Chemical Shift Tensors Research Articles

The influence of sulfur configuration in 1 H NMR chemical shifts of diasteromeric five-membered cyclic sulfites.

The effect of the stereochemistry of the sulfur atom on 1 H chemical shifts of the diasteromeric pair of cyclic sulfites of 4-[methoxy(4-nitrophenyl)methyl]-5-phenyl-1,3,2-dioxathiolan-2-oxide was investigated. The complete 1 H and 13 C NMR spectral assignment was achieved by the use of one-dimensional and two-dimensional NMR techniques in combination with X-ray data. A correlation of experimental data with theoretical calculations of chemical shift tensors using density functional theory and topological theory of atoms in molecules was made. Copyright © 2016 John Wiley & Sons, Ltd.

Local Electronic Structure in γ-LiAlO2 Studied by Single-Crystal 27Al NMR and DFT Calculations.

From single-crystal 27Al NMR experiments, the full tensors for both the electrical field gradient (EFG) and the chemical shift (CS) for the aluminum atoms in γ-LiAlO2 have been determined. A simultaneous fit of the quadrupolar splittings observed for the four 27Al in the unit cell gave the EFG tensor in the crystal frame, from which a quadrupolar coupling constant of χ = CQ = 3.330 ± 0.005 MHz and an asymmetry parameter of ηQ = 0.656 ± 0.002 were derived. The experimentally determined quadrupolar splittings were sufficiently sensitive to quantify small deviations of both rotation axis direction and starting direction by the data fitting routine. For determination of the CS tensor, the evolution of the outer satellite centers over the crystal rotation was tracked, and the contribution of the quadrupolar shift was subtracted according to the previously determined EFG tensor. The resulting CS tensor of 27Al yields an isotropic chemical shift of δiso = 81.8 ± 0.25 ppm and an asymmetry parameter of ηCS = 0.532 ± 0.004, in good agreement with the fit of a MAS NMR spectrum acquired at B0 = 21.1 T. From both experiments and DFT calculations using the Castep code, we find the eigenvectors of the EFG and CS tensors to be practically colinear.

Synthesis, Structure, and Local Molecular Dynamics for Crystalline Rotors Based on Hecogenin/Botogenin Steroidal Frameworks

The synthesis and solid-state characterization of a series of cyclic/acyclic molecular rotors derived from naturally occurring steroidal 12-oxosapogenins are described. The bridged molecular rotors with rigid steroidal frameworks were obtained by employing ring-closing metathesis (RCM) as a key step. The X-ray diffraction technique was employed for determination and refinement of the crystal and molecular structure of selected models giving good quality single crystals. In the case of the bridged hecogenin molecular rotor 11E for which poor quality crystals were obtained, an NMR crystallography approach was used for fine refinement of the structure. Solid state NMR spectroscopic techniques were applied for the study of local molecular dynamics of the featured acyclic/cyclic molecular rotors. Analysis of 13C principal components of chemical shift tensors and chemical shift anisotropy (CSA) as well as heteronuclear 1H–13C dipolar couplings (DC) unambiguously proved that aromatic rings located in the space within the rigid steroidal framework both for cyclic and acyclic rotors are under kHz exchange regime. Experimental results were confirmed by theoretical calculations of rotation barrier on the density functional theory level. Small distinctions in the values of CSA and DC for the rotors under investigation are explained on the basis of differences in their molecular structures.

Handling the influence of chemical shift in amplitude-modulated heteronuclear dipolar recoupling solid-state NMR.

We present a theoretical analysis of the influence of chemical shifts on amplitude-modulated heteronuclear dipolar recoupling experiments in solid-state NMR spectroscopy. The method is demonstrated using the Rotor Echo Short Pulse IRrAdiaTION mediated Cross-Polarization ((RESPIRATION)CP) experiment as an example. By going into the pulse sequence rf interaction frame and employing a quintuple-mode operator-based Floquet approach, we describe how chemical shift offset and anisotropic chemical shift affect the efficiency of heteronuclear polarization transfer. In this description, it becomes transparent that the main attribute leading to non-ideal performance is a fictitious field along the rf field axis, which is generated from second-order cross terms arising mainly between chemical shift tensors and themselves. This insight is useful for the development of improved recoupling experiments. We discuss the validity of this approach and present quaternion calculations to determine the effective resonance conditions in a combined rf field and chemical shift offset interaction frame transformation. Based on this, we derive a broad-banded version of the (RESPIRATION)CP experiment. The new sequence is experimentally verified using SNNFGAILSS amyloid fibrils where simultaneous (15)N → (13)CO and (15)N → (13)Cα coherence transfer is demonstrated on high-field NMR instrumentation, requiring great offset stability.

Determination of Relative Configuration from Residual Chemical Shift Anisotropy.

Determination of relative configuration is frequently a rate-limiting step in the characterization of small organic molecules. Solution NMR-based nuclear Overhauser effect and scalar J-coupling constants can provide useful spatial information but often fail when stereocenters are separated by more than 4-5 Å. Residual dipolar couplings (RDCs) can provide a means of assigning relative configuration without limits of distance between stereocenters. However, sensitivity limits their application. Chemical shift is the most readily measured NMR parameter, and partial molecular alignment can reveal the anisotropic component of the chemical shift tensor, manifested as residual chemical shift anisotropy (RCSA). Hence, (13)C RCSAs provide information on the relative orientations of specific structural moieties including nonprotonated carbons and can be used for stereochemical assignment. Herein, we present two robust and sensitive methods to accurately measure and apply (13)C RCSAs for stereochemical assignment. The complementary techniques are demonstrated with five molecules representing differing structural classes.

Structure Elucidation of an Yttrium Diethyldithiocarbamato‐Phenanthroline Complex by X‐ray Crystallography, Solid‐State NMR, and ab‐initio Quantum Chemical Calculations

We present a structural analysis method for molecular and electronic structure of yttrium diethyldithiocarbamato‐phenanthroline complex {[Y(S2CNR2)3PHEN] with R = C2H5 and PHEN = 1,10‐phenanthroline} combining solid‐state NMR spectroscopy, XRD, and first principles DFT calculations. Replacing the Nd3+ ion with Y3+ in the reported crystal structure of [Nd(S2CNR2)3PHEN] complex generated an approximate 3D structure of the title complex. The structure was then subjected to first principles quantum chemical geometry optimisation using periodic DFT method. The quality of the method is discussed by comparing predicted and experimental powder XRD patterns. Full assignment of 13C and 15N solid‐state CP‐MAS NMR spectra as well as analyses of the principal values of the chemical shift tensors were carried out using periodic scalar relativistic DFT modelling. Spin‐orbit relativistic effects, estimated by SO‐ZORA formalism for one molecular unit, were evaluated. Finally, the X‐ray structure of the title complex was determined, which proved that the former procedure is appropriate. The most important orbital interactions were investigated by Natural Bond Orbital analysis. The isotropic shielding values for S2CN‐carbons were analysed by Natural Localised Molecular Orbital analysis. The present approach can be further extended to study other rare earth metal complexes, particularly those having similar but not yet solved crystal structures.

NMR properties of hydrogen-bonded glycine cluster in gas phase

Density Functional Theory (DFT) calculations have been performed to study the effect of the hydrogen bond formation on the Nuclear Magnetic Resonance (NMR) parameters of hydrogen-bonded clusters of glycine molecules in gas-phase. DFT predicted isotropic chemical shifts of H, C, N and O of the isolated glycine with respect to standard reference materials are in reasonable agreement with available experimental data. The variations of isotropic and anisotropic chemical shifts for all atoms constituting these clusters containing up to four glycine molecules have been investigated systematically employing gradient corrected hybrid B3LYP functional with three different types of extended basis sets. The clusters are mainly stabilized by a network of strong hydrogen bonds among the carboxylic (COOH) groups of glycine monomers. The formation of hydrogen bond influences the molecular structure of the clusters significantly which, on the other hand, gets reflected in the variations of NMR properties. The carbon (C) atom of the COOH group, the bridging hydrogen (H) and the proton-donor oxygen (O) atom of the OH bond suffer downfield shift due to the formation of hydrogen bond. The hydrogen bond lengths and the structural complexity of the clusters are found to vary with the number of participating monomers. A direct correlation between the hydrogen bond length and isotropic chemical shift of the bridging hydrogen is observed in all cases. The individual variations of the principal axis elements in chemical shift tensor provide additional insight about the different nature of the monomers within the cluster.

Selenium Chain Length Distribution in GexSe100-x Glasses: Insights from (77)Se NMR Spectroscopy and Quantum Chemical Calculations.

The statistics of selenium chain length distribution in GexSe100-x glasses with 5 ≤ x ≤ 20 are investigated using a combination of high-resolution, two-dimensional (77)Se nuclear magnetic resonance (NMR) spectroscopy and quantum chemical calculations. This combined approach allows for the distinction of various selenium chain environments on the basis of subtle but systematic effects of next-nearest neighbors of Se atoms in -Se-Se-Se- linkages on the (77)Se chemical shift tensor parameters. Simulation of the experimental (77)Se NMR spectral line shapes indicates that Se chain speciation in these chalcogenide glasses follows the Flory-Schulz distribution, originally developed for organic chain polymers.

Effect of Hydrogen Bonding to Water on the31P Chemical Shift Tensor of Phenyl- and Trialkylphosphine Oxides and α-Amino Phosphonates

The effect of hydrogen bonding to water on the 31P NMR chemical shift tensor in the titled compounds has been studied experimentally and simulated theoretically using the DFT-GIAO approach. It has been shown that the effect of hydrogen bonding on the 31P NMR parameters of trialkylphosphine oxides can be simulated at the GIAO-B3LYP/cc-pVDZ approximation. The influence of molecular conformation on the NMR parameters is small in these compounds. In contrast, the effect of molecular conformation can totally mask the effect of hydrogen bonding in the case of phenylphosphine oxides. The limiting case is a hydrogen bond with water. In these compounds the DFT-GIAO approach can be useful for the analysis of stronger hydrogen bonds only. The GIAO-B3LYP/cc-pVDZ approximation fails to reproduce the 31P NMR parameters of α-amino phosphonates. Thus, a combination of 31P NMR and low-cost calculations can be used especially effectively in the study of noncovalent interactions in polycrystalline and noncrystalline materials based on trialkylphosphine oxides. The described research strategy is applicable to the study of the local chemical structures in amorphous materials, at surfaces, and in interfaces.

Nuclear magnetic resonance study of potassium dihydrophosphate

A powder sample of potassium dihydrophosphate KH2PO4 has been studied by the 31P NMR method in a wide temperature range covering the ferroelectric phase transition. Changes in the position and shape of the resonance line at the transition to the ferroelectric phase have been revealed. The parameters of the chemical shift tensor of 31P (isotropic shift, anisotropy, and asymmetry) in the ferroelectric phase have been calculated from the experimental data. A sharp increase in the anisotropy of the tensor at the phase transition has been demonstrated. Dielectric measurements have also been carried out to verify the transition temperature.

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.

Sterically Driven Olefin Metathesis: The Impact of Alkylidene Substitution on Catalyst Activity

The dramatic reactivity difference between the Grubbs metathesis catalysts and their resting-state methylidene derivatives was probed in an integrated crystallographic, solid-state NMR and localized molecular orbital analysis study. A principal focus was the second-generation Grubbs system RuCl2(H2IMes)(PCy3)(═CHR) (GII, R = Ph; GIIm, R = H); supporting studies were carried out with the first-generation species RuCl2(PCy3)2(═CHR) (GI, GIm). The compiled rate constants for PCy3 dissociation demonstrate the limited lability of the methylidene complexes (e.g., ca. 275-fold lower for GIIm than GII and nearly 2000 times lower for the IMes analogue GIIm′). This is important because it impedes catalyst re-entry from the resting state into the active cycle. The 31P chemical shift (CS) tensors for the PCy3 ligand exhibited the expected changes (i.e., those characteristic of an increased Ru–P orbital interaction) in GIIm relative to GII, as did GIm vs GI. Greater insight was offered by the 13C CS tensors. Whereas calculations on truncated models predict significant differences in 13C CS tensor values for GII compared with GIIm, the experimental values are equivalent, implying a compensating effect that weakens the Ru═C interaction in the benzylidene complex. Published X-ray crystallographic parameters for GII and GI reveal that one chloride ligand is displaced below the basal plane by steric interactions with the benzylidene phenyl group, an effect absent in GIIm and GIm. During PCy3 loss from the [Ru]═CHPh systems, established processes of alkylidene rotation transform Ph–Cl repulsion into Ph–PCy3 repulsion. Displacing the PCy3 ligand below the plane does not relieve this conflict, instead incurring steric interactions with the H2IMes ligand. Enhanced PCy3 lability in the benzylidene complexes, relative to their methylidene analogues, is hence proposed to originate in the steric pressure exerted by the Ph substituent.

Elucidating the Link between NMR Chemical Shifts and Electronic Structure in d(0) Olefin Metathesis Catalysts.

The nucleophilic carbon of d(0) Schrock alkylidene metathesis catalysts, [M] = CHR, display surprisingly low downfield chemical shift (δ(iso)) and large chemical shift anisotropy. State-of-the-art four-component relativistic calculations of the chemical shift tensors combined with a two-component analysis in terms of localized orbitals allow a molecular-level understanding of their orientations, the magnitude of their principal components (δ11 > δ22 > δ33) and associated δ(iso). This analysis reveals the dominating influence of the paramagnetic contribution yielding a highly deshielded alkylidene carbon. The largest paramagnetic contribution, which originates from the coupling of alkylidene σ(MC) and π*(MC) orbitals under the action of the magnetic field, is analogous to that resulting from coupling σ(CC) and π*(CC) in ethylene; thus, δ11 is in the MCH plane and is perpendicular to the MC internuclear direction. The higher value of carbon-13 δ(iso) in alkylidene complexes relative to ethylene is thus due to the smaller energy gap between σ(MC) and π*(MC) vs this between σ(CC) and π*(CC) in ethylene. This effect also explains why the highest value of δ(iso) is observed for Mo and the lowest for Ta, the values for W and Re being in between. In the presence of agostic interaction, the chemical shift tensor principal components orientation (δ22 or δ33 parallel or perpendicular to π(MX)) is influenced by the MCH angle because it determines the orientation of the alkylidene CHR fragment relative to the MC internuclear axis. The orbital analysis shows how the paramagnetic terms, understood with a localized bond model, determine the chemical shift tensor and thereby δ(iso).

Ultrafast Magic-Angle Spinning: Benefits for the Acquisition of Ultrawide-Line NMR Spectra of Heavy Spin-1/2 Nuclei.

The benefits of the ultrafast magic-angle spinning (MAS) approach for the acquisition of ultrawide-line NMR spectra-spectral simplification, increased mass sensitivity allowing the fast study of small amounts of material, efficient excitation, and application to multiple heavy nuclei-are demonstrated for tin(II) oxide (SnO) and the tin complex [(LB)Sn(II) Cl](+) [Sn(II) Cl3 ](-) [LB=2,6-diacetylpyridinebis(2,6-diisopropylanil)] containing two distinct tin environments. The ultrafast MAS experiments provide optimal conditions for the extraction of the chemical-shift anisotropy tensor parameters, anisotropy, and asymmetry for heavy spin-1/2 nuclei.

Ab Initio Quality NMR Parameters in Solid-State Materials Using a High-Dimensional Neural-Network Representation.

Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful experimental tools to probe the local atomic order of a wide range of solid-state compounds. However, due to the complexity of the related spectra, in particular for amorphous materials, their interpretation in terms of structural information is often challenging. These difficulties can be overcome by combining molecular dynamics simulations to generate realistic structural models with an ab initio evaluation of the corresponding chemical shift and quadrupolar coupling tensors. However, due to computational constraints, this approach is limited to relatively small system sizes which, for amorphous materials, prevents an adequate statistical sampling of the distribution of the local environments that is required to quantitatively describe the system. In this work, we present an approach to efficiently and accurately predict the NMR parameters of very large systems. This is achieved by using a high-dimensional neural-network representation of NMR parameters that are calculated using an ab initio formalism. To illustrate the potential of this approach, we applied this neural-network NMR (NN-NMR) method on the (17)O and (29)Si quadrupolar coupling and chemical shift parameters of various crystalline silica polymorphs and silica glasses. This approach is, in principal, general and has the potential to be applied to predict the NMR properties of various materials.

Synthesis, crystal structure, and advanced NMR characterization of a low temperature polymorph of SiSe2

A low temperature polymorphic modification of silicon diselenide was synthesized via solid-state reaction and its crystal structure was determined by single crystal X-ray diffraction. In contrast to the well-characterized high-temperature form of SiSe2, where edge-sharing SiSe4 tetrahedra form one-dimensional chains, in the crystal structure of the low-temperature polymorph two-dimensional puckered layers are formed via edge- and corner-sharing SiSe4 tetrahedra. Such rearrangements have a significant impact on the electronic structure of SiSe2 as revealed by quantum chemical calculations and optical spectroscopy. The experimentally determined indirect bandgap for the low-temperature modification of SiSe2 is 2.00(5) eV. Multinuclear (77Se, 29Si) NMR spectroscopy combined with first principles calculations is applied to characterize the NMR chemical shift tensor parameters for the Si and Se sites in the crystal structure. These results will enable quantitative application of combined 29Si and 77Se NMR spectroscopy in future structural studies of Si–Se glasses, whose structures consist of both corner- and edge-sharing SiSe4/2 tetrahedra.

(11)B Solid-State NMR Interaction Tensors of Linear Two-Coordinate Boron: The Dimesitylborinium Cation.

Borinium cations (R2B(+)) are of particular fundamental and applied interest in part due to their pronounced Lewis acidity which enables unique chemical transformations. Solid-state NMR spectroscopy of magic-angle spinning and stationary powdered samples of the dicoordinate boron cation in the recently reported dimesitylborinium tetrakis(pentafluorophenyl)borate compound (Shoji et al. Nature Chem. 2014, 6, 498) is applied to characterize the (11)B electric field gradient (EFG) and chemical shift (CS) tensors. The experimental data are consistent with linear C-B(+)-C geometry. The (11)B quadrupolar coupling constant, 5.44 ± 0.08 MHz, and the span of the CS tensor, 130 ± 1 ppm, are both particularly large relative to literature data for a variety of boron functional groups, and represent the first such data for the linear C-B(+)-C borinium moiety. The NMR data are similar to those for the neutral tricoordinate analogue, trimesitylborane, but contrast with those of the Cp*2B(+) cation. Quantum chemical calculations are applied to provide additional insights into the relationship between the NMR observables and the molecular and electronic structure of the dimesitylborinium cation.

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.

Monitoring the refinement of crystal structures with (15)N solid-state NMR shift tensor data.

The (15)N chemical shift tensor is shown to be extremely sensitive to lattice structure and a powerful metric for monitoring density functional theory refinements of crystal structures. These refinements include lattice effects and are applied here to five crystal structures. All structures improve based on a better agreement between experimental and calculated (15)N tensors, with an average improvement of 47.0 ppm. Structural improvement is further indicated by a decrease in forces on the atoms by 2-3 orders of magnitude and a greater similarity in atom positions to neutron diffraction structures. These refinements change bond lengths by more than the diffraction errors including adjustments to X-Y and X-H bonds (X, Y = C, N, and O) of 0.028 ± 0.002 Å and 0.144 ± 0.036 Å, respectively. The acquisition of (15)N tensors at natural abundance is challenging and this limitation is overcome by improved (1)H decoupling in the FIREMAT method. This decoupling dramatically narrows linewidths, improves signal-to-noise by up to 317%, and significantly improves the accuracy of measured tensors. A total of 39 tensors are measured with shifts distributed over a range of more than 400 ppm. Overall, experimental (15)N tensors are at least 5 times more sensitive to crystal structure than (13)C tensors due to nitrogen's greater polarizability and larger range of chemical shifts.

Critical Analysis of Cluster Models and Exchange-Correlation Functionals for Calculating Magnetic Shielding in Molecular Solids.

Calculations of the principal components of magnetic-shielding tensors in crystalline solids require the inclusion of the effects of lattice structure on the local electronic environment to obtain significant agreement with experimental NMR measurements. We assess periodic (GIPAW) and GIAO/symmetry-adapted cluster (SAC) models for computing magnetic-shielding tensors by calculations on a test set containing 72 insulating molecular solids, with a total of 393 principal components of chemical-shift tensors from 13C, 15N, 19F, and 31P sites. When clusters are carefully designed to represent the local solid-state environment and when periodic calculations include sufficient variability, both methods predict magnetic-shielding tensors that agree well with experimental chemical-shift values, demonstrating the correspondence of the two computational techniques. At the basis-set limit, we find that the small differences in the computed values have no statistical significance for three of the four nuclides considered. Subsequently, we explore the effects of additional DFT methods available only with the GIAO/cluster approach, particularly the use of hybrid-GGA functionals, meta-GGA functionals, and hybrid meta-GGA functionals that demonstrate improved agreement in calculations on symmetry-adapted clusters. We demonstrate that meta-GGA functionals improve computed NMR parameters over those obtained by GGA functionals in all cases, and that hybrid functionals improve computed results over the respective pure DFT functional for all nuclides except 15N.