Solid State Nuclear Magnetic Resonance 73 (2016) 1–14

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Solid State Nuclear Magnetic Resonance

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Trends

Solid-State 17O NMR studies of organic and biological molecules:Recent advances and future directions

Gang WuDepartment of Chemistry, Queen's University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6

a r t i c l e i n f o

Article history:Received 6 October 2015Received in revised form20 November 2015Accepted 24 November 2015Available online 30 November 2015

Keywords:Solid-state NMROxygen-17OrganicBiologicalMetal organic frameworksPharmaceuticalsMolecular motionDynamic nuclear polarizationParamagnetic compounds

x.doi.org/10.1016/j.ssnmr.2015.11.00140/& 2015 Elsevier Inc. All rights reserved.

a b s t r a c t

This Trends article highlights the recent advances published between 2012 and 2015 in solid-state 17ONMR for organic and biological molecules. New developments in the following areas are described:(1) new oxygen-containing functional groups, (2) metal organic frameworks, (3) pharmaceuticals,(4) probing molecular motion in organic solids, (5) dynamic nuclear polarization, and (6) paramagneticcoordination compounds. For each of these areas, the author offers his personal views on importantproblems to be solved and possible future directions.

& 2015 Elsevier Inc. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Recent advances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1. New oxygen-containing functional groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2. Metal organic frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3. Pharmaceuticals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.4. Probing molecular motion in organic solids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.5. Dynamic nuclear polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.6. Paramagnetic coordination compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1. Introduction

Solid-state 17O NMR spectroscopy is a useful technique for di-rect detection of local oxygen environments in organic and bio-logical molecules. The remarkable sensitivity of 17O quadrupolarcoupling (QC) and chemical shift (CS) tensors toward molecularinteractions such as hydrogen bonding and metal–ligand interac-tions makes 17O an important NMR probe capable of providinginformation that sometimes is difficult to obtain with the more

conventional 1H, 13C and 15N NMR techniques. Another aspect ofsolid-state 17O NMR, which has been largely unexplored in thepast, is that both 17O QC and CS interactions are also sensitive tomolecular motion. Thus, solid-state 17O NMR studies can yieldvaluable information about the dynamic processes occurring insolid materials. While the potential of 17O NMR in studies of or-ganic and biological molecules was recognized some time ago [1],progress has been rather slow. The technical difficulties inperforming 17O NMR experiments are linked to the unfavorable

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–142

nuclear properties of 17O, the only NMR-active stable oxygen iso-tope. First, the natural abundance of 17O is exceedingly low(0.037%). As a result, site-specific 17O-isotopic labeling of the mo-lecular system through chemical synthesis becomes a prerequisitefor nearly all 17O NMR studies. This could be a challenging task fororganic and biological molecules. Second, 17O has a nuclear spinquantum number (I) of 5/2, thus belonging to a class of isotopesknown as having quadrupolar nuclei (I41/2). From the NMRperspective, quadrupolar nuclei often experience a much largernuclear spin interaction known as the quadrupolar interaction, inaddition to the ones common for both spin-1/2 and quadrupolarnuclei (i.e., Zeeman, magnetic shielding, dipolar, and indirect spin–spin interactions). The strong nuclear quadrupolar interactiontends to cause severe line broadening in the NMR spectra ofquadrupolar nuclei. Third, the magnetogyric ratio (γ) of 17O isconsiderably lower than that of 1H. For example, at 14.1 T, theLarmor frequencies, (γ/2π)B0, of 1H and 17O are 600 and 81 MHz,respectively. This relatively low γ of 17O imposes additional lim-itations on the sensitivity of 17O NMR experiments. Since 2000, thefield of solid-state 17O NMR for organic and biological moleculeshas experienced a significant expansion, largely due to the avail-ability of ultrahigh magnetic fields (e.g., 21 T). This is because themajor obstacle in solid-state 17O NMR is related to the fact that 17Onuclei in most organic functional groups experience large second-order nuclear quadrupolar interactions, which can only be par-tially removed by the standard magic-angle spinning (MAS)technique. As a consequence, 17O MAS NMR spectra often displaypoor spectral resolution to the extent that signals linked to thedifferent chemical environments cannot be resolved, making itdifficult to find any chemical application. However, because thesecond-order nuclear quadrupolar interaction is inversely pro-portional to the applied magnetic field, performing solid-state 17ONMR experiments at very high magnetic fields would bring tre-mendous benefits in both sensitivity and resolution. These twoaspects are particularly important for studying organic and bio-logical molecules for which not only different oxygen-containingfunctional groups may be present, but the weight percentage of17O nuclei can also be exceedingly low. For example, if one is todetect a single oxygen atom (assuming 50% 17O enriched) out of a100-kDa protein, the weight percentage for 17O nuclei is onlyabout 0.01%. Furthermore, because most of the current solid-state17O NMR experiments rely on detection of the so-called centraltransition (CT, mI¼1/22mI¼�1/2), the maximum CT signal in-tensity for 17O nuclei is only 25.7% of the total signal intensityavailable from the Boltzmann population distribution. Despitethese difficulties, 17O QC and CS tensors have been experimentallydetermined for many oxygen-containing organic functional groupsin the past 15 years. Now it is also possible to obtain solid-state 17ONMR spectra for large protein-ligand complexes [2]. In the pastfew years, the pace of development in this particular field hasaccelerated considerably, as evidenced by the fact that solid-state17O NMR has found applications in several new areas. Thus it isperhaps appropriate to summarize these new advances in thisTrends article. The subject of solid-state 17O NMR of organic andbiological molecules has been reviewed several times in the pastdecade with the most recent account being published in 2014 [3–7]. Also in 2014, Theodorou et al. [8] published a comprehensivereview on the synthetic methodologies for preparing 17O-labeledcompounds. Therefore, another thorough review of the field isunnecessary at this time. Rather, this Trends article will highlightonly the new results published between 2012 and 2015 in the fieldof solid-state 17O NMR for organic and biological molecules. Wewill categorize the new developments into the following areas:(1) new oxygen-containing functional groups, (2) metal organicframeworks, (3) pharmaceuticals, (4) probing molecular motion inorganic solids, (5) dynamic nuclear polarization, and

(6) paramagnetic coordination compounds. Many of these areasrepresent new frontiers of solid-state 17O NMR. In each section,while all new reports will be described, in-depth discussion will bedevoted only to a few selected examples. The choice of examplesreflects only the personal preference of the author. At the end ofeach section, the author's views on important problems to besolved and possible future directions will be discussed.

2. Recent advances

2.1. New oxygen-containing functional groups

In general, solid-state 17O NMR studies allow characterizationof both QC and CS tensors. These NMR tensors provide importantinformation about the chemical bonding and molecular structure.While a large number of oxygen-containing organic functionalgroups have been characterized by solid-state 17O NMR [3–7],there are still many other oxygen-containing functional groupsthat have not yet been studied by this technique. Thus, continuingaccumulation of solid-state 17O NMR data for new oxygen-con-taining functional groups is of fundamental value.

In 2012, O'Dell and co-workers [9] reported a solid-state NMRstudy of crystalline taurine (2-aminoethane-1-sulfonic acid,H3Nþ–CH2CH2–SO3

–) in which they obtained solid-state NMRspectra for 1H, 13C, 14/15N and 17O nuclei. This is the first time that acomplete set of 17O NMR tensors were reported for a sulfonatefunctional group (–SO3

�). In combination with their previouslyreported solid-state 33S NMR data on taurine [10], they showedthat the DFT calculations can be evaluated by comparing withexperimental solid-state NMR data collected for all magnetic nu-clei in this organic molecule. In 2013, Hanna and co-workers [11]employed a combined solid-state NMR, DFT and X-ray diffractionapproach to study the hydrogen bonding interaction in a series ofacid salts of dibenzoates, MH(PhOO)2 (M¼Li, K, Rb, and Cs). Inaddition to collecting solid-state NMR data on 1H, 13C, 7Li, 39K,87Rb, and 133Cs nuclei, they also reported solid-state 17O MAS andDOR NMR spectra for this series of compounds. This powerfulcombination of MAS and DOR techniques allowed the authors toobtain reliable 17O NMR parameters for all crystallographicallydistinct oxygen sites in these compounds. The above two studiesare among a very few cases [12] where efforts were made to re-cord solid-state NMR spectra for all magnetic nuclei in a chemicalcompound. In principle, refinement of crystal structures by anNMR crystallography approach should utilize NMR informationfrom all magnetic nuclei available in a molecular system. However,in practice, quantum chemical calculations of NMR properties mayhave different degrees of accuracy for different types of chemicalbonds and for different elements across the periodic table becauseof the variations in electron correlation and relativistic effects.Thus it is still a challenge to find a unified treatment of all NMRactive nuclei within the molecule.

In addition to common H, C, N, and O atoms, many biologicalmolecules also contain metal ions for both structural and catalyticfunctions. Very often, the interaction between a metal center andorganic ligands is mediated through oxygen-containing functionalgroups. As such, solid-state 17O NMR is an important tool forstudying metal-ligand interactions [13–16]. In 2012, Zhu et al. [17]determined both the magnitudes and orientations of the 17O NMRtensors in two terminal oxo-metal complexes: 17O≡Ti(IV)(TMP)and 17O≡Cr(IV)(TMP), where TMP is 5,10,15,20-tetra-mesitylporphyrin. In each of these transition metal oxo com-pounds, the oxygen atom is triply bonded to a metal atom, i.e.,O≡MLn. For this type of metal complexes, no solid-state 17O NMRdata were available in the literature prior to this work. As seenfrom Fig. 1, these terminal oxo metal complexes exhibit rather

*

ppm-5000500100015002000 ppm-5000500100015002000

simulated

observed

14.09 T

*

*11.75 T

simulated

observed

simulated

observed

14.09 T

11.75 T

simulated

observed

*iso

iso

iso

iso

*

ppm-5000500100015002000 ppm-5002000

simulated

observed

14.09 T

*

*11.75 T

simulated

observed

simulated

observed

14.09 T

11.75 T

simulated

observed

*iso

*iso

iso

iso

iso

Fig. 1. Experimental (top trace) and simulated (lower trace) 17O MAS spectra of (a) 17O≡Ti(TMP) and (b) 17O≡Cr(TMP) at two magnetic fields. Reproduced with permissionfrom Ref. [17]. Copyright 2012 Royal Society of Chemistry.

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–14 3

small 17O quadrupole coupling constants (CQE2 MHz) but verylarge chemical shift anisotropies (Ω¼δ11–δ33E2000 ppm). This issimilar to what have been known for the oxygen atom involved inan O≡E triple bond where E is a main group element such as C(O≡C) and N (O≡Nþ). In these special cases, because the second-order quadrupolar broadenings are very small even at moderatemagnetic fields, the 17O MAS spectra display spinning sidebandmanifolds that resemble those from spin-1/2 nuclei, as seen fromFig. 1. In this study, the authors also presented a detailed analysisof the individual molecular orbital (MO) contributions to the totalmagnetic shielding. They identified the major paramagneticshielding contribution to arise from the magnetic-field-inducedmixing between s(O≡M) and π*(O≡M) MOs. This occurs when theexternal magnetic field is perpendicular to the O≡M triple bond. Infact, this is a common feature in all terminal phosphide [18], car-bide [19], nitrido [20] and oxo compounds. They also found a clearcorrelation between Ω and the mean inverse cube of the 2pelectron radius, or�342p, among the isoelectronic carbide (C4�),nitride (N3�), and oxo (O2�) compounds. This observation sug-gests that the increase in Ω from carbide, nitride, to oxo is due tothe fact that the increasing nuclear charge in the series (C, N, andO) causes a contraction of the 2p electrons. Furthermore, the verysmall CQ(17O) value in the O≡M triple bond indicates that theoxygen 2s electrons have very small contributions to the bonding.It is quite interesting to note that terminal oxo compounds re-present one of the rare cases where the oxygen nucleus simulta-neously experiences highly anisotropic magnetic shielding, but ahighly symmetric electric field distribution. This highlights thefundamentally different origins of these two types of NMR inter-actions. Also in 2012, Hagaman and co-workers [21] reported so-lid-state 17O NMR parameters for benzoic acid, p-anisic (4-meth-oxybenzoic) acid, and methyl-p-anisate. They further used solid-state 17O NMR to probe benzoic acid adsorption on metal oxidesurfaces. In particular, they showed that, when benzoic acid was

dry mixed with mesoporous silica, chemical reactions did notoccur. However, when benzoic acid was dry mixed with nonporoustitania and alumina, the observed 17O NMR spectral changes sug-gest the formation of metal-oxygen bonds on the metal oxidesurface. This study shows the potential of solid-state 17O NMR inproviding new insight into the chemistry occurring on amorphousmetal oxide materials.

Very recently, Kong et al. [22] reported a combined solid-state17O NMR, crystallographic, and computational study of N-acyl-imidazoles. These molecules were used as precursors to form acyl-enzyme complexes. They noticed that some of the N-acyl imida-zoles exhibit 17O isotropic chemical shifts in the range of 380-450 ppm. These 17O chemical shifts are clearly outside the normalrange found for amides including aromatic amides (o350 ppm);but they are quite similar to those observed for twisted amides(400–500 ppm). Is it possible that the C(O)–N amide bonds inthese N-acyl imidazoles are also twisted? The fact that theseN-acyl imidazoles can be readily hydrolyzed in aqueous solutionseems to be consistent with the known properties of twistedamides. To fully understand the relationship between 17O NMRtensor parameters and molecular structure, Kong et al. [22] carriedout solid-state 17O NMR and single-crystal X-ray diffraction mea-surements on three representative N-acyl imidazoles of the typeR–C(17O)-Im: R¼p-methoxycinnamoyl (MCA-Im), R¼4-(dimethy-lamino)benzoyl (DAB-Im), and R¼2,4,6-trimethylbenzoyl (TMB-Im), as shown in Scheme 1. The crystal structures of these com-pounds showed that, while the C(O)–N amide bond in DAB-Imexhibits a small twist, those in MCA-Im and TMB-Im are essen-tially planar. In these N-acyl imidazoles, the 17O QC and CS tensorswere found to depend critically on the torsion angle between theconjugated acyl group and the C(O)–N amide plane. The compu-tational results from a plane-wave DFT approach, which takes intoconsideration of the entire crystal lattice, are in excellent agree-ment with the experimental solid-state 17O NMR results. The

N

O

N

NCH

CH N

O

N

CH

CH

CHO

H

HO

CH

N

N

TMB-Im MCA-Im DAB-Im

Scheme 1. Molecular structures of N-acyl imidazoles.

Fig. 2. Experimental and simulated 17O MAS spectra of uniformly 17O-labeled L-cysteine �HCl �H2O at 21.1 T. The sample spinning frequency was 20 kHz. The threetypes of signals are: H2O (green), O¼C-OH (red), and C¼O (blue). Reproduced withpermission from Ref. [27]. Copyright 2015 American Chemical Society. (For inter-pretation of the references to color in this figure legend, the reader is referred tothe web version of this article.)

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–144

authors further performed quantum chemical computations toevaluate the dependence of 17O NMR parameters on the Ar–C(O) bond rotation. These calculations suggest that the angulardependence on the Ar–C(O) bond rotation is very similar to thatpreviously observed for the C(O)–N bond rotation in twistedamides. The major conclusion from this study is that, in general,one should be cautious when attributing the observed NMR che-mical shifts (13C, 15N, 17O) as well as other spectroscopic data suchas IR in carbonyl compounds to bond rotation of only one side ofthe carbonyl functional group.

In another recent study, Wu et al. [23] determined the 17O NMRtensors in two compounds each containing an O–N single bond:17O-labeled hydroxylammonium chloride ([H17O–NH3]Cl) and so-dium trioxodinitrate monohydrate (Na2[17ONNO2] �H2O, Angeli'ssalt). These two compounds are important in nitroxyl (HNO)chemistry [24]. In addition, [HO–NH3]þ is electronically related tohydrogen peroxide (HO–OH). They found that the characteristicfeature of an O–N single bond is that the 17O nucleus experiences alarge quadrupolar coupling constant (CQE13–15 MHz) but a ra-ther small chemical shift anisotropy (ΩE100–250 ppm). This isexactly opposite to what was discussed earlier for terminal oxometal compounds. It is also interesting to point out that, forcompounds containing an O¼N double bond (the nitroso func-tional group), both the 17O quadrupole coupling constant andchemical shift anisotropy are very large (e.g., CQE16 MHz andΩE3000 ppm) [25]. In the literature, very little has been reportedon the 17O NMR tensors in compounds containing O–N singlebonds. Therefore, the work by Wu et al. [23] provided muchneeded information about the 17O NMR tensors in this importantfunctional group. To certain extent, hydroxylammonium chlorideand Angeli's salt can also be considered as an acid/base pair (i.e.,H–O–N/–O–N). By comparing the observed 17O NMR tensors inthese two compounds, the authors were able to examine the effectof protonation/deprotonation on the oxygen nucleus. They foundthat the protonation/deprotonation effects for the O–N bond showa parallelism with those reported previously for phenolic oxygenatoms [26]. Another conclusion drawn from this work is that it isimportant to compare tensor components with the same or-ientation within the molecular frame of reference in order to gaininsight into the relationship between nuclear magnetic shieldingand chemical bonding.

Michaelis and co-workers [27] recently reported a compre-hensive solid-state 17O NMR study of bound water molecules incrystalline amino acids and dipeptides. While preliminary solid-state 17O NMR data on crystalline hydrates were reported pre-viously [28–30], this study represents the most thorough ex-amination of crystalline amino acid hydrates. In several cases, theyalso prepared amino acids or dipeptides where both the carbox-ylate group and water of hydration were 17O labeled. This leads tothe presence of multiple well-resolved 17O NMR signals in the MASspectra. For example, as shown in Fig. 2, the 17O MAS spectrum ofuniformly 17O-labeled L-cysteine �HCl �H2O (obtained at 21.1 Twith a sample spinning frequency of 20 kHz) exhibits three sets ofsignals: C¼O, δiso¼345 ppm, CQ¼8.45 MHz, ηQ¼0.05; C–O–H,δiso¼176 ppm, CQ¼7.2 MHz, ηQ¼0.26; H2O, δiso¼31 ppm,CQ¼7.0 MHz, ηQ¼0.90. By analyzing the solid-state 17O NMRspectra recorded at multiple magnetic fields, they were able to

report the first set of 17O CS tensor data for bound water mole-cules. As the 17O chemical shift anisotropy for water molecules isrelatively small, accurate measurement becomes a challenge evenat 21.1 T. They also noticed that, for the bound water molecules,there exists considerably discrepancy between experimentallyobserved CQ(17O) values and those calculated either by a plane-wave based DFT method such as CASTEP or by Gaussian09 onmolecular cluster models, regardless whether X-ray or neutronstructures were used. They attributed this disagreement to thedynamic effect on the observed 17O EFG tensor and proposed tocarry out a variable-temperature study on a model system. For thebound water molecules, the δiso(17O) values were found to vary ina range of approximately 40 ppm, which potentially can be used asfingerprints of the hydrogen bonding environment.

Future directions. As the scope of solid-state 17O NMR applica-tions continues to expand, accumulation of solid-state 17O NMRdata for new organic functional groups is still an important di-rection of research. It is useful to build up a comprehensive da-tabase of 17O NMR parameters, which can serve as a guide in newapplications. It can be anticipated that research along this line willcontinue in the future. While giving a long list of oxygen-con-taining functional groups for which no or very limited solid-state17O NMR data are available may not be practical here, severalgeneral areas are worth noting: (1) carbohydrates, (2) organicperoxides, (3) S-nitrosothiols (R–S–N¼O), and (4) nucleosides andnucleotides. Future developments in these specific areas will likelyproduce important and much needed data. One of the challengesis certainly the synthesis of 17O-labled molecules of interest. An-other important area that will continue to evolve in the future isthe utilization of solid-state 17O NMR for studying protein–ligandinteractions. The common approach used to date is to probe thisinteraction from the ligand side (by preparing 17O-labeled ligands)because it is relatively easy to synthesize 17O-labeled small organicmolecules than proteins [2,31]. However, as it is feasible (bothtechnically and economically) to introduce site-specific 17O-labelsinto proteins by recombinant protein expression techniques, oneimportant future direction is to demonstrate the unique in-formation that can be obtained from such 17O NMR studies. This isan area too important to be ignored in the future.

H3BTC H2BDC

H4DOT

H2BDC-OH

H3TATB H3BTB

H2ADC

Scheme 2. Molecular structures and abbreviations of common organic linkers used in MOFs.

Fig. 3. The 3D framework structure (top left), metal coordination environment (topright), 17O MAS and CP-MAS spectra of Zr–UiO-66. Reproduced with permissionfrom Ref. [34]. Copyright 2013 American Chemical Society.

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–14 5

2.2. Metal organic frameworks

Metal organic frameworks (MOFs) are a new class of three-di-mensional crystalline porous materials that may be considered asa hybrid between inorganic and organic materials [32]. MOFsgenerally have very high porosity and can be used in a wide rangeof applications such as gas adsorption, catalysis, sensors, drugstorage and delivery, etc. As shown in Scheme 2, the commonorganic linkers in MOFs often contain oxygen-containing func-tional groups that can interact with metal-containing units.Therefore, oxygen atoms usually form the organic/inorganic in-terface in MOFs. While solid-state NMR studies for 1H, 13C, 15N andNMR-active metal nuclei can provide information about organicand inorganic components respectively [33], solid-state 17O NMRseems to be an ideal tool for probing the organic/inorganic inter-face in MOFs.

In 2013, Huang et al. [34] reported the first detailed solid-state17O NMR study of MOFs. They demonstrated three syntheticstrategies to directly prepare 17O-labeled MOFs in a cost effectivefashion. They reported solid-state 17O NMR spectra for four 17O-labeled MOFs: Zr–UiO-66, MIL-53(Al), CPO-27-Mg, and micro-porous α-Mg3(HCOO)6. All spectra were acquired under the MAScondition at an ultrahigh magnetic field, 21.1 T. Because thesespectra are sensitive to the mode of bonding between the oxygen-containing group and the metal center, they can be used to iden-tify the presence of chemically different oxygen species. In somefavorable cases, it is also possible to distinguish chemically iden-tical but crystallographically non-equivalent oxygen sites. Asshown in Fig. 3, the 17O MAS NMR spectrum of Zr–UiO-66 exhibitsthree distinct sets of signals: δiso¼386 ppm, CQ¼2.0 MHz,ηQ¼0.50; δiso¼278 ppm, CQ¼7.1 MHz, ηQ¼0.85; δiso¼65 ppm,CQ¼6.5 MHz, ηQ¼0.60. This MOF has a 3D framework structureutilizing a basic building block Zr6O4(OH)4(1,4-BDC)6 where the sixzirconium atoms form the core octahedron and are linked by threetypes of oxygen-containing functional groups: carboxylate, μ3-O2�

anion, and μ3-OH. Thus, the authors assigned the three sets ofsignals to the three types of organic linkers on the basis of the wellestablished 17O NMR spectral signatures for these functionalgroups. They further confirmed the assignment for the capping μ3-OH groups by performing a 1H-17O cross polarization experiment

as also shown in Fig. 3.Many MOFs exhibit an exceptional degree of framework flex-

ibility. Upon adsorption of guest molecules, the unit cell of theMOF structure can change considerably, causing a phase trans-formation. Using MIL-53(Al) as an example, Huang et al. [34]showed that solid-state 17O NMR can be used to monitor such aphase transition in MOFs. The as-synthesized MIL-53(Al) crystal-lizes in an orthorhombic phase displaying a 1D channel structurewith a large pore size. Upon adsorption of water vapor, the or-thorhombic phase of MIL-53(Al) is completely transformed into amonoclinic phase with a one-third reduction in the unit cell vo-lume without breaking any chemical bond. Huang et al. [34] ob-served that such a phase transition causes an apparent signalsplitting in the 17O MAS spectrum of MIL-53(Al). Spectral analysesshowed quite different 17O NMR parameters (δiso, CQ, and ηQ) forthe same carboxylate group in the two phases. In the same study,the authors attempted to acquire 2D 17O MQMAS spectra for MOFsbut were unsuccessful due to the low 17O enrichment (o10%) intheir samples. Recently, Kong et al. [35] reported solid-state 17ONMR spectra for MOFs containing paramagnetic metal centers;

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–146

this study will be described in detail in a later section.Future directions. As the study by Huang et al. [34] is the first

solid-state 17O NMR study on MOFs, we anticipate that morestudies will appear in this new area of solid-state 17O NMR ap-plications. In many cases, it will be desirable to be able to applyMQMAS or DOR techniques to obtain high-resolution spectra.With the current strategies employed by Huang and co-workers[34], MOF samples were prepared directly using 40% 17O-enrichedwater as the source of 17O. However, the final 17O enrichment le-vels in the MOFs were all less than 10%. While the origin of thisfactor of 44 dilution of 17O labels in the MOF synthesis was un-clear, one possible alternative is to prepare 17O-labeled organiclinkers before the MOF synthesis. While this may increase the costof producing the 17O-labeled MOF materials, the benefit in in-creased NMR sensitivity will make many sophisticated experi-ments possible such as 2D MQMAS and heteronuclear correlationspectroscopy. Another future direction will be to combine solid-state 17O NMR with more conventional 1H and 13C NMR methodsin the so-called NMR crystallography approach to determine newMOF structures [36]. Since many MOFs contain paramagneticmetal centers, it will be important to further explore solid-state17O NMR applications in these paramagnetic systems.

2.3. Pharmaceuticals

Solid-state NMR in general has become an increasingly indis-pensable tool for characterization of active pharmaceutical in-gredients (APIs) [37,38]. In addition to the conventional solid-state13C NMR approach, applications of high-resolution solid-state 1HNMR [39–41] and studies of quadrupolar nuclei such as 14N (I¼1)[42,43], 23Na (I¼3/2) [44], and 35/37Cl (I¼3/2) [45,46] have alsoappeared. Recently, solid-state 17O NMR has emerged as a newaddition to the collection of NMR techniques that are useful in thestudy of pharmaceutical compounds. One of the main challenges isthe synthesis of 17O-labeled pharmaceutical molecules. Scheme 3summarizes the pharmaceutical compounds that have alreadybeen studied by solid-state 17O NMR.

In 2013, Wu et al. [47] reported a comprehensive solid-state 17ONMR study of two important pharmaceutical compounds: salicylicacid (SA) and o-acetylsalicylic acid (Aspirin). Between these twocompounds, there are a total of 7 oxygen sites that can be used as

OH

O

O

O

O1 O2

O3

O

OH

OH

OO1 O2

O3 7 1

2

3 4

5

6

7 1

2

3

4

5

6 8

Salicylic acid (SA) Aspirin

Carboplatin Oxalip

O1 O2

Scheme 3. Pharmaceutical molecules

17O NMR probes. The authors were able to selectively introduce17O labels onto these 7 oxygen sites. Fig. 4 shows the 17O MAS NMRspectra of SA and Aspirin recorded at two magnetic fields, 14.1 and21.1 T. One can see clearly that the spectra recorded at 21.1 T are ofconsiderably higher quality than those obtained at 14.1 T. Likemany other carboxylic acids, both SA and Aspirin molecules formcentrosymmetric hydrogen bonded dimers in the crystal lattice. Ineach dimer, the O � � �O separation is approximately 2.6 Å, corre-sponding to a medium-strength hydrogen bond. As seen fromFig. 4, in each case, two signals were observed for the carboxylicacid group, corresponding to the C¼O and C–O–H types of oxygenatoms. However, the 17O chemical shifts are significantly differentin these two seemingly similar compounds. For example, the C¼Oand C–O–H signals in [1,2-17O2]SA have 17O isotropic chemicalshifts of 284 and 168 ppm, respectively, while the correspondingsignals in [1,2-17O2]Aspirin appear at 273 and 215 ppm. The 17Oquadrupole parameters observed for the two types of oxygenatoms are also quite different in SA and Aspirin. The authors at-tributed the large differences to the fact that the double-well po-tential for proton movement within each dimer is nearly sym-metric in Aspirin, but very asymmetric in SA. As a result, theobserved 17O NMR signals for the C¼O and C–O–H groups in As-pirin at room temperature are subject to motional averaging dueto the proton movement. They further obtained solid-state 17ONMR spectra of SA and Aspirin at different temperatures. From thetemperature dependence of δ(17O), they were able to extract theenergy asymmetry (ΔE) of the double-well potential:ΔE¼3.070.5 kJ/mol for Aspirin and ΔE410 kJ/mol for SA. Forthe phenolic oxygen in SA, the observed 17O chemical shift was89 ppm. For the ester functional group in Aspirin, the observed 17Ochemical shifts were 369 and 203 ppm for the C¼O and C–O–Cgroups, respectively. In the same study, the authors also obtainedsolid-state 13C and 1H NMR data for Aspirin and SA. The 1H che-mical shifts for the hydrogen bonded carboxylic acid protons werefound to be 12.3 and 11.7 ppm for Aspirin and SA, respectively. Theauthors then compared all the experimental multinuclear NMRdata with those computed with the plane-wave DFT calculationsand found an excellent agreement between the experimental andcomputed results.

Also in 2013, Vogt et al. [48] used solid-state 17O NMR to studythe hydrogen bonding in four different forms of diflunisal

CH3

4

9

OH

OH

OO1 O2

O3

Diflunisal

F F

latin

O4

O3 O1

O2

studied by solid-state 17O NMR.

ppm

-500-400-300-200-10001002003004005006007008009001000

14.0 T 21.1 T

ppm

-200-1000100200300400500600700800

*

* *

* * *

* * * *

* * * *

* *

* *

* * * * * * *

*

*

* *

*

Fig. 4. Experimental (lower blue trace) and simulated (upper red trace) 17O MAS NMR spectra of (a) [1,2-17O2]SA, (b) [3-17O]SA, (c) [1,2-17O2]Aspirin, (d) [3-17O]Aspirin, and(e) [4-17O]Aspirin. The sample spinning frequency was 14.5 and 22.0 kHz for spectra collected at 14.1 and 21.1 T, respectively. Spinning sidebands are marked by *. Re-produced with permission from Ref. [47]. Copyright 2013 American Chemical Society.

Fig. 5. 2D 1H-17O CP-HETCOR spectrum (16.4 T) of the 17O-diflunisal pyrazinamidecocrystal. The sample spinning frequency was 25 kHz. Reproduced with permissionfrom Ref. [48]. Copyright 2013 American Chemical Society.

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–14 7

including a particular polymorph, a co-crystal with pyrazinamide,and two amorphous dispersions with polymers. They showed thatsolid-state 17O NMR spectra offer unique insight into hydrogenbonding interactions in these pharmaceutical solids, and should bea complementary tool to more conventional solid-state 1H, 13C and19F NMR studies. In this study, the authors also used 2D 1H-17O CP-HETCOR to help identify 1H NMR signals. As shown in Fig. 5, uti-lizing a short contact time (1 ms), they were able to observe twostrong 1H-17O correlation peaks for a [17O]diflunisal-pyrazinamideco-crystal sample. The stronger one was assigned to the directlybonded O2–H (rO–H¼1.0 Å) whereas the weaker one was assignedto an intramolecular hydrogen bond, O1 � � �H–O3 (rO � � �H¼1.7 Å).This observation allows elucidation of a key hydrogen-bondingfeature in a pharmaceutical co-crystal without the crystal struc-ture. This 2D 1H-17O CP-HETCOR spectrum also offers the spectralresolution of two overlapping 1H signals from phenolic and car-boxylic acid groups. Using this 17O NMR approach, the authorsshowed that the hydrogen-bonding environments are drasticallydifferent in the dispersions of disflunisal with two polymers.

In 2015, Kong et al. [49] reported a solid-state 17O NMR study oftwo platinum anticancer drugs: Carboplatin and Oxaliplatin. Al-though several thousand platinum-based anticancer drug mole-cules have been developed in research laboratories over the past30 years, only about two dozens have ever gone into clinical trials[50]. Carboplatin and Oxaliplatin are the only second-generationplatinum-based anticancer drugs that have gained internationalmarketing approval. In Carboplatin and Oxaliplatin, the mode ofbonding between the carboxylate group and the Pt(II) metal center

Fig. 6. Experimental (left) and simulated (right) static 17O NMR spectra of ABSAobtained at different temperatures at 21.1 T. Reproduced with permission from Ref.[56]. Copyright 2012 American Chemical Society.

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–148

can be described as being monodentate, O¼C–O–Pt(II). As a result,the non-Pt-coordinated oxygen atom can serve as an internal re-ference, making it easier to examine the effect of metal binding on17O NMR tensors. Kong et al. [49] experimentally determined the17O CS and QC tensors in these important Pt(II) anticancer drugs.With the aid of plane-wave DFT computations, the 17O CS and QCtensor orientations were determined in the molecular frame ofreference. They found that significant changes in the 17O CS and QCtensors occur for the carboxylate oxygen atom upon its co-ordination to Pt(II). In particular, the 17O isotropic chemical shiftsfor the oxygen atoms directly bonded to Pt(II) are found to besmaller (more shielded) by 200 ppm than those for the non-Pt-coordinated oxygens within the same carboxylate group. This isperhaps the largest 17O coordination shift reported to date. Carefulexamination of the 17O CS tensor components allowed the authorsto conclude that such a large 17O coordination shift is primarilydue to the shielding increase along the direction that is within theO¼C–O–Pt plane and perpendicular to the O–Pt bond. This resultis interpreted as due to the s donation from the oxygen non-bonding orbital (electron lone pair) to the Pt(II) empty dyz orbital,which results in large energy gaps between s(Pt–O) and un-occupied molecular orbitals thus reducing the paramagneticshielding contribution along the direction perpendicular to the O–Pt bond. This work represents the first time that solid-state 17ONMR is used to study metallodrugs. Compared with other NMR-active nuclei such as 14N, 35Cl, and 195Pt that have been previouslyused in the study of Pt(II) complexes [51], 17O NMR seems to beparticularly sensitive to the metal-carboxylate interactions.

Future directions. One drawback of the 2D 1H-17O CP-HETCORexperiment shown in Fig. 5 was that it took 55 h to acquire on a700 MHz spectrometer (16.4 T). One reason was the modest 17Oenrichment level (17.4%) used in the samples. However, the mainproblem is that the conventional CP sequence, which is based onthe Hartmann�Hahn match of the two spin-lock RF fields, hasvery low transfer efficiency between 1H and quadrupolar nuclei. Infact, the 17O signal is very often reduced rather than enhanced byCP. Further improvements are needed before this 2D method canbe routinely applied to pharmaceuticals. One possible solution, asPerras et al. [52] recently demonstrated, is to use the PRESTO(Phase-shifted Recoupling Effects a Smooth Transfer of Order) se-quence [53] for 1H-17O polarization transfer. As PRESTO wasshown by Perras et al. [52] to enhance the 17O NMR sensitivity by afactor of 3 over the conventional Hartmann–Hahn CP, this trans-lates into an experimental time saving of nearly an order ofmagnitude. Perras et al. [52] also showed that PRESTO can becoupled with QCPMG to further improve sensitivity in the 1H-17OHECTCOR experiment. Another possible direction is to utilize therobust D/J-HMQC type of experiments [54,55]. One can also en-visage that, in some cases, 17O NMR is perhaps uniquely sensitiveto polymorphism of pharmaceuticals either in pure form orcocrystal. 17O NMR is a direct probe to the state of water moleculesin solids, and thus can be used to study not only the hydrationstate of pharmaceutical compounds but also moisture uptake oftablets. Another area for future research is to use solid-state 17ONMR to study molecular motion in pharmaceutical materials.

2.4. Probing molecular motion in organic solids

One unique aspect of solid-state NMR is its ability to probemolecular motion in solid materials over a very large range oftimescale (from 10�12 to 102 s). In this regard, the most commonsolid-state NMR approach is 2H (I¼1) NMR. However, in manycases, the functional group of interest contains only non-hydrogenatoms. In 2012, Kong et al. [56] demonstrated the use of solid-state17O NMR to monitor molecular motion of sulfonate groups (–SO3

�) in organic solids. In particular, they studied three 17O-

labeled crystalline sulfonic acids: 2-aminoethane-1-sulfonic acid(taurine, T), 3-aminopropane-1-sulfonic acid (homotaurine, HT),and 4-aminobutane-1-sulfonic acid (ABSA). In the solid state, allthree compounds exist as zwitterionic structures, NH3

þ–R– −SO3 , inwhich the −SO3 groups participate in various degrees of O � � �H–Nhydrogen bonding. As an example, Fig. 6 shows the experimentaland simulated variable temperature (VT) 17O NMR spectra of astatic powder sample of ABSA at 21.1 T. The close agreement be-tween the observed and simulated spectra confirms that the – −SO3groups in these compounds undergo three-fold rotations. Theexperimentally determined jump rates are between 102 and105s�1. An Arrhenius analysis of these data yielded the activationenergies (Ea) for this process: Ea¼4877, 4273 and4571 kJ mol�1 for T, HT, and ABSA, respectively. This is the firsttime that −SO3 rotational dynamics were directly probed by solid-state 17O NMR. Using the experimental activation energies for −SO3rotation, they were able to further evaluate quantitatively the totalhydrogen bonding energy for each −SO3 group in the crystal lattice.This is based on the fact that, in order for the −SO3 group to rotateabout its three-fold axis interchanging oxygen positions, all threeO � � �H–N hydrogen bonds must break. This work provides a clearillustration of the utility of solid-state 17O NMR in quantifyingdynamic processes occurring in organic solids. Clearly, for studyingthe dynamics of – −SO3 functional group, 17O NMR is the only choice.

In 2014, Huang et al. [57] reported a novel application of solid-state 17O NMR in probing molecular dynamics. In this case, theyshowed that solid-state 17O NMR can be used to learn about thedynamics of gas molecules such as CO2 adsorbed in CPO-27-Mgand CPO-27-Zn (also known as MOF-74). They discovered that thedynamic motion of CO2 within the void spaces of these two closelyrelated MOFs can be best modeled by a combination of two typesof motion: localized wobbling and non-localized hopping. As seenfrom Fig. 7, the 17O NMR spectra of C17O2 adsorbed in CPO-27-Mg

Fig. 7. Experimental (a) and simulated (b, c, and d) static 17O NMR spectra of C17O2 adsorbed in CPO-27-Mg (with a molar ratio CO2:Mg of 0.1) obtained at differenttemperatures at 9.4 T. Reproduced with permission from Ref. [57]. Copyright 2014 American Chemical Society.

Fig. 8. (a) Partial crystal structure of [(TBA)2][CO3⊂mBDCA-5t-H6] illustrating the hydrogen bonding environment around the −CO32 anion inside the cryptate.

TBA¼tetrabutylammonium; mBDCA-5t-H6¼tert-butyl-substituted hexacarboxamide cryptand. (b) Experimental and simulated static 17O NMR spectra of [K2(DMF)2][CO17O2⊂mBDCA-5t-H6] at different temperatures. All spectra were recorded at 14.1 T. Reproduced with permission from Ref. [61]. Copyright 2015 American ChemicalSociety.

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–14 9

change drastically as a function of temperature. The authors wereable to simulate the experimental line shapes by using the com-bined motion model. It is clear that spectral simulations withneither wobbling nor hopping model alone can provide a sa-tisfactory match to the experimental line shapes. The authors ar-gued that solid-state 17O NMR is a particular sensitive probe to CO2

motion because of the presence of both 17O quadrupolar andchemical shift interactions. In comparison, solid-state 13C NMRspectra of these MOFs seem to be less useful in differentiatingbetween the one- and two-motion models of CO2 motion. Theauthors also showed that the metal-CO2 binding strength plays akey role in the CO2 dynamics, which can be further linked to theCO2 adsorption capability of the MOFs. It remains unclear whetherthe two-motion model is general in all MOFs. One may argue that,given the diversity in MOF structures and compositions, other

types of CO2 motion should also be possible. In any event, it isimportant to emphasize that a combined 13C and 17O solid-stateNMR approach will likely produce the most reliable picture aboutCO2 dynamics within MOFs. This is also in line with what wasdiscussed earlier in this article about utilizing all magnetic nucleiavailable within the molecule of interest.

In 2014, Adjei-Acheamfour and Böhmer [58] used the conven-tional stimulated echo time-domain 17O NMR technique to probethe ultra-slow motion of water molecules (with a correlation timeon the order of 10–3 s at 180 K) in hexagonal ice Ih. More recently,Böhmer et al. [59] employed 17O exchange spectroscopy (EXSY) toobtain 2D 17O CT exchange powder patterns for static solids. Theyshowed that such 2D patterns contain detailed information aboutthe dynamic process. As an example, they recorded the 2D 17OEXSY spectrum for THF-d8 clathrate hydrate. By using a dynamic

Fig. 9. Static 17O NMR spectra (5 T) of glycerol/D2O/H217O (60/30/10% by weight)

recorded at 82 K. The final concentration of H217O was �7.4%. (a) Simulated

spectrum, (b) CP-echo with microwave on (128 scans), (c) CPMG with microwaveon (16 scans), (d, e) CP-echo with microwave off (4864 scans). Reproduced withpermission from Ref. [65]. Copyright 2012 American Chemical Society.

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–1410

model developed by Ba et al. [60], the authors were able to si-mulate the observed 2D exchange pattern. The agreement be-tween calculated and experimental patterns was further improvedby inclusion of residual 1H–17O dipolar interactions in the simu-lation. In this study, the authors assumed that the 17O CT is subjectonly to the second-order quadrupolar interaction, which was validfor water molecules at the low magnetic field used in the study.However, a more general treatment including both second-orderquadrupolar and chemical shift anisotropy interactions is clearlyneeded, as the latter is quite large in many other oxygen-con-taining functional groups.

Very recently, Nava et al. [61] used solid-state 17O NMR toprobe the carbonate anion ( −CO2

2) dynamics inside a hex-acarboxamide cryptand cage. As seen from Fig. 8, each of the threeoxygen atoms of the carbonate anion is hydrogen bonded to twocarboxamide H–N groups with O � � �N distances ranging from2.611 to 2.977 Å, typical of intermediate strength hydrogen bonds.Analysis of the VT solid-state 17O NMR spectra shown in Fig. 8reveals that the carbonate anion undergoes three-fold jumps withan Ea of 22 kJ mol�1. It is interesting to note that this activationenergy is smaller than those found for the three-fold jumps for the– −SO3 group in crystalline sulfonic acids discussed earlier. Thisdiscrepancy can be attributed to the fact that sulfonic acids exhibitstronger ionic O� � � �H–Nþ hydrogen bonds.

Future directions. There are many important oxygen-containingfunctional groups for which the dynamic motion involves onlypositional interchange of oxygen atoms. To probe the motion ofthese functional groups, NMR studies of heteroatoms other than17O will not be very useful. These functional groups may includenitro (–NO2), carboxylate (–COO�), nitrite (NO2

�), and nitrate(NO3

�). Very often, these groups are directly involved in intra- and

inter-molecular hydrogen bonding interactions in the crystal lat-tice. Thus, solid-state 17O NMR can be used to obtain informationabout the energetics of local motion, which then can be used toinfer the strength of the hydrogen bonds involved. For example,this kind of information may be of particular importance in thedesign and synthesis of selective receptors for the anions listedabove. It may also be possible to use solid-state 17O NMR to probesegment motion in polymer materials. Another important direc-tion for future research is to combine solid-state 17O NMR studieswith plane-wave DFT molecular dynamics (MD) simulation. Ingeneral, as the molecular motion detected by solid-state NMR isrelatively slow (microseconds to seconds), the free energy barrierscan be considerably larger than the typical kinetic energy of themolecular system in question. Under these circumstances, regularbrute-force MD simulations may not achieve satisfactory samplingin the configuration space. Perhaps it is possible to incorporatesome modern free energy computation techniques well developedfor studying molecular systems into plane-wave DFT calculations.From a fundamental NMR theory point of view, because the 17Oquadrupolar coupling constants are on the order of 5–10 MHz formost organic functional groups, the entire NMR spectrum in-cluding both ST and CT should be sensitive to both fast and slowmotion. However, a unified treatment of molecular motion overthe entire 17O NMR timescale is to be developed in the future.

2.5. Dynamic nuclear polarization

It is well known that the Achilles heel of NMR spectroscopy isits intrinsically low sensitivity. This is because the nuclear Zeemanenergy levels have nearly the same populations under mostpractical conditions. Thus increasing the sensitivity in NMR ex-periments has always been a challenge since the very beginning ofthe field. Any new technique that can improve the sensitivitywould extend the range of NMR applications, thus opening up newfrontiers of research. In recent years, implementation of the dy-namic nuclear polarization (DNP) methodology at high magneticfields has gained tremendous momentum as a result of the pio-neering work by Griffin et al. [62–64]. However, most DNP-en-hanced solid-state NMR experiments are focused on detection ofspin-1/2 nuclei such as 13C and 15N.

In 2012, Griffin et al. [65] demonstrated for the first time thatDNP can be used to enhance solid-state 17O NMR signals. Using abiradical polarizing agent, TOTAPOL, they were able to observe asignal enhancement of 80 for the 17O NMR signal from a staticsample of water/glycerol glass at 82 K; see Fig. 9. This factor of 80in signal enhancement is equivalent to 46000-fold experimentaltime saving. In this study, DNP-enhanced 17O NMR signals wereachieved in two steps. In the first step, 1H nuclei are polarized viamicrowave irradiation at an appropriate frequency on polarizingagents. If the cross-effect (CE) is the dominant mechanism forpolarization transfer between electron and nuclear spins, the dif-ference between the Larmor frequencies of the two interactingelectrons must be equal to the 1H Larmor frequency. Then theenhanced 1H spin polarization is transferred to 17O nuclei by theconventional CP. They also demonstrated DNP-enhanced 17OQCPMG, 1H-17O SEDOR, and 1H-17O HETCOR experiments. Later,Griffin and co-workers [66] also showed that 17O NMR signal en-hancement can also be achieved by direct transfer of the electronpolarization to 17O nuclei. They found that direct polarization re-quires quite different polarizing agents from those used in indirectpolarization studies. For the molecular systems studied, the trityl(OX063) radical seems to give the largest 17O NMR signal en-hancement, a factor of 4100. They reported the DNP 17O NMRresults for three simple organic molecules: water, urea, and phe-nol. In both of these studies, solid-state 17O NMR experimentswere performed for static samples. Blanc et al. [67] extended the

Fig. 10. Molecular structures (a, d, where hydrogen atoms are omitted for clarity), experimental and simulated static (b, e) and MAS (c, f) 17O NMR spectra (21.1 T) of V([17O2]acac)3 and K3V([17O4]oxalate)3⋅3H2O. Simulated sub-spectra for individual sites are also shown. The sample spinning frequency was 62.5 and 55.0 kHz in (c) and (f),respectively. Reproduced with permission from Ref. [35]. Copyright 2015 Wiley.

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–14 11

utility of DNP-enhanced 17O NMR in two important directions. Oneis to perform the DNP experiment under the MAS condition andthe other is to study inorganic solids at the 17O natural abundance.Recently, Perras et al. [52] further improved the indirect DNPmethod by using the PRESTO sequence rather than the conven-tional CP for polarization transfer between 1H and 17O nuclei. Theyshowed that PRESTO not only yields undistorted line shapes forthe 17O signals, but also gives an enhanced sensitivity by a factor of3 as compared with CP. By using this improved scheme, they re-ported the first natural abundance 17O DNP-surface enhancedexperiment on a mesoporous silica sample. While the latter twostudies focused primarily on inorganic materials, the same meth-odologies should be applicable to organic and biological solids.

Future directions. So far DNP-enhanced 17O NMR has been de-monstrated for only a few simple organic molecules under thestatic condition. In addition, these 17O DNP NMR studies have allbeen performed at relatively low magnetic fields, 5.0 and 9.4 T. Atthese magnetic fields, it is still difficult to apply DNP to more

challenging oxygen-containing functional groups (with large CQvalues). However, as the high-frequency DNP technology con-tinues to develop [68], it is anticipated that most oxygen-con-taining functional groups in organic molecules will be accessibleby DNP in the near future. Another related issue is the feasibility ofvery fast MAS at cryogenic temperatures. While DNP offers hugegains in sensitivity, it does not solve the resolution problem insolid-state 17O NMR. In more complex organic and biologicalmolecules, there will be several chemically different oxygen-con-taining functional groups. As a result, different 17O NMR signals,each being broadened by the second-order quadrupolar interac-tion, would severely overlap even at currently available ultrahighmagnetic fields. Fortunately, with the significant sensitivity en-hancement gained by DNP, it should be possible to perform moretime-consuming 2D MQMAS experiments to improve spectral re-solution in the indirect dimension. However, this remains to bedemonstrated in the future.

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–1412

2.6. Paramagnetic coordination compounds

So far all the advances of solid-state 17O NMR that we havediscussed deal with diamagnetic molecules. It is well known thatNMR signals from paramagnetic substances are much more diffi-cult to detect than those from diamagnetic compounds. This isbecause the hyperfine interactions between magnetic dipoles ofunpaired electrons and atomic nuclei are substantially strongerthan the typical nuclear spin interactions such as nuclear quad-rupolar, magnetic shielding, and dipolar interactions, etc. As aresult, the NMR signals from paramagnetic compounds are sig-nificantly shifted and broadened compared with those from dia-magnetic compounds. In recent years, there have been consider-able interests in solution [69] and solid-state [70] NMR studies oforganic and biological systems containing paramagnetic metalions. To date most NMR studies of paramagnetic compounds haverelied on detection of 1H and 13C nuclei, because hydrogen andcarbon atoms are generally remote from the paramagnetic metalcenters, thus experiencing relatively weak hyperfine interactions.As discussed in the previous sections, solid-state 17O NMR studiesare already more difficult than 1H and 13C NMR experiments be-cause of the presence of 17O nuclear quadrupolar interactions. Inparamagnetic coordination compounds, as oxygen atoms are oftendirectly bonded to the paramagnetic metal center, detection ofsolid-state 17O NMR signals is expected to be far more challengingthan that for the 1H and 13C NMR signals in the same molecularsystem.

In 2015, Kong et al. [35] showed for the first time that high-quality solid-state 17O NMR spectra can be obtained for oxygenatoms that are directly bonded to paramagnetic metal ions. Inparticular, they reported both static and MAS 17O NMR spectra forseveral paramagnetic coordination compounds containing V(III)(d2, S¼1), Cu(II) (d9, S¼1/2), and Mn(III) (d4, S¼2) ions. This workconsiderably widens the landscape for solid-state 17O NMR ap-plications for organic and biological molecules. Now it is possibleto study a variety of paramagnetic substances that have previouslybeen considered unsuitable for solid-state 17O NMR studies. As anexample, Fig. 10 shows the 17O static and MAS NMR spectra ob-tained at 21.1 T for two paramagnetic V(III) coordination com-plexes. For V([17O2]acac)3, the 17O NMR signals in the static spec-trum are centered at around �1300 ppm covering a spectral rangeof over 1800 ppm. Remarkably, in the 17O MAS NMR spectrum of V([17O2]acac)3, all six crystallographically non-equivalent oxygensites are resolved with their paramagnetic shifts differing by morethan 500 ppm. In contrast, the isotropic 17O chemical shifts for thesix oxygen atoms in a diamagnetic analog, Al([17O2]acac)3, differ byonly 5 ppm [12]. The most positive paramagnetic shift reported inthis study was that from Mn(III)([17O2]acac)3, ca. 7500 ppm, whichis due to the high S value in this complex. K3V([17O4]oxalate)3 �3H2O is an interesting case, because it has two types ofoxygen atoms: direct chelating (O1, O2, O3) and non-chelating(O4, O5, O6) oxygen atoms. As seen from Fig. 10, the 17O NMRsignals from O1-O3 are found around �1200 ppm whereas thosefrom O4-O6 appear at 350 ppm. In diamagnetic metal oxalates, the17O NMR signals from chelating and non-chelating oxygen atomstypically appear at 220 and 280 ppm, respectively, thus differingby only ca. 60 ppm. It is also interesting to note that the smallparamagnetic shifts observed for non-chelating oxygen atoms forthe oxalate ligands reflect the small hyperfine coupling constants(HFCCs) at those “remote” oxygen sites. In general, one can definea paramagnetic shift tensor containing both the orbital (from allpaired electrons) and hyperfine (from unpaired electrons) con-tributions:

δ δ δ= + ( )1ii iiorb

iihf

where

δμ

γ= (

ℏ)

( + )

( )A g S S

kT

1

3 2iihf ii B

N

In Eq. (2), Aii is the principal component of the hyperfine cou-pling tensor, γN is the nuclear magnetogyric ratio, g is the freeelectron g-value, μB is the Bohr magneton, k is the Boltzmannconstant, and T is the absolute temperature. For example, the di-rect chelating oxygen atoms in K3V([17O4]oxalate)3 �3H2O exhibitan isotropic paramagnetic shift of �1200 ppm at 353 K, whichcorresponds to an isotropic hyperfine shift of –1200 – (220)¼–

1420 ppm. This translates to Aiso/hE3.3 MHz for the V(III)–Obond. On the other hand, the non-chelating oxygen atoms ex-perience a very small hyperfine shift of ca. 70 ppm, correspondingto Aiso/hE0.2 MHz. These very weak hyperfine interactions arevery difficult if not impossible to measure with the conventionaltechniques such as electron nuclear double resonance (ENDOR)and electron spin echo envelop modulation (ESEEM) in electronparamagnetic resonance (EPR) spectroscopy [71].

In the same study, Kong et al. [35] showed that the oxygenatoms directly bonded to a Cu(II) center are generally undetectableby solid-state 17O NMR, simply because the electron spin-latticerelaxation times for Cu(II) ions are relatively long (typically10�9 s), so that the fast-exchange condition of 2πAτe « 1 (where τeis the electron relaxation time and A is the hyperfine couplingconstant) is not satisfied for Cu(II) complexes. However, they alsomade an important discovery that the direct chelating oxygenatoms in an antiferromagnetically coupled di-Cu(II) MOF (HKUST-1) can actually be detected by solid-state 17O NMR. They hy-pothesized that the antiferromagnetic coupling between the twoCu(II) ions significantly shortens the electron relaxation times inHKUST-1. In this study, Kong et al. [35] also demonstrated thatquantum chemical calculations can qualitatively reproduce theobserved 17O paramagnetic shifts over a range of more than10000 ppm. In general, the information obtained from solid-state17O NMR for paramagnetic coordination compounds can be com-plementary to EPR studies.

Future directions. In the initial solid-state 17O NMR study ofparamagnetic coordination compounds, systems containing V(III),Cu(II), and Mn(III) were examined. One future direction is tosurvey 17O hyperfine shifts in coordination complexes containingother paramagnetic metal ions. Several metal ions are of particularimportance in biological systems such as Fe(II, III), Co(II), Mn(II),Mo(IV, V, VI), and Ni(II, III). Another area of importance is to ex-plore whether 2D 17O MQMAS spectra can be obtained for para-magnetic solids. This will further push the limit in spectral re-solution. One potential problem may arise from the fact that veryfast sample spinning (e.g., 50–70 kHz) is often required to removethe significant anisotropic hyperfine interaction. It is well knownthat the frictional heating from such rapid sample spinning willgenerate a temperature gradient across the sample. While such atemperature gradient may not cause any problem for diamagneticcompounds, it could give rise to additional line broadenings inparamagnetic solids because the hyperfine shift is very sensitive tosample temperature. As already mentioned, Kong et al. [35]showed that antiferromagnetic coupling may drastically changethe NMR detectability of oxygen-containing ligands bound to theparamagnetic metal ions. It is an important future direction tofurther investigate such systems. Another reason is that anti-ferromagnetically coupled multinuclear metal complexes areprevalent in biological systems. Finally, it is necessary to extendsolid-state 17O NMR to studies of paramagnetic proteins. On thebasis of reported solid-state 17O NMR data for proteins [2], thisseems to be entirely feasible as far as the NMR sensitivity is con-cerned. In a few cases, because the 17O hyperfine coupling tensors

G. Wu / Solid State Nuclear Magnetic Resonance 73 (2016) 1–14 13

are known from ENDOR or more recently EDNMR measurementsfor metalloproteins [71–73], they can be used to estimate the 17Ohyperfine shifts expected in solid-state 17O NMR spectra. The mostimportant advantage of solid-state 17O NMR lies in its potential toprovide complementary information about the ligand-metal in-teractions, as compared with traditional EPR studies, because thetwo techniques have different requirements for electron spin re-laxation. In some cases when EPR cannot be easily applied, solid-state NMR may turn out to be preferable. Solid-state NMR shouldbe especially useful for detecting ligand atoms experiencing rela-tively small hyperfine couplings. This could include ligand atomsthat are two or three bonds away from the paramagnetic metalcenter.

3. Concluding remarks

In this Trends article, we have presented a brief overview of themost recent advances in the field of solid-state 17O NMR for or-ganic and biological molecules. It is clear that the field has ex-panded considerably in the last four years. One common feature ofthe new reports discussed in this article is that, nearly in all cases,they represent new frontiers of solid-state 17O NMR applications.These initial studies will undoubtedly inspire more investigations.While we have made an effort to predict future directions, it is theintrinsic nature of scientific research that we cannot know withcertainty what new techniques will come along and what newdiscoveries will be made in the future. However, there are strongindications suggesting that, after a rather long period of steadygrowth, the field will experience a rapid expansion. The significantadvances in methodology and technology in several areas de-scribed in this article have laid a solid foundation for a rapid burstof future solid-state 17O NMR applications. As some major ob-stacles hampering solid-state 17O NMR studies of organic andbiological molecules are quickly disappearing, it is anticipated thatthe field will flourish in the very near future.

Acknowledgments

Our research in solid-state 17O NMR of organic and biologicalmolecules has been supported by the Natural Sciences and En-gineering Research Council (NSERC) of Canada (Grant no. 203308-11). Many former and current students and postdoctoral fellowshave contributed to the results described in this article. In parti-cular, I wish to thank Shuan Dong, Amanda Geris, Xianqi Kong,Irene Kwan, Justin Lau, Jiasheng Lu, Xin Mo, Mellisa Shan, AaronTang, Abouzar Toubaei, Alan Wong, and Jianfeng Zhu. I would alsolike to thank Drs. Victor Terskikh, Eric Ye, Zhehong Gan, Ivan Hung,Luke O'Dell, Yining Huang, Yong Zhang, Kit Cummins, Hiroshi Fujii,and Ruiyao Wang for collaborations.

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