17O Quadrupole Coupling Research Articles

Magnetically aligned nanodiscs enable direct measurement of 17O residual quadrupolar coupling for small molecules

The use of 17O in NMR spectroscopy for structural studies has been limited due to its low natural abundance, low gyromagnetic ratio, and quadrupolar relaxation. Previous solution 17O work has primarily focused on studies of liquids where the 17O quadrupolar coupling is averaged to zero by isotropic molecular tumbling, and therefore has ignored the structural information contained in this parameter. Here, we use magnetically aligned polymer nanodiscs as an alignment medium to measure residual quadrupolar couplings (RQCs) for 17O-labelled benzoic acid in the aqueous phase. We show that increasing the magnetic field strength improves spectral sensitivity and resolution and that each satellite peak of the expected pentet pattern resolves clearly at 18.8 T. We observed no significant dependence of the RQC magnitudes on the magnetic field strength. However, changing the orientation of the alignment medium alters the RQC by a consistent factor, suggesting that 17O RQCs measured in this way can provide reliable orientational information for elucidations of molecular structures.

Fast and Accurate Electric Field Gradient Calculations in Molecular Solids With Density Functional Theory.

Modern approaches for calculating electric field gradient (EFF) tensors in molecular solids rely upon plane-wave calculations employing periodic boundary conditions (PBC). In practice, models employing PBCs are limited to generalized gradient approximation (GGA) density functionals. Hybrid density functionals applied in the context of gauge-including atomic orbital (GIAO) calculations have been shown to substantially improve the accuracy of predicted NMR parameters. Here we propose an efficient method that effectively combines the benefits of both periodic calculations and single-molecule techniques for predicting electric field gradient tensors in molecular solids. Periodic calculations using plane-wave basis sets were used to model the crystalline environment. We then introduce a molecular correction to the periodic result obtained from a single-molecule calculation performed with a hybrid density functional. Single-molecule calculations performed using hybrid density functionals were found to significantly improve the agreement of predicted 17O quadrupolar coupling constants (C q ) with experiment. We demonstrate a 31% reduction in the RMS error for the predicted 17O C q values relative to standard plane-wave methods using a carefully constructed test set comprised of 22 oxygen-containing molecular crystals. We show comparable improvements in accuracy using five different hybrid density functionals and find predicted C q values to be relatively insensitive to the choice of basis set used in the single molecule calculation. Finally, the utility of high-accuracy 17O C q predictions is demonstrated by examining the disordered 4-Nitrobenzaldehyde crystal structure.

A combined solid-state 17O NMR, crystallographic, and computational study of oxiranes

We report the synthesis and solid-state 17O NMR characterization of three 17O-labeled oxiranes: (2S*,3S*)-2,3-bis(4-nitrophenyl)-[17O]oxirane, (2S*,3R*)-2,3-bis(4-nitrophenyl)-[17O]oxirane, and 2,2,3-triphenyl-[17O]oxirane. In addition, we have determined the crystal structure of (2S*,3R*)-2,3-bis(4-nitrophenyl)oxirane by X-ray crystallography. When the experimentally determined 17O NMR tensors for oxiranes (where the C–O–C bond angle is about 60°) are compared with those for dimethyl ether (where the C–O–C bond angle is 113°) and other R–O–R′ functional groups, we found that the highly constrained geometry of oxiranes results in distinct tensor orientations in the molecular frame of reference. The experimental results are complemented by quantum chemical computations. This study represents the first time that 17O chemical shift and quadrupole coupling tensors are simultaneously determined for oxirane compounds.

Single-Crystal NMR Characterization of Halogen Bonds.

Oxygen-17-enriched triphenylphosphine oxide and three of its halogen-bonded cocrystals featuring 1,4-diiodotetrafluorobenzene and 1,3,5-trifluoro-2,4,6-triiodobenzene as halogen bond donors have been characterized by 31P and 17O single-crystal NMR spectroscopy. Single-crystal NMR allows for the measurement of not only the magnitudes of various NMR interaction tensors, but also their orientations relative to the crystal lattice and therefore relative to the halogen bonds themselves. 31P chemical shift tensors, 17O chemical shift tensors, 17O quadrupolar coupling tensors, and 31P-17O indirect nuclear spin-spin (J) coupling tensors are reported here for P═O···I halogen bonds. The angular deviations in the directions of the pseudo-unique components of the 31P chemical shift tensors, the 17O chemical shift tensors, and the 17O quadrupolar coupling tensors from the direction of the oxygen-iodine halogen bond correlate with the deviations in linearity of the P═O···I halogen bond. There is also a clear decrease in anisotropy and an increase in asymmetry of the J(31P,17O) coupling tensors attributable to the formation of iodine-oxygen halogen bonds. The small but quantifiable changes in the tensors are consistent with the weak nature of these halogen bonds relative to the P═O motif. Overall, this work establishes single-crystal NMR as a novel probe of halogen bonds in solids. Analysis of the results has provided insights into the correlations between the magnitude and orientation of various NMR interaction tensors and the local geometry of the halogen bond. Gauge-including projector-augmented wave computations corroborate the experimental findings.

Oxygen transfer in electrophilic epoxidation probed by 17O NMR: differentiating between oxidants and role of spectator metal oxo.

Peroxide compounds are used both in laboratory and industrial processes for the electrophilic epoxidation of olefins. Using NMR-spectroscopy, we investigate why certain peroxides engage in this type of reaction while others require activation by metal catalysts, e.g. methyltrioxorhenium (MTO). More precisely, an analysis of 17O NMR chemical shift and quadrupolar coupling parameters provides insights into the relative energy of specific frontier molecular orbitals relevant for reactivity. For organic peroxides or H2O2 a large deshielding is indicative of an energetically high-lying lone-pair on oxygen in combination with a low-lying σ*(O-O) orbital. This feature is particularly pronounced in species that engage in electrophilic epoxidation, such as peracids or dimethyldioxirane (DMDO), and much less pronounced in unreactive peroxides such as H2O2 and ROOH, which can however be activated by transition-metal catalysts. In fact, for the proposed active peroxo species in MTO-catalyzed electrophilic epoxidation with H2O2 an analysis of the 17O NMR chemical shift highlights specific π- and δ-type orbital interactions between the so-called metal spectator oxo and the peroxo moieties that raise the energy of the high-lying lone-pair on oxygen, thus increasing the reactivity of the peroxo species.

Study of Defect Chemistry in the System La2–xSrxNiO4+δ by 17O Solid-State NMR Spectroscopy and Ni K-Edge XANES

The properties of mixed ionic–electronic conductors (MIECs) are most conveniently controlled through site-specific aliovalent substitution, yet few techniques can report directly on the local structure and defect chemistry underpinning changes in ionic and electronic conductivity. In this work, we perform high-resolution 17O (I = 5/2) solid-state NMR spectroscopy of La2–xSrxNiO4+δ, an MIEC and prospective solid oxide fuel cell (SOFC) cathode material, showing the sensitivity of 17O hyperfine (Fermi contact) shifts and quadrupolar coupling constants due to local structural changes arising from Sr substitution (x). Previously, we resolved resonances from three distinct oxygen sites (interstitial, axial, and equatorial) in the unsubstituted x = 0 material (Halat et al., J. Am. Chem. Soc. 2016, 138, 11958). Here, substitution-induced changes in these three spectral features indirectly report on the ionic conductivity, local octahedral tilting, and electronic conductivity, respectively, of the (substituted) ma...

Solid-State 15N and 17O NMR Studies of S-Nitrosothiols.

We report a solid-state 15N and 17O NMR study of two representative S-nitrosothiols (RSNO): S-nitroso N-acetylpenicillamine (SNAP) and S-nitrosoglutathione (GSNO). The 15N and 17O NMR tensors are experimentally determined for the first time for this important class of nitric-oxide (NO)-related compounds. The observed NMR characteristics for RSNO include large 15N and 17O chemical shift anisotropies and large 17O quadrupole coupling constants. Quantum chemical calculations are also performed for the 15N and 17O NMR tensors in two simple RSNO models: t-BuSNO and MeSNO. On the basis of computational results, we have identified the molecular orbitals that are responsible for the observed large chemical shift anisotropies in RSNO compounds.

Probing nitrite ion dynamics in NaNO2 crystals by solid‐state 17O NMR

AbstractWe report a solid‐state 17O (I = 5/2) NMR study of the nitrite ion dynamics in crystalline NaNO2. Variable temperature (VT) 17O NMR spectra were recorded at 3 magnetic fields, 11.7, 14.1, and 21.1 T. The VT 17O NMR data suggest that the ion in the ferroelectric phase of NaNO2 undergoes 2‐fold flip motion about the crystallographic b axis and the corresponding rotational barrier is 68 ± 5 kJ mol−1. We also obtained a 2D 17O EXSY spectrum for a stationary sample of NaNO2 at 250 K, which, in combination with 1D 17O NMR spectral analyses, allowed precise determination of the relative orientation between the 17O quadrupolar coupling and chemical shift tensors in the molecular frame of reference. The experimentally determined 17O NMR tensors for NaNO2 were in agreement with quantum chemical calculations produced by a periodic DFT code BAND.

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.

(17)O NMR Investigation of Water Structure and Dynamics.

The structure and dynamics of the bound water in barium chlorate monohydrate were studied with (17)O nuclear magnetic resonance (NMR) spectroscopy in samples that are stationary and spinning at the magic-angle in magnetic fields ranging from 14.1 to 21.1 T. (17)O NMR parameters of the water were determined, and the effects of torsional oscillations of the water molecule on the (17)O quadrupolar coupling constant (CQ) were delineated with variable temperature MAS NMR. With decreasing temperature and reduction of the librational motion, we observe an increase in the experimentally measured CQ explaining the discrepancy between experiments and predictions from density functional theory. In addition, at low temperatures and in the absence of (1)H decoupling, we observe a well-resolved (1)H-(17)O dipole splitting in the spectra, which provides information on the structure of the H2O molecule. The splitting arises because of the homogeneous nature of the coupling between the two (1)H-(17)O dipoles and the (1)H-(1)H dipole.

A theoretical study on the characteristics of the intermolecular interactions in the active site of human androsterone sulphotransferase: DFT calculations of NQR and NMR parameters and QTAIM analysis

A theoretical study at the level of density functional theory (DFT) was performed to characterize noncovalent intermolecular interactions, especially hydrogen bond interactions, in the active site of enzyme human androsterone sulphotransferase (SULT2A1/ADT). Geometry optimization, interaction energy, 2H, 14N, and 17O electric field gradient (EFG) tensors, 1H, 13C, 17O, and 15N chemical shielding (CS) tensors, Natural Bonding Orbital (NBO) analysis, and quantum theory of atoms in molecules (QTAIM) analysis of this active site were investigated. It was found that androsterone (ADT) is able to form hydrogen bonds with residues Ser80, Ile82, and His99 of the active site. The interaction energy calculations and NBO analysis revealed that the ADT molecule forms the strongest hydrogen bond with Ser80. Results revealed that ADT interacts with the other residues through electrostatic and Van der Waals interactions. Results showed that these hydrogen bonds influence on the calculated 2H, 14N, and 17O quadrupole coupling constants (QCCs), as well as 1H, 13C, 17O, and 15N CS tensors. The magnitude of the QCC and CS changes at each nucleus depends directly on its amount of contribution to the hydrogen bond interaction.

Oxygen-17 NMR spectroscopy of water molecules in solid hydrates

17O solid-state NMR studies of waters of hydration in crystalline solids are presented. The 17O quadrupolar coupling and chemical shift (CS) tensors, and their relative orientations, are measured experimentally at room temperature for α-oxalic acid dihydrate, barium chlorate monohydrate, lithium sulfate monohydrate, potassium oxalate monohydrate, and sodium perchlorate monohydrate. The 17O quadrupolar coupling constants (CQ) range from 6.6 to 7.35 MHz and the isotropic chemical shifts range from –17 to 19.7 ppm. The oxygen CS tensor spans vary from 25 to 78 ppm. These represent the first complete CS and electric field gradient tensor measurements for water coordinated to metals in the solid state. Gauge-including projector-augmented wave density functional theory calculations overestimate the values of CQ, likely due to librational dynamics of the water molecules. Computed CS tensors only qualitatively match the experimental data. The lack of strong correlations between the experimental and computed data, and between these data and any single structural feature, is attributed to motion of the water molecules and to the relatively small overall range in the NMR parameters relative to their measurement precision. Nevertheless, the isotropic chemical shift, quadrupolar coupling constant, and CS tensor span clearly differentiate between the samples studied and establish a ‘fingerprint’ 17O spectral region for water coordinated to metals in solids.

Mechanochemical Production of Halogen-Bonded Solids Featuring P═O···I–C Motifs and Characterization via X-ray Diffraction, Solid-State Multinuclear Magnetic Resonance, and Density Functional Theory

A series of phosphine oxide-iodofluorobenzene cocrystals featuring relatively strongly halogen-bonded P═O···I–C motifs has been prepared mechanochemically and characterized by 31P and 17O solid-state nuclear magnetic resonance (SSNMR) spectroscopy and single-crystal X-ray diffraction. Direct insights were obtained into the correlation between several NMR observables (chemical shift tensors, electric field gradient tensors, and J(31P, 17O) coupling) and the molecular and electronic structure of the halogen bonds. The 31P isotropic chemical shift and span, the 17O quadrupolar coupling constant and asymmetry parameter, as well as J(31P, 17O) coupling can be strong indicators of the presence of a halogen bond. Density functional theory calculations using periodic boundary conditions have revealed several notable trends. For example, the oxygen isotropic chemical shift and J(31P, 17O) values were found to correlate linearly with the strength of the halogen bond. An additional natural localized molecular orbital (NLMO) investigation shows that the three main contributions to J(31P, 17O) coupling are the P═O bonding orbitals, the oxygen lone pair orbital, and oxygen core orbitals, and that they all together determine the overall trend observed experimentally. Further NLMO analysis revealed a linear correlation between the contribution of the oxygen pz lone pair orbital to the quadrupolar coupling constant and the strength of the halogen bond. This work, encompassing the first 17O SSNMR studies of halogen bonds, demonstrates how NMR observables provide a useful means to characterize halogen bonds and also improve our understanding of the correlation between the electronic structure of the halogen bond and NMR properties.

Effective orientation of water in 1,4-dioxane···water: the rotational spectrum of the H217O isotopologue

The measurements of the rotational spectrum of 1,4-dioxane-water complex, performed by pulsed jet Fourier transform microwave spectroscopy, have been extended to the 7–18.5 GHz frequency region and to two additional isotopologues, 1,4-dioxane···H217O and 1,4-dioxane···HOD. The effective orientation of water in the complex has been obtained from the 17O quadrupole coupling constants.

Average orientation of water in CH2F2⋯H2O from the 17O quadrupole effects in the rotational spectrum of CH2F2⋯H217O

The rotational spectrum of the CH2F2⋯H217O adduct has been measured in the 6.5–18.5GHz range using a pulsed jet Fourier-transform microwave spectrometer. The experimental values of the 17O quadrupole coupling constants have been determined to be χaa=−3.853(6), χbb=−3.259(1) and χcc=7.112(9) MHz, respectively. These data correspond to an average value of the angle between the water plane and the FCF plane of about 23°.

Hydrogen Bonds in Cocrystals and Salts of 2-Amino-4,6-dimethylpyrimidine and Carboxylic Acids Studied by Nuclear Quadrupole Resonance

(14)N and (17)O nuclear quadrupole resonance frequencies have been measured in 1:1 cocrystals and salts of 2-amino-4,6-dimethylpyrimidine and several carboxylic acids. A systematic decrease of the (17)O quadrupole coupling constant on increasing strength of the hydrogen bond is observed in cocrystals bound by O-H···N hydrogen bonds. The O-H distances deduced from the line widths of the (17)O NQR lines show that the hydrogen atom is in a hydrogen bond formed by a carboxylic groups for about 0.01 nm displaced from the oxygen atom toward the center of the hydrogen bond. In the O-H···N hydrogen bond formed by the hydroxyl group, which is only slightly longer than the hydrogen bonds formed by the carboxyl group, the hydrogen atom is much less displaced. A linear relation between the (14)N quadrupole coupling constant and the sum of the inverse third powers of the H···A (A = O or N) distances is deduced for the amino group. A linear correlation of the principal values of the (14)N quadrupole coupling tensor in -NH2, as observed in the solid phase and in the gas phase, is analyzed in a simple model assuming a displacement of the electron charge in the N-H σ bond and simultaneous deformation of the nitrogen lone pair electron orbital. At the ring nitrogen position, hydrogen bonding and proton transfer produce a large decrease of the (14)N quadrupole coupling constant. A linear correlation of the principal values of the (14)N quadrupole coupling tensor is observed in cocrystals and salts of 2-amino-4,6-dimethylpyrimidine. This correlation differs from the correlation observed in substituted pyrimidine, where the hydrogen atoms are replaced by other atoms or functional groups. The difference is analyzed in a model, which assumes that the hydrogen bonding and substituents affect the nitrogen lone pair and π electron orbitals. The analysis shows that the two effects are nearly independent.

Unusual Electron Charge Density in Carboxylic Acid. 17O Quadrupole Coupling in cis-Cyclobutane-1,2-dicarboxylic Acid

The (17)O NQR frequencies have been measured in cis-cyclobutane-1,2-dicarboxylic acid and the quadrupole coupling tensors have been determined at various temperatures. Two O···H oxygen positions and two O-H oxygen positions are observed, showing the presence of two different types of O-H···O hydrogen bonds in the unit cell. The quadrupole coupling constants at the O-H oxygen positions are approximately 30% lower than the lowest quadrupole coupling constants experimentally observed at the C-O-H positions in other carboxylic acids with either ordered or disordered hydrogen bonds. The O-H distances have been calculated from the (17)O-(1)H dipole-dipole interaction at the O-H oxygen positions. The obtained values are longer than the O-H distances usually found in O-H···O hydrogen bonds with comparable O···O distance, in agreement with the proposed proton exchange O-H···O ↔ O···H-O, which partially averages the dipole-dipole interaction. The energy difference of the two proton configurations, O-H···O and O···H-O, is calculated from the O-H distances determined by NQR. The temperature dependence of the (17)O quadrupole coupling tensors at the (17)O···H-O oxygen positions is analyzed in the model of proton exchange and the energy differences of the two proton configurations obtained by this analysis agree with the values obtained from the O-H distances. The quadrupole coupling tensors are analyzed in a model based on the Townes and Dailey model. The model shows that the population of an oxygen lone pair orbital is at this oxygen position reduced from 2 to approximately 1.3. The electron electric charge is most probably transferred to the oxygen σ and π electron orbitals. This may be associated with the structure of the cyclobutane ring, where the X-ray data show the presence of two unusually short C-C bonds.

Solid-state 17O NMR and computational studies of terminal transition metal oxo compounds

We report solid-state NMR characterization of the 17O (I = 5/2) chemical shift and quadrupole coupling tensors in two terminal oxo compounds, 17O≡Ti(IV)(TMP) and 17O≡Cr(IV)(TMP), in which TMP is 5,10,15,20-tetramesitylporphyrin and the oxo ligand is enriched by 17O (ca. 40%). This is the first time that 17O NMR tensors are determined for this important class of compounds. The 17O nuclei in the O≡Ti and O≡Cr triple bonds are found to exhibit very large chemical shift anisotropies but rather small 17O quadrupole coupling constants. Terminal oxo compounds represent one of the rare cases where the atomic nucleus under study simultaneously experiences a highly anisotropic magnetic shielding environment, but a highly symmetrical electric field distribution. The solid-state 17O NMR data are consistent with solid-state 13C, 15N and 31P NMR results on carbido, nitrido and phosphido compounds. The density functional theory (DFT) computations using the zeroth-order regular approximation (ZORA) to account for spin-orbital relativistic effects reproduce the experimental 17O NMR results reasonably well. DFT computations also reveal the origins at the molecular orbital level of the observed 17O NMR properties for terminal oxo compounds.

Ionothermal 17O enrichment of oxides using microlitre quantities of labelled water

We present an ionothermal-based method for the simple and low-cost enrichment in 17O of oxide materials. This is demonstrated for the case of SIZ-4, an ionothermally-prepared aluminophosphate framework with the CHA topology. A preliminary study of unenriched samples of SIZ-4 highlights the importance of the careful choice of template in order to obtain an ordered structure. We then show how an ionothermal synthesis procedure incorporating microlitre quantities of 17O-enriched H2O enables as-prepared and calcined samples of SIZ-4 to be obtained with 17O enrichment levels that are sufficient to enable the recording of high-quality 17O solid-state NMR spectra. While second-order quadrupolar-broadened resonances are unresolved in 17O MAS NMR spectra, 17O double-rotation (DOR) and multiple-quantum (MQ)MAS NMR spectra reveal distinct resonances that are partially assigned by comparison with NMR parameters derived using first-principles calculations. The calculations also enable an investigation of the dependence of 17O NMR parameters on the local structural environment. We find that both the 17O isotropic chemical shift and quadrupolar coupling constant show clear dependencies on Al–O–P bond lengths, and angles and will therefore provide a sensitive probe of structure and geometry in aluminophosphate frameworks in future studies.

Synthesis of 17O-Labeled Cs2WO4 and Its Ambient- and Low-Temperature Solid-State 17O MAS NMR Spectra

Following several seemingly straightforward but unsuccessful attempts to prepare a sample of (17)O-enriched Cs(2)WO(4), we here report a simple, aqueous procedure for synthesis of pure Cs(2)WO(4), if so desired, enriched in (17)O. The purpose for the preparation of (17)O-enriched Cs(2)WO(4) is to record its solid-state (17)O MAS NMR spectrum, which would allow for a determination of its quadrupole coupling and chemical shift anisotropy (CSA) parameters and thereby for a comparison with the corresponding (33)S and (77)Se parameters in the related compounds M(2)WS(4) and M(2)WSe(4). These compounds are isomorphous and crystallize in the orthorhombic space group Pnma, and Cs(2)WO(4) turns out to be the only alkali metal tungstate with the Pnma crystal structure. Therefore, it has been mandatory to use Cs(2)WO(4) and not K(2)WO(4) (space group C2/m) for which CSA data have previously been published, to achieve a reliable comparison with the (33)S and (77)Se data and thus allow assignment of the three different sets of (17)O NMR parameters to the three distinct oxygen sites (O(1,1), O(2), and O(3)) in the Pnma crystal structure of Cs(2)WO(4). Because the ambient temperature (17)O MAS NMR spectrum of Cs(2)WO(4) exhibits a dynamically broadened singlet, resorting to low-temperature (-83 °C) conditions at 21.15 T was necessary and resulted in a high-resolution (17)O MAS spectrum that allowed both (17)O quadrupole coupling and CSA parameters to be determined. As no quadrupole coupling data were obtained from the earlier investigation on K(2)WO(4), the present results for Cs(2)WO(4) prompted a reinvestigation of the (17)O MAS spectrum for K(2)WO(4), which actually also shows the presence of (17)O quadrupole couplings for all three oxygen sites. These data for Cs(2)WO(4) and K(2)WO(4) are consistent and result in unambiguous assignments of the parameters to the three distinct oxygen sites in their crystal structures.