Oxygen-17 NMR spectroscopy: basic principles and applications (part I)

Mobility of sodium ions in agarose gels probed through combined single- and triple-quantum NMR

Metal ions, including biologically prevalent sodium ions, can modulate electrostatic interactions frequently involved in the stability of condensed compartments in cells. Quantitative characterization of heterogeneous ion dynamics inside biomolecular condensates demands new experimental approaches. Here we develop a 23Na NMR relaxation-based integrative approach to probe dynamics of sodium ions inside agarose gels as a model system. We exploit the electric quadrupole moment of spin-3/2 23Na nuclei and, through combination of single-quantum and triple-quantum-filtered 23Na NMR relaxation methods, disentangle the relaxation contribution of different populations of sodium ions inside gels. Three populations of sodium ions are identified: a population with bi-exponential relaxation representing ions within the slow motion regime and two populations with mono-exponential relaxation but at different rates. Our study demonstrates the dynamical heterogeneity of sodium ions inside agarose gels and presents a new experimental approach for monitoring dynamics of sodium and other spin-3/2 ions (e.g. chloride) in condensed environments.

Exploration of the Existence Forms and Patterns of Dissolved Oxygen Molecules in Water

The structure of liquid water is primarily composed of three-dimensional networks of water clusters formed by hydrogen bonds, and dissolved oxygen is one of the most important indicators for assessing water quality. In this work, distilled water with different concentration of dissolved oxygen were prepared, and a clear negative correlation between the size of water clusters and dissolved oxygen concentration was observed. Besides, a phenomenon of rapid absorption and release of oxygen at the water interfaces was unveiled, suggesting that oxygen molecules predominantly exist at the interfaces of water clusters. Oxygen molecules can move rapidly through the interfaces among water clusters, allowing dissolved oxygen to quickly reach a saturation level at certain partial pressure of oxygen and temperature. Further exploration into the mechanism by molecular dynamics simulations of oxygen and water clusters found that oxygen molecules can only exist stably at the interfaces among water clusters. A semi-empirical formula relating the average number of water molecules in a cluster (n) to 17O NMR half-peak width (W) was summarized: n = 0.1 W + 0.85. These findings provide a foundation for exploring the structure and properties of water.

Origin of 17O NMR chemical shifts based on molecular orbital theory: paramagnetic terms of the pre-α, α and β effects from orbital-to-orbital transitions, along with the effects from vinyl, carbonyl and carboxyl groups

17O NMR chemical shifts (δ(O)) were analysed based on the molecular orbital (MO) theory, using the diamagnetic, paramagnetic and total absolute magnetic shielding tensors (σd(O), σp(O) and σt(O), respectively). O2- was selected as the standard for the analysis. An excellent relationship was observed between σd(O) and the charges on O for O6+, O4+, O2+, O0 and O2-. The data from H2O, HO+, HO- and H3O+ were on the correlation line. However, such relationship was not observed for the oxygen species, other than above. The pre-α, α and β effects were evaluated bases on σt(O), where the pre-α effect arises from the protonation to a lone pair orbital on O2-, for an example. The 30-40 ppm and 20-40 ppm (downfield shifts) were predicted for the pre-α and β effects, respectively, whereas the values for the α effect was very small in magnitude, where the effect from the hydrogen bond formation should be considered. Similarly, the carbonyl effect in H2C[double bond, length as m-dash]O and the carboxyl effects in H(HO)C[double bond, length as m-dash]O were evaluated from MeOH, together with H2C[double bond, length as m-dash]CHOH from CH3CH2OH. Very large downfield shifts of 752, 425 and 207 ppm were predicted for H2C[double bond, length as m-dash]O*, H(HO)C[double bond, length as m-dash]O* and H(HO*)C[double bond, length as m-dash]O, respectively, together with the 81 ppm downfield shift for H2C[double bond, length as m-dash]CHO*H. The origin of the effect were visualized based on the occupied-to-unoccupied orbital transitions. As a result, the origin of the 17O NMR chemical shifts (δ(17O)) can be more easily imaged and understand through the image of the effects. The results would help to understand the role of O in the specific position of a compound in question and the mechanisms to arise the shift values also for the experimental scientists. The aim of this study is to establish the plain rules founded in theory for δ(17O), containing the origin, which has been achieved through the treatments.

Range and sensitivity of 17O nuclear spin-lattice relaxation as a probe of aqueous electrolyte dynamics

The study of electrolytic solutions is of relevance in many research fields, ranging from biophysics, materials, and colloid science to catalysis and electrochemistry. The dependence of solution dynamics on the nature of electrolytes and their concentrations has been the subject of many experimental and computational studies, yet it remains challenging to obtain a full understanding of the factors that govern solution behavior. Here, we provide additional insights into the behavior of aqueous solutions of alkali chlorides by combining 17O relaxation data with diffusion and viscosity data and contrast their behavior with 1H nuclear magnetic resonance relaxation data. The main findings are that 17O relaxation correlates well with viscosity data but not with diffusion data, while 1H relaxation correlates with neither. Certain ionic trends match known ion-specific series behavior, especially at high concentrations. Notably, we also examine the ranges of the interactions and conclude that the majority of the effects are tied to local water reorientation dynamics.

Advanced solid-state NMR spectroscopy and its applications in zeolite chemistry

Solid-state NMR spectroscopy (ssNMR) can provide details about the structure, host-guest/guest-guest interactions and dynamic behavior of materials at atomic length scales. A crucial use of ssNMR is for the characterization of zeolite catalysts that are extensively employed in industrial catalytic processes. This review aims to spotlight the recent advancements in ssNMR spectroscopy and its application to zeolite chemistry. We first review the current ssNMR methods and techniques that are relevant to characterize zeolite catalysts, including advanced multinuclear and multidimensional experiments, in situ NMR techniques and hyperpolarization methods. Of these, the methodology development on half-integer quadrupolar nuclei is emphasized, which represent about two-thirds of stable NMR-active nuclei and are widely present in catalytic materials. Subsequently, we introduce the recent progress in understanding zeolite chemistry with the aid of these ssNMR methods and techniques, with a specific focus on the investigation of zeolite framework structures, zeolite crystallization mechanisms, surface active/acidic sites, host-guest/guest-guest interactions, and catalytic reaction mechanisms.

Origin of Oxygen in Graphene Oxide Revealed by 17O and 18O Isotopic Labeling

Wet-chemical oxidation of graphite in a mixture of sulfuric acid with a strong oxidizer, such as potassium permanganate, leads to the formation of graphene oxide with hydroxyl and epoxide groups as the major functional groups. Nevertheless, the reaction mechanism remains unclear and the source of oxygen is a subject of debate. It could theoretically originate from the oxidizer, water, or sulfuric acid. In this study, we employed 18O and 17O labeled reagents to experimentally elucidate the reaction mechanism and, thus, determine the origin of oxo-functional groups. Our findings reveal the multifaceted roles of sulfuric acid, acting as a dispersion medium, a dehydrating agent for potassium permanganate, and an intercalant. Additionally, it significantly acts as a source of oxygen next to manganese oxides. Through 17O solid-state magic-angle spinning (MAS) NMR experiments, we exclude water as a direct reaction partner during oxygenation. With labeling experiments, we conclude on mechanistic insights, which may be exploited for the synthesis of novel graphene derivatives.

Water dynamics in eutectic solutions of sodium chloride and magnesium sulfate: implications for life in Europa's subsurface ocean and ice shell

Liquid water is essential for life as we know it and the coupling between water and biomolecular dynamics is crucial for life processes. Jupiter's moon Europa is a good candidate for searching for extraterrestrial life in our outer solar system, mainly because a liquid water salty ocean in contact with a rocky seafloor underlies its ice shell. Little, however, is known about the chemical composition of the subglacial ocean of Europa or the brine pockets within its ice shell and their impacts on water dynamics. Here, we employ 1H, 17O, 23Na and 35Cl NMR spectroscopy, especially NMR spin relaxation and diffusion methods, and investigate the mobility of water molecules and ions in eutectic solutions of magnesium sulfate and sodium chloride, two salts ubiquitously present on the surface of Europa, over a range of temperatures and pressures pertinent to Europa's subglacial ocean. The NMR data demonstrate the more pronounced effect of magnesium sulfate compared with sodium chloride on the mobility of water molecules. Even at its much lower eutectic temperature, the sodium chloride solution retains a relatively large level of water mobility. Our results highlight the higher potential of a sodium chloride-rich than magnesium sulfate-rich Europa's ocean to accommodate life and support life origination within the eutectic melts of Europa's ice shell.

17 O nuclear magnetic resonance: Recent advances and applications

The present review is focused on the most recent achievements in the application of liquid phase 17 O nuclear magnetic resonance (NMR) to inorganic, organic, and biochemical molecules focusing on their structure, conformations, and (bio)chemical behavior. The review is composed of four basic parts, namely, (1) simple molecules; (2) water and hydrogen bonding; (3) metal oxides, clusters, and complexes; and (4) biological molecules. Experimental 17 O NMR chemical shifts are thoroughly tabulated. They span a range of as much as almost 650 ppm (from -35.6 to +610.0 ppm) for inorganic and organic molecules, whereas this range is much wider for biological species being of about 1350 ppm (from -12 to +1332 ppm), and in the case of hemoproteins and heme-model compounds, isotropic chemical shifts of up to 2500 ppm were observed. The general prospects and caveats in the modern development of the liquid phase 17 O NMR in chemistry and biochemistry are critically discussed and briefly outlined in view of their future applications.

On the use of 17O NMR for understanding molecular and silica-grafted tungsten oxo siloxide complexes

¹⁷O-labelled tungsten siloxide complexes [WOCl₂(OSitBu₃)₂] (1-Cl) and [WOMe₂(OSitBu₃)₂] (1-Me) were prepared and characterized by ¹⁷O MAS NMR, with input from theoretical calculations of NMR parameters. Guidelines linking ¹⁷O NMR parameters and the coordination sphere of molecular and silica-grafted tungsten oxo species are proposed. The grafting of 1-Me on SiO_2-700 afforded material 2, with surface species [(SiO)WOMe₂(OSitBu₃)] as shown by elemental analysis, IR and ¹H and ¹³C MAS NMR. The DFT calculations of the grafting mechanism are in line with the observed reactivity. They indicate the occurrence of several isomeric species of close energy for the grafted W centers, precluding efficient ¹⁷O MAS NMR studies. The lack of catalytic activity in olefin metathesis and ring-opening olefin metathesis polymerization indicates that initiation by α-H elimination is not operative in 2, contrary to related tungsten surface species, which illustrates the crucial influence of the nature of the metal coordination sphere.

Ex Situ Generation of 18O2 and 17O2 from Endoperoxides for *O-Labeling and Mechanistic Studies of Oxidations by Dioxygen

Near-stoichiometric amounts of ¹⁸O₂ and ¹⁷O₂ were generated ex situ from endoperoxides in a two-chamber glassware to oxidize various substrates. This strategy gave [*O₂]endoperoxides, [*O₁]quinones, [*O₁]phenols, and [*O_x]artemisin in moderate to good yields and high isotopic enrichments (up to 84%) at affordable costs. Moreover, mass spectrometry and ¹⁷O NMR of the [*O]products provided valuable information about the chemical mechanisms involved.

Fast and Cost-Efficient 17 O-Isotopic Labeling of Carboxylic Groups in Biomolecules: From Free Amino Acids to Peptide Chains

¹⁷ O NMR spectroscopy is a powerful technique, which can provide unique information regarding the structure and reactivity of biomolecules. However, the low natural abundance of ¹⁷ O (0.04 %) generally requires working with enriched samples, which are not easily accessible. Here, we present simple, fast and cost-efficient ¹⁷ O-enrichment strategies for amino acids and peptides by using mechanochemistry. First, five unprotected amino acids were enriched under ambient conditions, consuming only microliter amounts of costly labeled water, and producing pure molecules with enrichment levels up to ∼40 %, yields ∼60-85 %, and no loss of optical purity. Subsequently, ¹⁷ O-enriched Fmoc/tBu-protected amino acids were produced on a 1 g/day scale with high enrichment levels. Lastly, a site-selective ¹⁷ O-labeling of carboxylic functions in peptide side-chains was achieved for RGD and GRGDS peptides, with ∼28 % enrichment level. For all molecules, ¹⁷ O ssNMR spectra were recorded at 14.1 T in reasonable times, making this an important step forward for future NMR studies of biomolecules.

Computational simulation of 1 H NMR profiles of complex biofluid analyte mixtures at differential operating frequencies: Applications to low-field benchtop spectra

Estimations of accurate and reliable NMR chemical shift values, coupling patterns and constants within a reasonable timeframe remain significantly challenging, and the unavailability of reliable software strategies for the prediction of low-field (e.g., 60 MHz) spectra from those acquired at higher operating frequencies hampers their direct comparison. Hence, this study explored the applications of accessible software options for predicting these parameters in the ¹ H NMR profiles of analytes as a function of magnetic field strength; this was performed for individual analytes and also for complex biofluid matrices featured in metabolomics investigations. For this purpose, results from the very first successful experimental acquisition and simulation of the ¹ H NMR profiles of intact human salivary supernatant samples on a 60 MHz benchtop spectrometer were evaluated. Using salivary metabolite concentrations determined at 400 MHz, it was demonstrated that simulation of the low-field spectra of five biomolecules with the most prominent ¹ H resonances detectable allowed multiple component fits to be applied to experimental spectra. Hence, these salivary ¹ H NMR profiles could be successfully predicted throughout the 45-600 MHz operating frequency range. With the exception of propionate resonance multiplets, which revealed more complex coupling patterns at low field and required more astute computational and fitting options, valuable quantitative metabolomics data on salivary acetate, formate, methanol and glycine could be attained from low-field spectrometres. These studies are both timely and pertinent in view of the recent advancement of low-field benchtop NMR facilities for diagnostically significant biomarker tracking in biofluids. Experiments performed with added ammonium chloride to facilitate the release of salivary metabolites from biopolymer binding sites provided evidence that a small but nevertheless significant proportion of propionate, but not lactate, was bound to such sites, an observation of much relevance to biomolecule quantification in salivary metabolomics investigations.

Harnessing Water to Enhance Quadrupolar NMR Spectroscopy and Imaging

¹⁷ O and ¹⁴ N are attractive targets for in vivo NMR spectroscopy and imaging, but low gyromagnetic ratios γ and fast spin relaxation complicate observations. This work explores indirect ways of detecting some of these sites with the help of proton-detected double resonance techniques. As standard coherence transfer methods are of limited use for such indirect detection, alternative routes for probing the quadrupolar spectra on ¹ H were tested. These centered on modulating the broadening effects imparted onto protons adjacent to the low-γ species through J couplings through either continuous wave or spin-echo double-resonance decoupling/recoupling sequences. As in all cases, the changes imparted by these double-resonance strategies were small due to the fast relaxation undergone by the quadrupoles, the sensitivity of these approaches was amplified by transferring their effects onto the abundant water ¹ H signal. These amplifications were mediated by the spontaneous exchanges that the labile ¹ Hs bound to ¹⁷ O or ¹⁴ N undergo with the water protons. In experiments designed on the basis of double-resonance spin echoes, these enhancements were imparted by looping the transverse encodings together with multiple longitudinal storage periods, leading to decoupling-recoupling with exchange (D-REX) sequences. In experiments designed on the basis of continuous on/off quadrupolar decoupling, these solvent exchanges were incorporated into chemical-exchange saturation transfer schemes, leading to decoupling-recoupling with saturation transfer (D-REST) sequences. Both of these variants harnessed sizable proportions of the easily detectable water signals, in order to characterize the NMR spectra and/or to image with atomic-site specificity the ¹⁷ O and ¹⁴ N species.

Nuclear Magnetic Resonance and Metadynamics Simulations Reveal the Atomistic Binding of l-Serine and O-Phospho-l-Serine at Disordered Calcium Phosphate Surfaces of Biocements

Interactions between biomolecules and structurally disordered calcium phosphate (CaP) surfaces are crucial for the regulation of bone mineralization by noncollagenous proteins, the organization of complexes of casein and amorphous calcium phosphate (ACP) in milk, as well as for structure-function relationships of hybrid organic/inorganic interfaces in biomaterials. By a combination of advanced solid-state NMR experiments and metadynamics simulations, we examine the detailed binding of O-phospho-l-serine (Pser) and l-serine (Ser) with ACP in bone-adhesive CaP cements, whose capacity of gluing fractured bone together stems from the close integration of the organic molecules with ACP over a subnanometer scale. The proximity of each carboxy, aliphatic, and amino group of Pser/Ser to the Ca²⁺ and phosphate species of ACP observed from the metadynamics-derived models agreed well with results from heteronuclear solid-state NMR experiments that are sensitive to the ¹³C-³¹P and ¹⁵N-³¹P distances. The inorganic/organic contacts in Pser-doped cements are also contrasted with experimental and modeled data on the Pser binding at nanocrystalline HA particles grown from a Pser-bearing aqueous solution. The molecular adsorption is driven mainly by electrostatic interactions between the negatively charged carboxy/phosphate groups and Ca²⁺ cations of ACP, along with H bonds to either protonated or nonprotonated inorganic phosphate groups. The Pser and Ser molecules anchor at their phosphate/amino and carboxy/amino moieties, respectively, leading to an extended molecular conformation across the surface, as opposed to an "upright standing" molecule that would result from the binding of one sole functional group.

17O NMR Spectroscopy in Lithium-Ion Battery Cathode Materials: Challenges and Interpretation

Modern studies of lithium-ion battery (LIB) cathode materials employ a large range of experimental and theoretical techniques to understand the changes in bulk and local chemical and electronic structures during electrochemical cycling (charge and discharge). Despite its being rich in useful chemical information, few studies to date have used ¹⁷O NMR spectroscopy. Many LIB cathode materials contain paramagnetic ions, and their NMR spectra are dominated by hyperfine and quadrupolar interactions, giving rise to broad resonances with extensive spinning sideband manifolds. In principle, careful analysis of these spectra can reveal information about local structural distortions, magnetic exchange interactions, structural inhomogeneities (Li⁺ concentration gradients), and even the presence of redox-active O anions. In this Perspective, we examine the primary interactions governing ¹⁷O NMR spectroscopy of LIB cathodes and outline how ¹⁷O NMR may be used to elucidate the structure of pristine cathodes and their structural evolution on cycling, providing insight into the challenges in obtaining and interpreting the spectra. We also discuss the use of ¹⁷O NMR in the context of anionic redox and the role this technique may play in understanding the charge compensation mechanisms in high-capacity cathodes, and we provide suggestions for employing ¹⁷O NMR in future avenues of research.

Rapid, Quantitative Nuclear Magnetic Resonance Test for Oxygen-17 Enrichment in Water

Nuclear magnetic resonance (NMR) studies involving ¹⁷O are increasingly important in molecular biology, material science, and other disciplines. A large number of these studies employ H₂¹⁷O as a source of ¹⁷O, and this reliance can be limiting because the high cost of H₂¹⁷O. To overcome this constraint, a recent study proposed a distillation scheme capable of producing significant quantities of H₂¹⁷O at a low cost. Although this method is reported to be effective, the reactions proposed to quantify percent of ¹⁷O enrichment are either time intensive or have a risk of errors due to the isotope effect. Here, an alternative reaction scheme is described to measure ¹⁷O water that ultimately creates methyl benzoate as the sole ¹⁷O-containing product. The proposed reaction is completed in a matter of minutes at room temperature, produces only one ¹⁷O product, and requires no clean-up step. The large isotope shift observed in solution NMR between the ¹³C═¹⁶O and ¹³C═¹⁷O resonances allows for integration of the individual peaks. This ¹³C NMR analysis is found to be highly accurate over a wide enrichment range and is accessible to most NMR spectroscopists.

Theoretical Investigation of Glycine Micro-Solvated. Energy and NMR Spin Spin Coupling Constants Calculations

Glycine in its neutral form can exist in the gas phase while its zwitterion form is more stable in water solution, but how many waters are actually necessary to stabilize the zwitterionic structure in the gas phase? Are the intramolecular isotropic spin spin coupling constants sensitive enough to accuse the change in the environment? or the conformer observed? These and related questions have been investigated by a computational study at the level of density functional theory employing the B3LYP functional and the 6-31++G**-J basis set. We found that at least two water molecules explicitly accounted for in the super-molecule structure are necessary to stabilize both conformers of glycine within a water polarizable continuum model. At least half of the SSCCs of both conformers are very stable to changes in the environment and at least four of them differ significantly between Neutral and Zwitterion conformation.

Electrolyte Structure of Lithium Polysulfides with Anti-Reductive Solvent Shells for Practical Lithium-Sulfur Batteries

The lithium-sulfur (Li-S) battery is regarded as a promising secondary battery. However, constant parasitic reactions between the Li anode and soluble polysulfide (PS) intermediates significantly deteriorate the working Li anode. The rational design to inhibit the parasitic reactions is plagued by the inability to understand and regulate the electrolyte structure of PSs. Herein, the electrolyte structure of PSs with anti-reductive solvent shells was unveiled by molecular dynamics simulations and nuclear magnetic resonance. The reduction resistance of the solvent shell is proven to be a key reason for the decreased reactivity of PSs towards Li. With isopropyl ether (DIPE) as a cosolvent, DIPE molecules tend to distribute in the outer solvent shell due to poor solvating power. Furthermore, DIPE is more stable than conventional ether solvents against Li metal. The reactivity of PSs is suppressed by encapsulating PSs into anti-reductive solvent shells. Consequently, the cycling performance of working Li-S batteries was significantly improved and a pouch cell of 300 Wh kg^-1 was demonstrated. The fundamental understanding in this work provides an unprecedented ground to understand the electrolyte structure of PSs and the rational electrolyte design in Li-S batteries.

Expanding the NMR toolkit for biological solids: oxygen-17 enriched Fmoc-amino acids

We report the solid-state ¹⁷O NMR parameters for five previously uncharacterized N-α-fluoren-9-yl-methoxycarbonyl-O-t-butyl (Fmoc) protected amino acids.

Unveiling the Structure and Reactivity of Fatty-Acid Based (Nano)materials Thanks to Efficient and Scalable 17O and 18O-Isotopic Labeling Schemes

Fatty acids are ubiquitous in biological systems and widely used in materials science, including for the formulation of drugs and the surface-functionalization of nanoparticles. However, important questions regarding the structure and reactivity of these molecules are still to be elucidated, including their mode of binding to certain metal cations or materials surfaces. In this context, we have developed novel, efficient, user-friendly, and cost-effective synthetic protocols based on ball-milling, for the 17O and 18O isotopic labeling of two key fatty acids which are widely used in (nano)materials science, namely stearic and oleic acid. Labeled molecules were analyzed by 1H and 13C solution NMR, IR spectroscopy, and mass spectrometry (ESI-TOF and LC-MS), as well as 17O solid state NMR (for the 17O labeled species). In both cases, the labeling procedures were scaled-up to produce up to gram quantities of 17O- or 18O-enriched molecules in just half-a-day, with very good synthetic yields (all ≥84%) and enrichment levels (up to an average of 46% per carboxylic oxygen). The 17O-labeled oleic acid was then used for the synthesis of a metal soap (Zn-oleate) and the surface-functionalization of ZnO nanoparticles (NPs), which were characterized for the first time by high-resolution 17O NMR (at 14.1 and 35.2 T). This allowed very detailed insight into (i) the coordination mode of the oleate ligand in Zn-oleate to be achieved (including information on Zn···O distances) and (ii) the mode of attachment of oleic-acid at the surface of ZnO (including novel information on its photoreactivity upon UV-irradiation). Overall, this work demonstrates the high interest of these fatty acid-enrichment protocols for understanding the structure and reactivity of a variety of functional (nano)materials systems using high resolution analyses like 17O NMR.

Electrophilicity Scale of Activated Amides: 17 O NMR and 15 N NMR Chemical Shifts of Acyclic Twisted Amides in N-C(O) Cross-Coupling

The structure and properties of amides are of tremendous interest in organic synthesis and biochemistry. Traditional amides are planar and the carbonyl group non-electrophilic due to n_N →π*_C=O conjugation. In this study, we report electrophilicity scale by exploiting ¹⁷ O NMR and ¹⁵ N NMR chemical shifts of acyclic twisted and destabilized acyclic amides that have recently received major attention as precursors in N-C(O) cross-coupling by selective oxidative addition as well as precursors in electrophilic activation of N-C(O) bonds. Most crucially, we demonstrate that acyclic twisted amides feature electrophilicity of the carbonyl group that ranges between that of acid anhydrides and acid chlorides. Furthermore, a wide range of electrophilic amides is possible with gradually varying carbonyl electrophilicity by steric and electronic tuning of amide bond properties. Overall, the study quantifies for the first time that steric and electronic destabilization of the amide bond in common acyclic amides renders the amide bond as electrophilic as acid anhydrides and chlorides. These findings should have major implications on the fundamental properties of amide bonds.

Mapping the oxygen structure of γ-Al2O3 by high-field solid-state NMR spectroscopy

γ-Al₂O₃ is one of the most widely used catalysts or catalyst supports in numerous industrial catalytic processes. Understanding the structure of γ-Al₂O₃ is essential to tuning its physicochemical property, which still remains a great challenge. We report a strategy for the observation and determination of oxygen structure of γ-Al₂O₃ by using two-dimensional (2D) solid-state NMR spectroscopy at high field. 2D ¹⁷O double-quantum single-quantum homonuclear correlation NMR experiment is conducted at an ultra-high magnetic field of 35.2 T to reveal the spatial proximities between different oxygen species from the bulk to surface. Furthermore, 2D proton-detected ¹H-¹⁷O heteronuclear correlation NMR experiments allow for a rapid identification and differentiation of surface hydroxyl groups and (sub-)surface oxygen species. Our experimental results demonstrate a non-random distribution of oxygen species in γ-Al₂O₃.

γ-Al₂O₃ is widely used in catalytic processes, but understanding its detailed structure remains a challenge. The authors, using two-dimensional solid-state NMR spectroscopy at a high magnetic field, characterize the spatial proximity and connectivity between oxygen species from the bulk to the surface.

17O NMR as a measure of basicity of alkaline-earth oxide surfaces: A theoretical study

The surface basicity of the alkaline-earth metal oxides has been investigated by studying the properties of ¹⁷O nuclear magnetic resonance (NMR). To this end, we performed density functional theory calculations and determined the ¹⁷O chemical shift and the quadrupolar coupling constants of the regular and stepped surfaces of MO (M = Mg, Ca, Sr, and Ba) oxides. The computed average chemical shift (δ_iso ^av) for ¹⁷O NMR of bulk MgO, CaO, SrO, and BaO is 46, 301, 394, and 636 ppm, respectively, in excellent agreement with the experiment. The ¹⁷O NMR chemical shifts correlate linearly with the Madelung potential in the four oxides. Next, we considered the changes in the ¹⁷O chemical shift due to the adsorption of BR₃ (R = F and OCH₃) and pyrrole as probe molecules. We found that the ¹⁷O NMR signal of the O ion directly bound to the probe molecule shifts considerably compared to the clean surface. This is due to a change in the polarization of the O charge distribution due to the molecular adsorption. This change is the largest for BaO, with the strongest bond and the shortest surface-adsorbate distance, and the smallest for MgO, thus showing a direct correlation between ¹⁷O NMR and surface basicity. The ¹⁷O chemical shift of the basic site correlates linearly also with several properties of the adsorbed molecules, providing a direct measure of the surface basicity.

A facile oxygen-17 NMR method to determine effective viscosity in dilute, molecularly crowded and confined aqueous media

We present an NMR method based on natural abundance 17O relaxation of water to determine effective viscosity in biological aqueous samples. The method accurately captures viscosity of dilute and crowded protein solutions and offers a fairly simple way to quantify the internal fluidity of biological condensates formed through phase separation.

17O NMR studies of organic and biological molecules in aqueous solution and in the solid state

This review describes the latest developments in the field of ¹⁷O NMR spectroscopy of organic and biological molecules both in aqueous solution and in the solid state. In the first part of the review, a general theoretical description of the nuclear quadrupole relaxation process in isotropic liquids is presented at a mathematical level suitable for non-specialists. In addition to the first-order quadrupole interaction, the theory also includes additional relaxation mechanisms such as the second-order quadrupole interaction and its cross correlation with shielding anisotropy. This complete theoretical treatment allows one to assess the transverse relaxation rate (thus the line width) of NMR signals from half-integer quadrupolar nuclei in solution over the entire range of motion. On the basis of this theoretical framework, we discuss general features of quadrupole-central-transition (QCT) NMR, which is a particularly powerful method of studying biomolecules in the slow motion regime. Then we review recent advances in ¹⁷O QCT NMR studies of biological macromolecules in aqueous solution. The second part of the review is concerned with solid-state ¹⁷O NMR studies of organic and biological molecules. As a sequel to the previous review on the same subject [G. Wu, Prog. Nucl. Magn. Reson. Spectrosc. 52 (2008) 118-169], the current review provides a complete coverage of the literature published since 2008 in this area.

A 17O paramagnetic NMR study of Sm2O3, Eu2O3, and Sm/Eu-substituted CeO2

Paramagnetic solid-state NMR of lanthanide (Ln) containing materials can be challenging due to the high electron spin states possible for the Ln f electrons, which result in large paramagnetic shifts, and these difficulties are compounded for ¹⁷O due to the low natural abundance and quadrupolar character. In this work, we present examples of ¹⁷O NMR experiments for lanthanide oxides and strategies to overcome these difficulties. In particular, we record and assign the ¹⁷O NMR spectra of monoclinic Sm₂O₃ and Eu₂O₃ for the first time, as well as performing density functional theory (DFT) calculations to gain further insight into the spectra. The temperature dependence of the Sm³⁺ and Eu³⁺ magnetic susceptibilities are investigated by measuring the ¹⁷O shift of the cubic sesquioxides over a wide temperature range, which reveal non-Curie temperature dependence due to the presence of low-lying electronic states. This behaviour is reproduced by calculating the electron spin as a function of temperature, yielding shifts which agree well with the experimental values. Using the understanding of the magnetic behaviour gained from the sesquioxides, we then explore the local oxygen environments in 15 at% Sm- and Eu-substituted CeO₂, with the ¹⁷O NMR spectrum exhibiting signals due to environments with zero, one and two nearest neighbour Ln ions, as well as further splitting due to oxygen vacancies. Finally, we extract an activation energy for oxygen vacancy motion in these systems of 0.35 ± 0.02 eV from the Arrhenius temperature dependence of the ¹⁷O T₁ relaxation constants, which is found to be independent of the Ln ion within error. The relation of this activation energy to literature values for oxygen diffusion in Ln-substituted CeO₂ is discussed to infer mechanistic information which can be applied to further develop these materials as solid-state oxide-ion conductors.

Quo Vadis qNMR?

The quantification of a drug, its impurities, and e.g. components of a mixture has become routine in NMR laboratories and many applications have been described in the literature. However, besides simply using 1D ¹H or ¹³C NMR, a number of more advanced methods has been developed and used in the past. Here, we want to describe the applicability of nuclei beyond the classical ones ¹H and ¹³C. Mixtures can be characterized much better by applying various chemometric methods and separating the signals of mixture components can be achieved by DOSY experiments. All these methods contribute to the platform of qNMR methods and extend the possibilities of NMR for quantification and quality evaluation of drugs, excipients, polymers, and plant extracts. However, for quantification purposes, validation is always an issue and it is necessary to think about taking NMR related measures which might be different from the ones considered for chromatographic methods.

Natural abundance 17O MAS NMR and DFT simulations: New insights into the atomic structure of designed micas

Combining ¹⁷O Magic-Angle Spinning (MAS) NMR at natural abundance with DFT calculations is a promising methodology to shed light on the structure and disorder in tetrahedral sheets of designed micas with enhanced properties. Among brittle micas, synthetic mica is an important alternative to natural ones with a swelling sheet-like structure that results in many applications, by exploiting unique characteristics. Lowenstein's rule is one of the main chemical factor that determines the atomic structure of aluminosilicates and furthermore their properties. In the present article, ¹⁷O MAS NMR spectroscopy is used to validate (or not) the agreement of the Lowenstein's rule with the distribution of Si and Al sites in the tetrahedral sheets of synthetic micas. ¹⁷O MAS spectra of synthetic high-charged micas exhibit two regions of signals that revealed two distinguishable oxygen environments, namely Si-O-X (with X = Si, Al^tet, Mg) and Al^tet-O-Y (Y=Mg or Al^tet). DFT calculations were also conducted to obtain the ¹⁷O chemical shift and other NMR features like the quadrupolar coupling constant, C_Q, for all of the oxygen environments encountered in the two model structures, one respecting the Lowenstein's rule and the other involving Al^tet-O-Al^tet and Si-O-Si environments. Our DFT calculations support the ¹⁷O assignment, by confirming that Al^tet-O-3Mg and Al^tet-O-Al^tet oxygen environments show chemical shifts under 30 ppm and more important, with quadrupolar coupling constants of about 1 MHz, in line with the spectral observation. By quantifying the ¹⁷O MAS NMR spectra at natural abundance, we demonstrate that one of the synthetic mica compositions does not meet the Lowenstein's rule.

17O NMR and 15N NMR chemical shifts of sterically-hindered amides: ground-state destabilization in amide electrophilicity

The structure and spectroscopic properties of the amide bond are a topic of fundamental interest in chemistry and biology. Herein, we report 17O NMR and 15N NMR spectroscopic data for four series of sterically-hindered acyclic amides. Despite the utility of 17O NMR and 15N NMR spectroscopy, these methods are severely underutilized in the experimental determination of electronic properties of the amide bond. The data demonstrate that a combined use of 17O NMR and 15N NMR serves as a powerful tool in assessing electronic effects of the amide bond substitution as a measure of electrophilicity of the amide bond. Notably, we demonstrate that steric destabilization of the amide bond results in electronically-activated amides that are comparable in terms of electrophilicity to acyl fluorides and carboxylic acid anhydrides.

A Solution NMR Approach To Determine the Chemical Structures of Carbohydrates Using the Hydroxyl Groups as Starting Points

An efficient NMR approach is described for determining the chemical structures of the monosaccharide glucose and four disaccharides, namely, nigerose, gentiobiose, leucrose and isomaltulose. This approach uses the 1H resonances of the -OH groups, which are observable in the NMR spectrum of a supercooled aqueous solution, as the starting point for further analysis. The 2D-NMR technique, HSQC-TOCSY, is then applied to fully define the covalent structure (i.e., the topological relationship between C-C, C-H, and O-H bonds) that must be established for a novel carbohydrate before proceeding to further conformational studies. This process also leads to complete assignment of all 1H and 13C resonances. The approach is exemplified by analyzing the monosaccharide glucose, which is treated as if it were an "unknown", and also by fully assigning all the NMR resonances for the four disaccharides that contain glucose. It is proposed that this technique should be equally applicable to the determination of chemical structures for larger carbohydrates of unknown composition, including those that are only available in limited quantities from biological studies. The advantages of commencing the structure elucidation of a carbohydrate at the -OH groups are discussed with reference to the now well-established 2D-/3D-NMR strategy for investigation of peptides/proteins, which employs the -NH resonances as the starting point.

An 17 O NMR study of diamagnetic and paramagnetic lanthanide-tris(oxydiacetate) complexes in aqueous solution

¹⁷ O-enriched complexes between oxydiacetate ligand and several diamagnetic and paramagnetic lanthanide(III) metal ions (Ln) were investigated by solution-state ¹⁷ O NMR spectroscopy. The bound-state signals of chelating (O_in ) and nonchelating (O_out ) oxygen atoms of the carboxylate groups were observed for all the samples investigated. The data indicate that the ¹⁷ O line width is dominated by contributions from both quadrupole relaxation and chemical exchange in the case of Pr and Nd complexes. Dissection of the chemical shift induced by metal ions on O_in  into Fermi contact and pseudocontact contributions was performed , and the hyperfine coupling constant (A/ℏ) was estimated. No evidence of structural changes within the series was detected.

Intramolecular O-H⋯O and C-H⋯O hydrogen bond cooperativity in D-glucopyranose and D-galactopyranose-A DFT/GIAO, QTAIM/IQA, and NCI approach

Density functional theory calculations are used to compute proton nuclear magnetic resonance (NMR) chemical shifts, interatomic distances, atom-atom interaction energies, and atomic charges for partial structures and conformers of α-D-glucopyranose, β-D-glucopyranose, and α-D-galactopyranose built up by introducing OH groups into 2-methyltetrahydropyran stepwisely. For the counterclockwise conformers, the most marked effects on the NMR shift and the charge on the OH1 proton are produced by OH2, those of OH3 and OH4 being somewhat smaller. This argues for a diminishing cooperative effect. The effect of OH6 depends on the configuration of the hydroxymethyl group and the position, axial or equatorial, of OH4, which controls hydrogen bonding in the 1,3-diol motif. Variations in the interaction energies reveal that a "new" hydrogen bond is sometimes formed at the expense of a preexisting one, probably due to geometrical constraints. Whereas previous work showed that complexing a conformer with pyridine affects only the nearest neighbour, successive OH groups increase the interaction energy of the N⋯H1 hydrogen bond and reduce its length. Analogous results are obtained for the clockwise conformers. The interaction energies for C-H⋯OH hydrogen bonding between axial CH protons and OH groups in certain conformers are much smaller than for O-H⋯OH bonds but they are largely covalent, whereas those of the latter are predominantly coulombic. These interactions are modified by complexation with pyridine in the same way as O-H⋯OH interactions: the computed NMR shifts of the CH protons increase, the atom-atom distances are shorter, and interaction energies are enhanced.

Probing the surface of γ-Al2O3 by oxygen-17 dynamic nuclear polarization enhanced solid-state NMR spectroscopy

γ-Al2O3 is an important catalyst and catalyst support of industrial interest. Its acid/base characteristics are correlated to the surface structure, which has always been an issue of concern. In this work, the complex (sub-)surface oxygen species on surface-selectively labelled γ-Al2O3 were probed by 17O dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS). Direct 17O MAS and indirect 1H-17O cross-polarization (CP)/MAS DNP experiments enable observation of the (sub-)surface bare oxygen species and hydroxyl groups. In particular, a two-dimensional (2D) 17O 3QMAS DNP spectrum was for the first time achieved for γ-Al2O3, in which two O(Al)4 and one O(Al)3 bare oxygen species were identified. The 17O isotropic chemical shifts (δcs) vary from 56.7 to 81.0 ppm and the quadrupolar coupling constants (CQ) range from 0.6 to 2.5 MHz for the three oxygen species. The coordinatively unsaturated O(Al)3 species is characterized by a higher field chemical shift (56.7 ppm) and the largest CQ value (2.5 MHz) among these oxygen sites. 2D 1H → 17O HETCOR DNP experiments allow us to discriminate three bridging (Aln)-μ2-OH and two terminal (Aln)-μ1-OH hydroxyl groups. The structural features of the bare oxygen species and hydroxyl groups are similar for the γ-Al2O3 samples isotopically labelled by 17O2 gas or H217O. The results presented here show that the combination of surface-selective labelling and DNP-SENS is an effective approach for characterizing oxides with complex surface species.

The Strong β-CF3 Shielding Effect in Hexafluoroisopropanol and 100 Other Organic Solvents Revisited with 17O NMR Spectroscopy

An ¹⁷O NMR spectroscopy survey of more than 100 ubiquitous organic solvents and compounds, including some typical oxofluorinated solvents such as hexafluoroisopropanol, trifluoroethanol, trifluoroacetic acid, and others, is presented with D₂O as a reference. A strong alternating α,β−CF₃‐substituent chemical shift effect was thus observed. This alternating deshielding–shielding effect is suspected to have a role in the exceptional properties of these oxofluorinated solvents, notably in oxidative cross‐coupling reactions.

Dynamic NMR for coordination chemistry

Kinetic information that cannot be acquired with other techniques can be obtained by carefully planned and dynamic NMR experiments.

High Resolution NMR Spectroscopy as a Structural and Analytical Tool for Unsaturated Lipids in Solution

Mono- and polyunsaturated lipids are widely distributed in Nature, and are structurally and functionally a diverse class of molecules with a variety of physicochemical, biological, medicinal and nutritional properties. High resolution NMR spectroscopic techniques including 1H-, 13C- and 31P-NMR have been successfully employed as a structural and analytical tool for unsaturated lipids. The objective of this review article is to provide: (i) an overview of the critical 1H-, 13C- and 31P-NMR parameters for structural and analytical investigations; (ii) an overview of various 1D and 2D NMR techniques that have been used for resonance assignments; (iii) selected analytical and structural studies with emphasis in the identification of major and minor unsaturated fatty acids in complex lipid extracts without the need for the isolation of the individual components; (iv) selected investigations of oxidation products of lipids; (v) applications in the emerging field of lipidomics; (vi) studies of protein-lipid interactions at a molecular level; (vii) practical considerations and (viii) an overview of future developments in the field.

Initial investigation of glucose metabolism in mouse brain using enriched 17 O-glucose and dynamic 17 O-MRS

In this initial work, the in vivo degradation of ¹⁷ O-labeled glucose was studied during cellular glycolysis. To monitor cellular glucose metabolism, direct ¹⁷ O-magnetic resonance spectroscopy (MRS) was used in the mouse brain at 9.4 T. Non-localized spectra were acquired with a custom-built transmit/receive (Tx/Rx) two-turn surface coil and a free induction decay (FID) sequence with a short TR of 5.4 ms. The dynamics of labeled oxygen in the anomeric 1-OH and 6-CH₂ OH groups was detected using a Hankel-Lanczos singular value decomposition (HLSVD) algorithm for water suppression. Time-resolved ¹⁷ O-MRS (temporal resolution, 42/10.5 s) was performed in 10 anesthetized (1.25% isoflurane) mice after injection of a 2.2 M solution containing 2.5 mg/g body weight of differently labeled ¹⁷ O-glucose dissolved in 0.9% physiological saline. From a pharmacokinetic model fit of the H₂¹⁷ O concentration-time course, a mean apparent cerebral metabolic rate of ¹⁷ O-labeled glucose in mouse brain of CMR_Glc  = 0.07 ± 0.02 μmol/g/min was extracted, which is of the same order of magnitude as a literature value of 0.26 ± 0.06 μmol/g/min reported by ¹⁸ F-fluorodeoxyglucose (¹⁸ F-FDG) positron emission tomography (PET). In addition, we studied the chemical exchange kinetics of aqueous solutions of ¹⁷ O-labeled glucose at the C1 and C6 positions with dynamic ¹⁷ O-MRS. In conclusion, the results of the exchange and in vivo experiments demonstrate that the C6-¹⁷ OH label in the 6-CH₂ OH group is transformed only glycolytically by the enzyme enolase into the metabolic end-product H₂¹⁷ O, whereas C1-¹⁷ OH ends up in water via direct hydrolysis as well as glycolysis. Therefore, dynamic ¹⁷ O-MRS of highly labeled ¹⁷ O-glucose could provide a valuable non-radioactive alternative to FDG PET in order to investigate glucose metabolism.

Unleashing the Potential of 17 O NMR Spectroscopy Using Mechanochemistry

¹⁷ O NMR spectroscopy has been the subject of vivid interest in recent years, because there is increasing evidence that it can provide unique insight into the structure and reactivity of many molecules and materials. However, due to the very poor natural abundance of oxygen-17, ¹⁷ O labeling is generally a prerequisite. This is a real obstacle for most research groups, because of the high costs and/or strong experimental constraints of the most frequently used ¹⁷ O-labeling schemes. Here, we show for the first time that mechanosynthesis offers unique opportunities for enriching in ¹⁷ O a variety of organic and inorganic precursors of synthetic interest. The protocols are fast, user-friendly, and low-cost, which makes them highly attractive for a broad research community, and their suitability for ¹⁷ O solid-state NMR applications is demonstrated.

Hydrogen Atomic Positions of O-H···O Hydrogen Bonds in Solution and in the Solid State: The Synergy of Quantum Chemical Calculations with ¹H-NMR Chemical Shifts and X-ray Diffraction Methods

The exact knowledge of hydrogen atomic positions of O-H···O hydrogen bonds in solution and in the solid state has been a major challenge in structural and physical organic chemistry. The objective of this review article is to summarize recent developments in the refinement of labile hydrogen positions with the use of: (i) density functional theory (DFT) calculations after a structure has been determined by X-ray from single crystals or from powders; (ii) ¹H-NMR chemical shifts as constraints in DFT calculations, and (iii) use of root-mean-square deviation between experimentally determined and DFT calculated ¹H-NMR chemical shifts considering the great sensitivity of ¹H-NMR shielding to hydrogen bonding properties.

On the nanosecond proton dynamics in phosphoric acid–benzimidazole and phosphoric acid–water mixtures

Combining ¹H-NMR, ¹⁷O-NMR, and high-resolution backscattering QENS hydrodynamic and structural proton transport in phosphoric acid is separated. The rate limiting steps for structural proton diffusion in mixtures of acid with Brønsted bases are found to occur below the nanosecond timescale.

Solid-State 17O NMR of Unstable Acyl-Enzyme Intermediates: A Direct Probe of Hydrogen Bonding Interactions in the Oxyanion Hole of Serine Proteases

We report preparation, trapping, and solid-state ¹⁷O NMR characterization of three unstable acyl-enzyme intermediates (≈ 26 kDa): p-N,N-dimethylamino-[¹⁷O]benzoyl-chymotrypsin, trans-o-methoxy-[¹⁷O]cinnamoyl-chymotrypsin, and trans-p-methoxy-[¹⁷O]cinnamoyl-chymotrypsin. We show that both the ¹⁷O chemical shifts and nuclear quadrupolar parameters obtained for these acyl-enzyme intermediates in the solid state are correlated with their deacylation rate constants measured in aqueous solution. With the aid of quantum mechanical calculations, the experimental ¹⁷O NMR parameters were interpreted as to reflect the hydrogen bonding interactions between the carbonyl (C═¹⁷O) functional group of the acyl moiety and the two NH groups from the protein backbone (Ser195 and Gly193) in the oxyanion hole, a general feature of all serine proteases. Our results further suggest that the ¹⁷O chemical shift and quadrupole coupling constant display distinctly different sensitivities toward different aspects of hydrogen bonding, such as hydrogen bond distance and direction. This work demonstrates the utility of ¹⁷O as a useful nuclear probe in NMR studies of enzymes.

H-Bond: Τhe Chemistry-Biology H-Bridge

H-bonding, as a non covalent stabilizing interaction of diverse nature, has a central role in the structure, function and dynamics of chemical and biological processes, pivotal to molecular recognition and eventually to drug design. Types of conventional and non conventional (H-H, dihydrogen, H- π, CH- π, anti- , proton coordination and H-S) H-bonding interactions are discussed as well as features emerging from their interplay, such as cooperativity (σ- and π-) effects and allostery. Its utility in many applications is described. Catalysis, proton and electron transfer processes in various materials or supramolecular architectures of preorganized hosts for guest binding, are front-line technology. The H-bond-related concept of proton transfer (PT) addresses energy issues or deciphering the mechanism of many natural and synthetic processes. PT is also of paramount importance in the functions of cells and is assisted by large complex proteins embedded in membranes. Both intermolecular and intramolecular PT in H-bonded systems has received attention, theoretically and experimentally, using prototype molecules. It is found in rearrangement reactions, protein functions, and enzyme reactions or across proton channels and pumps. Investigations on the competition between intra- and intermolecular H bonding are discussed. Of particular interest is the H-bond furcation, a common phenomenon in protein-ligand binding. Multiple H-bonding (H-bond furcation) is observed in supramolecular structures.

In vivo17O MRS imaging - Quantitative assessment of regional oxygen consumption and perfusion rates in living brain

In the last decade, in vivo oxygen-17 (¹⁷O) MRS has evolved into a promising MR technique for noninvasively studying oxygen metabolism and perfusion in aerobic organs with the capability of imaging the regional metabolic rate of oxygen and its changes. In this chapter, we will briefly review the methodology of the in vivo¹⁷O MRS technique and its recent development and applications; we will also discuss the advantages of the high/ultrahigh magnetic field for ¹⁷O MR detection, as well as the challenges and potential of this unique MRS method for biomedical research of oxygen metabolism, mitochondrial function and tissue energetics in health and disease.

Simple (17) O NMR method for studying electron self-exchange reaction between UO2 (2+) and U(4+) aqua ions in acidic solution

(17) O NMR spectroscopy is proven to be suitable and convenient method for studying the electron exchange by following the decrease of (17) O-enrichment in U(17) OO(2+) ion in the presence of U(4+) ion in aqueous solution. The reactions have been performed at room temperature using I = 5 M ClO4 (-) ionic medium in acidic solutions in order to determine the kinetics of electron exchange between the U(4+) and UO2 (2+) aqua ions. The rate equation is given as R = a[H(+) ](-2)  + R', where R' is an acid independent parallel path. R' depends on the concentration of the uranium species according to the following empirical rate equation: R' = k1 [UO(2 +) ](1/2) [U(4 +) ](1/2)  + k2 [UO(2 +) ](3/2) [U(4 +) ](1/2) . The mechanism of the inverse H(+) concentration-dependent path is interpreted as equilibrium formation of reactive UO2 (+) species from UO2 (2+) and U(4+) aqua ions and its electron exchange with UO2 (2+) . The determined rate constant of this reaction path is in agreement with the rate constant of UO2 (2+) -UO2 (+) , one electron exchange step calculated by Marcus theory, match the range given experimentally of it in an early study. Our value lies in the same order of magnitude as the recently calculated ones by quantum chemical methods. The acid independent part is attributed to the formation of less hydrolyzed U(V) species, i.e. UO(3+) , which loses enrichment mainly by electron exchange with UO2 (2+) ions. One can also conclude that (17) O NMR spectroscopy, or in general NMR spectroscopy with careful kinetic analysis, is a powerful tool for studying isotope exchange reactions without the use of sophisticated separation processes. Copyright © 2015 John Wiley & Sons, Ltd.