyl (Fmoc)-protected amino acid building blocks. This approach allows for inexpensive ($0.25 USD/mg) insertion of 17O labels, an important barrier to overcome for future biomolecular studies. The 17O NMR results of these building blocks and a site-specific strategy for labeled N-acetyl-MLF-OH and N-formyl-MLF-OH tripeptides are presented. We showcase growth in NMR development for maximizing sensitivity gains using emerging sensitivity enhancement techniques including population transfer, high-field dynamic nuclear polarization, and cross-polarization magic-angle spinning cryoprobes.

Constraints on the superconducting order parameter in Sr2RuO4 from oxygen-17 nuclear magnetic resonance.

Phases of matter are usually identified through spontaneous symmetry breaking, especially regarding unconventional superconductivity and the interactions from which it originates. In that context, the superconducting state of the quasi-two-dimensional and strongly correlated perovskite Sr2RuO4 is considered to be the only solid-state analogue to the superfluid 3He-A phase1,2, with an odd-parity order parameter that is unidirectional in spin space for all electron momenta and breaks time-reversal symmetry. This characterization was recently called into question by a search for an expected 'split' transition in a Sr2RuO4 crystal under in-plane uniaxial pressure, which failed to find any such evidence; instead, a dramatic rise and a peak in a single-transition temperature were observed3,4. Here we use nuclear magnetic resonance (NMR) spectroscopy of oxygen-17, which is directly sensitive to the order parameter via hyperfine coupling to the electronic spin degrees of freedom, to probe the nature of superconductivity in Sr2RuO4 and its evolution under strain. A reduction of the Knight shift is observed for all strain values and at temperatures below the critical temperature, consistent with a drop in spin polarization in the superconducting state. In unstrained samples, our results contradict a body of previous NMR work reporting no change in the Knight shift5 and the most prevalent theoretical interpretation of the order parameter as a chiral p-wave state. Sr2RuO4 is an extremely clean layered perovskite and its superconductivity emerges from a strongly correlated Fermi liquid, and our work imposes tight constraints on the order parameter symmetry of this archetypal system.

Effect of Lewis Acids on Graphite-Electrode Properties in EC-Based Electrolyte Solutions with Organophosphorus Compounds

Compatibility of self-extinguishing property and electrochemical stability of electrolyte solutions containing organophosphorus compounds as flame retardants has been required for practical application of electrolyte solutions to lithium-ion batteries. By adding a Lewis acid, Ca2+, Mg2+, or Na+, to ethylene carbonate (EC)-based electrolyte solutions containing 50 vol.% trimethylphosphate or dimethyl methylphosphonate, the electrolyte solutions are able to suppress the co-intercalation of the organophosphorus compounds with Li+ into graphite. However, the coulombic efficiency in the first charge-discharge in a graphite/Li metal half-cell with an electrolyte solution depends on the species of the added Lewis acid and organophosphorus compound. The difference between the chemical-shift changes of the oxygen of the C=O bond in EC and the oxygen of the P=O bond in organophosphorus compounds in the oxygen-17 nuclear magnetic resonance (17O NMR) spectra of the electrolyte solutions was investigated. The investigation revealed that the difference in the acidities of the Lewis acids, electron-donating abilities of the organophosphorus compounds, and solvated structures with a Lewis acid affect the electrochemical stability of self-extinguishing electrolyte solutions.

Structural Role of Alkali Cations in Calcium Aluminosilicate Glasses as Examined Using Oxygen-17 Solid-State Nuclear Magnetic Resonance Spectroscopy

The structural roles of alkali and calcium cations are important for understanding the physical and chemical properties of aluminosilicate melts and glasses. Recently, oxygen-17 nuclear magnetic resonance (17O NMR) studies of calcium–sodium aluminosilicate glasses showed that these structural roles are not randomly given, but rather each cation has its own preferential role. However, the relationship between cation type and role preference in calcium aluminosilicate glass is not completely understood. In the present study, the structural roles of lithium, sodium, and potassium cations in selected calcium aluminosilicate glasses are investigated using 17O solid-state NMR experiments. Data from these experiments clearly show that potassium cations have a notably stronger tendency to act as charge compensators within the network structure, compared to sodium and lithium cations. The result of 17O NMR experiment also showed that sodium and lithium cations in part act as network modifier alongside with calcium cations.

Water dynamics on ice and hydrate lattices studied by second-order central-line stimulated-echo oxygen-17 nuclear magnetic resonance.

Oxygen-17 stimulated-echo spectroscopy is a novel nuclear magnetic resonance (NMR) technique that allows one to investigate the time scale and geometry of ultraslow molecular motions in materials containing oxygen. The method is based on detecting orientationally encoded frequency changes within oxygen's central-transition NMR line that are caused by second-order quadrupolar interactions. In addition to the latter, the present theoretical analysis of various two-pulse echo and stimulated-echo pulse sequences takes also heteronuclear dipolar interactions into account. As an experimental example, the ultraslow water motion in polycrystals of tetrahydrofuran clathrate hydrate is studied via two-time oxygen-17 stimulated-echo correlation functions. The resulting correlation times and those of hexagonal ice are similar to those from previous deuteron NMR measurements. Calculations of the echo functions' final-state correlations for various motional models are compared with the experimental data of the clathrate hydrate. It is found that a six-site model including the oxygen-proton dipolar interaction describes the present results.

Evidence for a gapped spin-liquid ground state in a kagome Heisenberg antiferromagnet.

The kagome Heisenberg antiferromagnet is a leading candidate in the search for a spin system with a quantum spin-liquid ground state. The nature of its ground state remains a matter of active debate. We conducted oxygen-17 single-crystal nuclear magnetic resonance (NMR) measurements of the spin-1/2 kagome lattice in herbertsmithite [ZnCu3(OH)6Cl2], which is known to exhibit a spinon continuum in the spin excitation spectrum. We demonstrated that the intrinsic local spin susceptibility χ(kagome), deduced from the oxygen-17 NMR frequency shift, asymptotes to zero below temperatures of 0.03J, where J ~ 200 kelvin is the copper-copper superexchange interaction. Combined with the magnetic field dependence of χ(kagome) that we observed at low temperatures, these results imply that the kagome Heisenberg antiferromagnet has a spin-liquid ground state with a finite gap.

Oxygen-17 Nuclear Magnetic Resonance Study of the Structure of Mixed Cation Calcium−Sodium Silicate Glasses at High Pressure: Implications for Molecular Link to Element Partitioning between Silicate Liquids and Crystals

The structure of silicate glasses and the corresponding liquids at high pressure and their structure-property relations remain difficult questions in modern physical chemistry, geochemistry, and condensed matter physics. Here we report high- resolution solid-state O-17 3QMAS NMR spectra for mixed cation Ca-Na silicate glasses quenched from melts at high pressure up to 8 GPa. The spectra provide the experimental evidence for the varying pressure-dependence in two different types of nonbridging oxygen (NBO) environments (i.e., Na-O-Si and mixed (Ca,Na)-O-Si) in the single glass composition. The percentage of NBO drops significantly with increasing pressure and is a complex function of melt composition, including cation field strength of network modifying cations. A decrease in NBO fraction with pressure is negatively correlated with the element partitioning coefficient between crystals and liquids at high pressure.

Relationship Between Water Mobility and Viscosity of Nanometric Alumina Suspensions

Addition of fructose has been shown to reduce the viscosity of aqueous suspensions of nanometric alumina powders. Highly concentrated and flowable aqueous nanometric alumina suspensions were achieved by fructose addition. Oxygen‐17 nuclear magnetic resonance spectroscopy was used to investigate the relaxation time and the molecular mobility of water in the suspensions with and without fructose. It was found that a significant fraction of water is bound to the surface with lower mobility. Average water mobility increases and the viscosity of the suspension decreases with fructose addition, both of which are because of displacement of water from the alumina surface by fructose molecules. The results were related to rheological behavior of the suspensions.

Kinetics of oxygen exchange between the two isomers of bisulfite ion, disulfite Ion (S(2)O(5)(2-)), and water as studied by oxygen-17 nuclear magnetic resonance spectroscopy.

The nuclear magnetic transverse relaxation time of oxygen-17 in aqueous sodium bisulfite solutions in the pH range from 2.5 to 5 was measured over a range of temperatures, pH, and S(IV) concentrations at an ionic strength of 1.0 m. From these data the rate law for oxygen exchange between bisulfite ion and water was determined and found to be consistent with oxygen exchange occurring via the reactions SHO(3)(-) + H(+) SO(2) + H(2)O, SO(3)H(-) + SHO(3-) SO(3)(2-) + SO(2) + H(2)O, and SO(3)H(-) + SHO(3-) S(2)O(5)(2-) + H(2)O, where the symbol SHO(3-) refers to both isomeric forms of bisulfite ion, one in which the hydrogen is bonded to the sulfur (denoted HSO(3-)) and another in which the hydrogen is bonded to an oxygen atom (denoted SO(3)H(-)). The SO(3)H(-) isomer exchanges oxygen atoms with water much more rapidly than does the HSO(3-) isomer. The value of k(-1) was determined and is in essential agreement with the results of a previous determination by relaxation measurements. The value of k(16a) + k(16b) was also found, and k(16b) is at least as large as k(16a). The rate and mechanism of oxygen exchange between the two bisulfite ion environments were studied by analyzing the broadening of the HSO(3-) resonance. Oxygen exchange occurs through isomerization caused by proton transfers.

Water Mobility in Glassy and Rubbery Solids as Determined by Oxygen-17 Nuclear Magnetic Resonance: Impact on Chemical Stability

Although chemical reactivity in solid food systems has been studied as a function of water activity and glass transition, the possible effects of water mobility on chemical reactions have not been investigated. The effect of the glass transition on water mobility at constant temperature and water content was determined. The relationship between the experimentally determined water mobility in polyvinylpyrrolidone (PVP) systems and chemical reactivity data from the same PVP systems was evaluated. Water mobility, as determined via 17O-NMR, was not affected by the glass transition; PVP systems at constant water activities and water contents, but different physical states (glassy and rubbery), had the same water mobility. An evaluation of four chemical reactions showed no relation between water mobility and kinetic data. The effect of water on chemical reactions is multidimensional and cannot be reduced to a single physicochemical parameter.

Extent of intermixing among framework units in silicate glasses and melts

One aspect of the inherent disorder in silicate glasses and melts is the intermixing among framework units, which has important implications to the macroscopic properties of silicate magmas. We present experimental evidence of extensive mixing of these framework units in silicate glasses including aluminosilicate and borosilicate glasses from oxygen-17 nuclear magnetic resonance (NMR), which shows remarkable similarity to predictions from ab initio molecular orbital calculations. We quantify the extent of framework disorder by introducing the degree of interdispersion ( P) ranging from complete phase separation to random distribution for borosilicate glasses and the degree of Al avoidance ( Q) for aluminosilicate glasses. Boron-11 triple quantum magic angle spinning NMR results show that the fraction of the boroxol ring group increases with increasing boron content in binary borosilicate glasses, affecting the corresponding configurational enthalpy. We demonstrate that a more complete description of the macroscopic thermodynamic properties of silicate glasses can be successfully derived from detailed information on the degree of framework disorder.

Hydration of Native and Rennin-Treated, Cold-Solubilized Caprine Caseins as Determined by Oxygen-17 Nuclear Magnetic Resonance

The hydration of native and rennin-coagulated bovine and caprine caseins was investigated by oxygen-17 nuclear magnetic resonance (NMR) and fitted by nonlinear regression analysis. A charge-charge interaction model (B0) was employed to analyze the transverse relaxation (1/T2) data. Relaxation differences between reconstituted native micelles at 10°C and rennin-coagulated, cold-solubilized micelles of caseins strongly suggest that important structural dissimilarities exist between these milk proteins that are due to differences in the ratios of αs1- to β-CN. Variants of αs1-CN had significant effects on caprine casein hydration. The differences were more pronounced in rennin-coagulated than in native casein micelles. All rennin-coagulated, cold-solubilized casein micelles demonstrated significant decreases in hydration. Micelles containing low αs1-CN retain an open, highly hydrated structure, in comparison with similarly treated bovine and caprine casein micelles containing high αs1-CN. Second virial coefficients (B0 values) derived from oxygen-17 NMR data suggest that reconstituted bovine and caprine casein micelles containing high αs1-CN exhibit strong interactions, which at first decrease then increase during clotting. In contrast, the native caprine casein micelles containing low αs1-CN exhibit strong charge repulsions, which increase with clotting. The compositional differences are reflected in differences in the extent of protein-protein aggregation during casein clotting by rennin. Our results demonstrate that alteration in casein composition can dramatically alter cheese-making properties.

Oxygen sites in silicate glasses: a new view from oxygen-17 NMR

Nuclear Magnetic Resonance (NMR) studies of 17O in silicate glasses is beginning to provide new information on structure from an “anionic” viewpoint, which is an important complement to more traditional discussions of cation coordination and connectivities. A variety of techniques are discussed here, but the most important recent advances have used the new, two-dimensional, high resolution techniques of Dynamic Angle Spinning (DAS) and Triple Quantum Magic Angle Spinning (3QMAS), which have resolved and distinguished peaks for bridging oxygens linking Si, Al and B cations, as well as non-bridging oxygens. These approaches have provided some of the first quantitative constraints on the extent of ordering, both among network modifier cations and among network former cations.

A Multi-nuclear Multiple-Field Nuclear Magnetic Resonance Study of the Y2O3−Al2O3 Phase Diagram

We have been investigating several compounds belonging to the Y2O3−Al2O3 phase diagram by means of aluminum-27, oxygen-17, and yttrium-89 high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy. All aluminum, oxygen and yttrium sites of the five crystalline compounds C-Y2O3, Y4Al2O9, YAlO3, Y3Al5O12, and α-Al2O3) have been resolved and the NMR parameters deduced. AlVI sites exhibit isotropic chemical shift δiso between 0 and 10 ppm and small quadrupolar coupling constant CQ, whereas AlIV are located between 70 and 80 ppm and show large CQ. 17O resonances are characterized by small CQ and δiso ranging from 72 ppm (Al2O3) to 358 ppm (Y2O3) and showing a strong sensitivity to the nature of the second coordination sphere of oxygen as well as its coordination number. 89Y δiso are found between 184 and 314 ppm and are not strongly correlated to the coordination state of yttrium, the chemical shift anisotropy being found moderate (∼100 ppm). On the basis of the results obtained with the crystalline phases, we have characterized the oxygen environment in a glassy sample of composition Y3Al5O12 and described the variation of the local structure during the crystallization process of YAlO3 from the sol−gel raw product. In the vitreous state, oxygen's environments can be described in terms of OYk Al4-k sites, with respective populations not distributed in a purely random fashion, and two types of 5-fold-like aluminum environments can be evidenced. A quantitative description of the heat treatments of the YAlO3 precursor is given by 17O NMR experiments in terms of Y4Al2O9, Y3Al5O12, and amorphous phases formed. New oxygen environments are also evidenced and may be attributed to the hexagonal metastable form of YAlO3.

Sugar-casein interaction in deuterated solutions of bovine and caprine casein as determined by oxygen-17 and carbon-13 nuclear magnetic resonance: a case of preferential interactions.

17O NMR spectroscopy and (13)C NMR spectroscopy have been used to study the mechanism of interaction of sugars with bovine and caprine caseins in D(2)O. The (17)O NMR relaxation results showed in all cases an increase in water of hydration, as a result of added sugar; this was predominantly associated with "trapped" water in the caseins. Analysis of the vir al coefficients, obtained from the (17)O relaxation data, suggested that preferential interactions occur in the sugar-protein solutions. This could be the result of either sugar binding or a solute-solute thermodynamic effect, preferential hydration. The addition of sugars to deuterated solutions of bovine casein and caprine casein high in alpha(s1)-casein had little or no effect on either line width or chemical shifts of the (13)C NMR spectra of these milk proteins. (13)C NMR studies of sucrose, at various concentrations (100-300 mM) in the presence of caprine casein high in alpha(s1)-casein, showed no changes in either chemical shifts or T(1) values. This indicates that the sugar molecules tumble isotropically and therefore neither bind to the protein nor affect viscosity in the protein-sugar studies. All of these data suggest that the preferential exclusion of the sugar from the domain of the caseins results in preferential hydration of the caseins.

Oxygen-17 Nuclear Magnetic Resonance of Organic Solids

We report solid-state 17O NMR determinations of the oxygen chemical shift (CS) and electric field gradient (EFG) tensors for a series of 17O-enriched organic compounds containing various functional groups. In several cases, analysis of the n O magic-angle-spinning (MAS) and static NMR spectra yields both the magnitude and relative orientations of the 17O CS and EFG tensors. We also demonstrate the feasibility of solid-state 17O NMR as a potentially useful technique for studying molecular structure and hydrogen bonding.

Reactive Al-O-Al sites in a natural zeolite; triple-quantum oxygen-17 nuclear magnetic resonance

In framework aluminosilicate materials (e.g., feldspars, zeolites and many others), it is generally assumed that AlO 4 tetrahedra do not share comers, i.e., that Al-O-Al oxygen sites are avoided when stoichiometry permits. This assumption plays a key role in models of thermodynamic properties and of reaction kinetics. We present a new approach to directly test this assumption, based on 17 O triple­quantum magic-angle spinning nuclear magnetic resonance (3QMAS NMR). We show that thermo-dynamically significant concentrations of Al-O-Al sites can be observed in a naturally occurring zeolite (stilbite), and that these sites react faster with H 2 O vapor than do Si-O-Si and Si-O-Al sites.

Does six-coordinate germanium exist in Na 2O–GeO 2 glasses? Oxygen-17 nuclear magnetic resonance measurements

Sodium germanate glasses of compositions Na 2O · 9GeO 2 and 2Na 2O · 9GeO 2 have been studied using solid-state 17O nuclear magnetic resonance (NMR). Quartz-type and rutile-type GeO 2 were used as examples of oxygen connected to 4-fold and 6-fold coordinated germanium. The 17O spectra yielded nuclear quadrupole coupling constants, ( e 2 qQ/ h), asymmetry parameters, ( η), and isotropic chemical shifts, ( δ i ), from line shape simulations. The chemical shifts of oxygen in [GeO 4] and [GeO 6] units in germania are 70 ± 5 and 160 ± 5 ppm, respectively, with only a small difference in the quadrupole coupling constant but a larger difference in the asymmetry parameter. In the Na 2O–GeO 2 glasses, the quadrupole coupling constant decreases with addition of Na 2O to the glass with no evidence for any oxygen with a smaller electric field gradient, indicating that there are very few non-bridging oxygens (NBOs) present. The 17O chemical shift in Na 2O · 9GeO 2 is similar to that in [GeO 6] while in 2Na 2O · 9GeO 2 the chemical shift is close to that of [GeO 4].