Isotropic Chemical Shift Values Research Articles

Low-Temperature Thermal Treatment and Boron Speciation Analysis from Coals

Despite urgent calls for decarbonization, the continued increasing demand for electricity, primarily from coals, has presented challenges in managing coal-derived wastes such as coal fly ash (CFA), which are enriched with environmentally hazardous substances like boron. This study explores a low-temperature heating process to remove boron from coal, aimed at preventing its condensation and enrichment into CFA during combustion. Initial boron concentrations in coals varied widely from 50 to 500 ppm by weight and were found to correlate with fixed carbon content (FC) through the following polynomial equation: [B]o = 0.0929(FC)2 − 14.388(FC) + 601.85; R2 = 0.9173. This relationship suggests that as coal undergoes coalification, boron-containing compounds are decomposed and released, resulting in a decline in boron levels as the coal matures. Boron-removal efficiency was investigated by drying coal samples at 110 °C, 160 °C, and 210 °C under natural air convection, and nuclear magnetic resonance (NMR) spectroscopy was used to assess changes in boron speciation during heating. Our results demonstrate that boron removal ranged from 5% to 82%, with minimal improvements observed beyond 110 °C. In addition, the 11B MAS-NMR spectra of the coal samples showed four peaks at isotropic chemical shift values of −1.0, 2.0, 8.0, and 14.0 ppm and suggested that the species of boron volatilized at low temperatures is the inorganic BO4 assigned to peak no. 0 at −1.0 ppm. The association of boron with inorganic components in coal suggests potential for efficient removal, particularly in coals with higher fixed carbon content. These findings highlight the viability of low-temperature thermal treatment as a cost-effective method for boron removal, which is crucial in mitigating the risks associated with coal combustion by-products.

The germanides ScTGe2 (T = Fe, Co, Ru, Rh) – crystal chemistry, 45Sc solid-state NMR and 57Fe Mössbauer spectroscopy

Abstract The TiMnSi2-type (space group Pbam) germanides ScTGe2 (T = Fe, Co, Ru, Rh) were synthesized from the elements by arc-melting. Single crystals were grown by annealing sequences of the arc-melted buttons in an induction furnace. The structures of ScFeGe2, ScRuGe2 and ScRhGe2 were refined from single-crystal X-ray diffraction data. In ScRuGe2, the ruthenium atoms have distorted octahedral germanium coordination (242–268 pm Ru–Ge). Three trans-face-sharing octahedra form a sub-unit which is condensed via common edges in c direction and connected via common corners with four adjacent blocks, forming a three-dimensional [RuGe2 type] substructure. The two crystallographically independent scandium sites have coordination numbers 15 (Sc1@Ge8Ru4Sc3) and 17 (Sc2@Ge7Ru6Sc4). Electronic band structure calculations for ScCoGe2 and ScRuGe2 show a net charge transfer from the scandium to the transition metal and germanium atoms, leading to a description with polyanionic networks Sc δ+[TGe2]δ−. The two crystallographically independent Sc sites are easily distinguishable by 45Sc magic-angle spinning (MAS)-NMR spectroscopy. Isotropic chemical shift values and nuclear electric quadrupolar interaction parameters were deduced from an analysis of the triple-quantum (TQ)-MAS NMR spectra. The electric field gradient parameters deduced from these experiments are in good agreement with quantum-chemical calculations using the Wien2k code. Likewise, the two crystallographically independent iron sites in ScFeGe2 could be discriminated in the 57Fe Mößbauer spectra through their isomer shifts and quadrupole splitting parameters: δ = 0.369(1) mm s−1 and ∆E Q = 0.232(2) mm s−1 for Fe1 and δ = 0.375(2) mm s−1 and ∆E Q = 0.435(4) mm s−1 for Fe2 (data at T = 78 K).

Structural Modeling of O/F Correlated Disorder in TaOF3 and NbOF3-x(OH)x by Coupling Solid-State NMR and DFT Calculations.

The structure of MOF3 (M = Nb, Ta) compounds was precisely modeled by combining powder X-ray diffraction, solid-state NMR spectroscopy, and semiempirical dispersion-corrected DFT calculations. It consists of stacked ∞(MOF3) layers along the c⃗ direction formed by heteroleptic corner-connected MX6 (X = O, F) octahedra. 19F NMR resonance assignments and occupancy rates of the anionic crystallographic sites have been revised. The bridging site is shared equally by the anions, and the terminal site is occupied by F only. An O/F correlated disorder is expected since cis-MO2F4 octahedra are favored, resulting in one-dimensional -F-M-O-M- strings along the <100> and <010> directions. Ten different 2 × 2 × 1 supercells per compound, fulfilling these characteristics, were built. Using DFT calculations and the GIPAW approach, the supercells were relaxed and the 19F isotropic chemical shift values were determined. The agreement between the experimental and calculated 19F spectra is excellent for TaOF3. The 1H and 19F experimental NMR spectra revealed that some of the bridging F atoms are substituted by OH groups, especially in NbOF3. New supercells involving OH groups were generated. Remarkably, the best agreement is obtained for the supercells with the composition closest to that estimated from the 19F NMR spectra, i.e., NbOF2.85(OH)0.15.

Spectral, quantum chemical, biological activity, and molecular docking analysis of 1,8-dihydroxyanthraquinone: A target drug exploration

In this current effort, the 1H and 13C NMR spectra as well as quantum chemical computational effort of 1,8 DHAQ molecule analyzed based on density functional theory (DFT) with the help of Gauss-View 6.0.6 and Gaussian 09W program package. The molecular structure of 1,8 DHAQ optimized first through DFT/B3LYP hybrid functional/6-311++G (d,p) basis-set. The NMR (1H and 13C) isotropic chemical shift values of the titled molecule (TMS as reference) was examined through gauge-invariant-atomic orbital (GIAO) process in DMSO‑d6 polar solvent using above similar basis set, then was correlated with the experimental outcomes. All the findings of DHAQ molecule correlated very well with each other. Reduced Density Gradients (RDG Scatter plot) give an evidence of various non-covalent intramolecular interactions inside molecule. ELF and LOL analysis proof a chemical association of molecules. It also reveals the main localization & delocalization of boding and non-bonding electrons within molecule. An in-silico ADMET profiles (absorption, distribution, metabolism, excretion, and toxicity), bioactivity score, and drug-likeness (based on the Lipinski rules of five) of 1,8 DHAQ molecule was executed via various computational online server tools. This study predicts that titled molecule have noble physicochemical and pharmacokinetic properties. Furthermore, the inhibitory nature of the DHAQ molecule has estimated by means of molecular docking analysis using various selected target proteins. The molecular docking is an effective computational analysis in estimating the binding affinity and binding sites of ligand together with the target proteins, which is much effective for future structure-based drug designing. Thus, the formation of numerous hydrogen bonds and docked binding affinities depicts that hydroxy-substituted Anthraquinone might serve as main structure for the development of upcoming drugs. As a result, this effort provide a summary of the chemistry, biology, and toxicology of 1,8 DHAQ molecule, focusing on innovative drugs discovery for treatment of various cancer diseases in pharmaceutical industries.

Synthesis, crystal structure and 19F NMR parameters modelling of CaTiF6(H2O)2 yielding to a revision of the bond-valence parameters for the Ti4+/F− ion pair

CaTiF6(H2O)2 was synthetized by a solvothermal method and was found to be isostructural to SrTiF6⋅2H2O. The structure, refined from Powder X-Ray Diffraction (PXRD) data, is built from the connectivity of dimers [Ca2F10(H2O)4] of square antiprisms (SAP) [CaF5(H2O)3] and TiF6 octahedra. To assign the six 19F NMR resonances to the six fluorine crystallographic sites of same multiplicity, 19F magnetic shielding tensors have been calculated using the GIPAW method. Unusually, the plot of the experimental 19F isotropic chemical shift (δiso) values, as a function of the calculated 19F isotropic shielding (σiso) values, shows a much better alignment for the experimental structure than for the DFT-optimized one. Whereas, the dynamics of the structural water molecules under experimental conditions are not captured by the optimized structure, the experimental averaged structure provides a good account of the experimental data and a reliable assignment of the 19F resonances to the F sites. Finally, the previously established bond-valence (BV) parameter Rij for the Ti4+/F− ion pair (1.76 ​Å) leads to overestimated BV sum values for Ti. This trend has been observed for almost all other fluorotitanates. We then undertook to refine its value, using the Ti–F bond lengths from 70 carefully selected structures containing 134 Ti sites forming TiF6 octahedra, and leading to Rij ​= ​1.706 ​Å.

Quantum chemical, spectroscopic and molecular docking investigations of potential pulmonary fibrosis drug methyl 2-chloro 4-iodonicotinate.

In this work, the methyl 2-chloro 4-iodonicotinate (MCIN) was investigated to study the structural, spectroscopic and electronic properties using density functional theory (DFT) quantum chemical calculations. The most stable structure of MCIN was optimized by DFT/B3LYP method with a LanLD2Z basis set. The optimized parameters and vibrational wavenumbers were determined. The vibrational task of the molecule was done by potential energy distribution calculations. The 13 C NMR spectrum of the MCIN molecule was simulated by the Gauge-Invariant-Atomic Orbital method using a dimethyl sulfoxide solution and the isotropic chemical shift values of the molecule were calculated and observed. Ultraviolet-visible spectra were simulated and observed. The pharmaceutical activity was predicted using frontier molecular orbital and natural bond orbital analysis. The reactive sites of the MCIN molecule were determined using Mulliken atomic charge distribution, molecular electrostatic potential surface and the local reactivity analysis. The molecular docking analysis confirms that the title molecule can be used in drug design for the treatment of pulmonary fibrosis.

Characterization of the Binary Nitrides VN and ScN by Solid‐State NMR Spectroscopy

AbstractNMR spectra of polycrystalline samples of the binary nitrides ScN and VN were acquired under magic‐angle spinning. The observed nuclides 45Sc, 51V and 14N are all quadrupolar nuclei with a spin . However, due to the high symmetry of their rock‐salt type structures, the spectra of the nitrides do not exhibit effects of quadrupolar or other anisotropic interactions of significant magnitude. This allows a relatively straightforward evaluation of the acquired spectra, leading to isotropic chemical shift values (δiso) of −213 ppm (51VN), 378 ppm (V14N), 290 ppm (45ScN) and 442 ppm (Sc14N) against commonly used reference standards. In the wider context of 14N‐NMR of binary nitrides, it is shown that the distance of nitrogen to the nearest neighbour cation can be correlated to the observed chemical shift.

Spectroscopic, Quantum chemical and Molecular docking Studies on Methyl 6-aminopyridine-3-carboxylate: A potent bioactive agent for the treatment of sarcoidosis

In the present study, the structural and spectroscopic investigations of Methyl 6-Aminopyridine-3-carboxylate (MAPC) molecule were performed using density functional theory (DFT) quantum chemical calculations. Molecular docking analysis was used to find out the inhibitory nature of the title molecule. The most stable molecular structure was predicted using conformational analysis at DFT/B3LYP level of cc-pVTZ basis set. The optimization was carried out using DFT/B3LYP method with cc-pVTZ basis to find out the most stable molecular structure of the molecule. The optimized molecular structure was used to determine the vibrational frequencies, which were assigned and compared with the experimentally observed vibrational frequencies. The Gauge-Invariant-atomic orbital (GIAO) method was used to calculate the 13C NMR isotropic chemical shift values of the molecule in DMSO solution and compared with experimental data. The ultraviolet-visible spectra of the molecule were used to find out the electronic properties of the molecule and are confirmed experimentally. The HOMO-LUMO energies, energy gap, chemical potential (μ) electronegativity (χ), hardness (η) and softness (S) values were calculated which results in the stability and molecular reactivity of the molecule. The second order perturbation energy E(2) values of the molecule were calculated using natural bond orbital analysis, which indicates the bioactivity of the molecule. Mullikan atomic charge distribution was calculated. Molecular Electrostatic potential (MEP) surface was computed and visualized by GaussView 05. The obtained lower binding energy and intermolecular energy values of MAPC ligand docked with HLA-DRB1 protein compared to the other selected proteins confirms that the title molecule can be useful for treatment of sarcoidosis disease. Hence, the present study paves the way for the development of novel drugs associated with sarcoidosis disease.

Assessment of the Performance of Optimally Tuned Range‐Separated Hybrid Functionals for Nuclear Magnetic Shielding Calculations

AbstractThe performance of optimally tuned range‐separated hybrid (OT‐RSH) functional calculations in predicting accurate isotropic nuclear magnetic shielding (σ) and chemical shift values is examined. To that end, the results of OT‐RSH and other approximate density functional theory calculations are assessed against recently published benchmark CCSD(T) calculations for a test set consisting of several molecules and bond types. It is found that for atoms in single bonds with a large paramagnetic contribution to σ, OT‐RSH offers a significant improvement in prediction of shielding constants over popular semi‐local and hybrid density functionals, yielding non‐empirical results that are as accurate as those of semi‐empirical density functionals often used for prediction of shielding constants. This success is attributed to the improved fundamental gap prediction of the OT‐RSH approach. For atoms in multiple bonds, however, larger errors often persist. By comparing OT‐RSH and recently reported double‐hybrid functional results, the remaining difficulties are traced to significant non‐local correlation.

Spectroscopic and molecular docking studies on 1-Hydroxyanthraquinone: A potent ovarian cancer drug

In the present study, the structural and spectroscopic investigations of1-Hydroxyanthraquinone molecule were performed using density functional theory quantum chemical calculations and validated by experimentally. In addition, the inhibitory nature of the title molecule was evaluated using molecular docking analysis. The potential energy surface scan was performed and the most stable molecular structure of the molecule was predicted. The molecular structure of the molecule was optimized and the harmonic vibrational wavenumbers of the molecule were calculated. The vibrational wavenumbers were also observed using FT-IR and FT-Raman spectra of the molecule. The calculated and observed vibrational wavenumbers were assigned. The 13C NMR isotropic chemical shift values of the molecule were calculated in DMSO solution and compared with the experimental data. The UV–Vis absorption spectrum of the molecule was computed in liquid phase (ethanol) and compared with the observed spectrum. Frontier molecular orbitals analysis reveals the molecular reactivity and kinetic stability of the molecule. The Mulliken atomic charge distribution, Fukui function and molecular electrostatic potential surface analysis of the molecule validate the reactive nature of the HAQ molecule. The natural bond orbital analysis indicates that the title molecule is bioactive in nature. The molecular docking analysis confirms that the title molecule acts as a good inhibitor of cyclooxygenase-1 protein than other selected proteins, which is overexpressed in some ovarian cancer cells. Hence, the present study paves the way for designing the novel drugs for the treatment of ovarian cancer.

3D Double-Quantum/Double-Quantum Exchange Spectroscopy of Protons under 100 kHz Magic Angle Spinning.

Solid-state 1H NMR spectroscopy has attracted much attention in the recent years due to the remarkable spectral resolution improvement by ultrafast magic-angle-spinning (MAS) as well as due to the sensitivity enhancement rendered by proton detection. Although these developments have enabled the investigation of a variety of challenging chemical and biological solids, the proton spectral resolution is still poor for many rigid solid systems owing to the presence of conformational heterogeneity and the unsuppressed residual proton-proton dipolar couplings even with the use of the highest currently feasible sample spinning speed of ∼130 kHz. Although a further increase in the spinning speed of the sample could be beneficial to some extent, there is a need for alternate approaches to enhance the spectral resolution. Herein, by fully utilizing the benefits of double-quantum (DQ) coherences, we propose a single radio frequency channel proton-based 3D pulse sequence that correlates double-quantum (DQ), DQ, and single-quantum (SQ) chemical shifts of protons. In addition to the two-spin homonuclear proximity information, the proposed 3D DQ/DQ/SQ experiment also enables the extraction of three-spin and four-spin proximities, which could be beneficial for revealing the dipolar coupled proton network in the solid state. Besides, the 2D DQ/DQ spectrum sliced at different isotropic SQ chemical shift values of the 3D DQ/DQ/SQ spectrum will also facilitate the identification of DQ correlation peaks and improve the spectral resolution, as it only provides the local homonuclear correlation information associated with the specific protons selected by the SQ chemical shift frequency. The 3D pulse sequence and its efficiency are demonstrated experimentally on small molecular compounds in the solid state. We expect that this approach would create avenues for further developments by suitably combining the benefits of partial deuteration of samples, selective excitation/decoupling pulses, heteronuclear spins for spectral editing, and nonuniform sampling.

Mechanochemical synthesis of MgF2 – MF2 composite systems (M = Ca, Sr, Ba)

The capability of mechanochemical synthesis for the formation of MgF2–MF2 (M: Ca, Sr, Ba) composites, solid solutions or well-defined compounds was tested applying a fluorination of different fluorine-free metal sources with NH4F directly at milling. No evidence was found for a substitution of Mg2+ with Ca2+ (Sr2+, Ba2+) ions, or vice versa, in rutile or fluorite structure. However, an equimolar ratio of Mg2+ to the second cation allows the mechanochemical synthesis of tetrafluoromagnesates, MMgF4, which is more and more hampered the smaller the radius of the cation M2+ is. BaMgF4 is formed even phase pure from the acetates, SrMgF4 can only be observed in a mixture accompanied by the binary fluorides. In addition, 19F MAS NMR spectra along with calculations of 19F isotropic chemical shift values according to the superposition model point to the formation of a metastable phase of CaMgF4, which disappears at thermal treatment and decomposes into the binary fluorides CaF2 and MgF2.

Structure and orientation of antibiotic peptide alamethicin in phospholipid bilayers as revealed by chemical shift oscillation analysis of solid state nuclear magnetic resonance and molecular dynamics simulation

The structure, topology and orientation of membrane-bound antibiotic alamethicin were studied using solid state nuclear magnetic resonance (NMR) spectroscopy. 13C chemical shift interaction was observed in [1-13C]-labeled alamethicin. The isotropic chemical shift values indicated that alamethicin forms a helical structure in the entire region. The chemical shift anisotropy of the carbonyl carbon of isotopically labeled alamethicin was also analyzed with the assumption that alamethicin molecules rotate rapidly about the bilayer normal of the phospholipid bilayers. It is considered that the adjacent peptide planes form an angle of 100° or 120° when it forms α-helix or 310-helix, respectively. These properties lead to an oscillation of the chemical shift anisotropy with respect to the phase angle of the peptide plane. Anisotropic data were acquired for the 4 and 7 sites of the N- and C-termini, respectively. The results indicated that the helical axes for the N- and C-termini were tilted 17° and 32° to the bilayer normal, respectively. The chemical shift oscillation curves indicate that the N- and C-termini form the α-helix and 310-helix, respectively. The C-terminal 310-helix of alamethicin in the bilayer was experimentally observed and the unique bending structure of alamethicin was further confirmed by measuring the internuclear distances of [1-13C] and [15N] doubly-labeled alamethicin. Molecular dynamics simulation of alamethicin embedded into dimyristoyl phophatidylcholine (DMPC) bilayers indicates that the helical axes for α-helical N- and 310-helical C-termini are tilted 12° and 32° to the bilayer normal, respectively, which is in good agreement with the solid state NMR results.

HP-CsB5O8 : synthesis and characterization of an outstanding borate exhibiting the simultaneous linkage of all structural units of borates.

The new cesium pentaborate HP-CsB5 O8 is synthesized under high-pressure/high-temperature conditions of 6 GPa and 900 °C in a Walker-type multianvil apparatus. The compound crystallizes in the orthorhombic space group Pnma (Z=4) with the parameters a=789.7(1), b=961.2(1), c=836.3(1) pm, V=0.6348(1) nm(3) , R1 =0.0359 and wR2 =0.0440 (all data). The new structure type of HP-CsB5 O8 exhibits the simultaneous linkage of trigonal BO3 groups, corner-sharing BO4 tetrahedra, and edge-sharing BO4 tetrahedra including the presence of threefold-coordinated oxygen atoms. With respect to the rich structural chemistry of borates, HP-CsB5 O8 is the second structure type possessing this outstanding combination of the main structural units of borates in one compound. The structure consists of corrugated chains of corner- and edge-sharing BO4 tetrahedra interconnected through BO3 groups forming octagonal channels. Inside these channels, cesium is 13+3-fold coordinated by oxygen atoms. (11) B MQMAS NMR spectra are analyzed to estimate the isotropic chemical shift values and quadrupolar parameters. IR and Raman spectra are obtained and compared to the calculated vibrational frequencies at the Γ-point. The high-temperature behavior is examined by means of temperature-programmed powder diffraction.

Powder Crystallography by Combining NMR and Crystal Structure Predictions

State-of-the-art work in the field of NMR crystallography for molecular systems at natural abundance has recently focused on the accurate measurement of 1H chemical shift values. We will show how when coupled with crystal structure prediction (CSP) methods, this protocol is well-suited for solving the crystal structures of small to medium sized organic molecules, including cocaine and the de-novo structure determination of AZD8329.[1,2] As complementary 1D and 2D NMR experiments are needed for the 1H assignment process, other information, such as isotropic 13C chemical shift values (δiso) are measured. Unfortunately, 13C chemical shifts are not generally useful for structure determination. Additional NMR parameters that are sensitive to structure would ensure that the structure determination procedure is robust, and would provide more accurate refinements when studying larger or more challenging systems. Here, we measure 13C chemical shift tensors for a variety of prototypical organic pharmaceuticals and use density functional theory computations under the gauge-including projector augmented-wave (GIPAW) formalism to probe whether these parameters may be discriminatory for unit cell determinations and structure determination (notably when added to the CSP + 1H chemical shifts protocol).

Identifying secondary structures in proteins using NMR chemical shift 3D correlation maps

NMR chemical shifts are accurate indicators of molecular environment and have been extensively used as aids in protein structure determination. This work focuses on creating empirical 3D correlation maps of backbone chemical shift nuclei for use as identifiers of secondary structure elements in proteins. A correlated database of backbone nuclei chemical shifts was constructed from experimental structural data gathered from entries in the Protein Data Bank (PDB) as well as isotropic chemical shift values from the RefDB database. Rigorous statistical analysis of the maps led to the conclusion that specific correlations between triplets of backbone chemical shifts are best able to differentiate between different secondary structures such as α-helices, β-strands and turns. The method is compared with similar techniques that use NMR chemical shift information as aids in biomolecular structure determination and performs well in tests done on experimental data determined for different types of proteins, including large multi-domain proteins and membrane proteins.

Revealing Structural Detail in the High Temperature La2Si2O7–Y2Si2O7 Phase Diagram by Synchrotron Powder Diffraction and Nuclear Magnetic Resonance Spectroscopy

High resolution synchrotron powder XRD, 89Y CPMG NMR, and 139La MAS NMR spectroscopy have been applied to eventually draw the phase diagram of the La2Si2O7–Y2Si2O7 system. The diagram presents a solid solubility region of G-(La,Y)2Si2O7, which extends to the La0.9Y1.1Si2O7 composition at any temperature of this study. Compositions richer in Y show two-phase domains, with G + α at T < 1450 °C and G + δ at T > 1450 °C. The Y-rich extreme is more complex, showing two solid solution regions of δ- and γ-(La,Y)2Si2O7 polymorphs which appear with increasing Y content, respectively. It is interesting to note that the La for Y substitution mechanism in the G-(La,Y)2Si2O7 polymorph is not homogeneous, but a preferential occupation of Y for the RE2 site is observed. Finally, the 89Y and 139La isotropic chemical shift values in G-(La,Y)2Si2O7 have been described here for the first time and assigned to the different RE crystallographic sites of the unit cell.

Binding of the three-repeat domain of tau to phospholipid membranes induces an aggregated-like state of the protein

In patients with Alzheimer's disease, the microtubule-associated protein tau is found aggregated into paired helical filaments (PHFs) in neurofibrillary deposits. In solution, tau is intrinsically unstructured. However, the tubulin binding domain consisting of three or four 31–32 amino acid repeat regions exhibits both helical and β-structure propensity and makes up the proteolysis resistant core of PHFs. Here, we studied the structure and dynamics of the three-repeat domain of tau (i.e. K19) when bound to membranes consisting of a phosphatidylcholine and phosphatidylserine mixture or phosphatidylserine alone. Tau K19 binds to phospholipid vesicles with submicromolar affinity as measured by fluorescence spectroscopy. The interaction is driven by electrostatic forces between the positively charged protein and the phospholipid head groups. The structure of the membrane-bound state of K19 was studied using CD spectroscopy and solid-state magic-angle spinning NMR spectroscopy. To this end, the protein was selectively 13C-labeled at all valine and leucine residues. Isotropic chemical shift values of tau K19 were consistent with a β-structure. In addition, motionally averaged 1H–13C dipolar couplings indicated a high rigidity of the protein backbone. The structure formation of K19 was also shown to depend on the charge density of the membrane. Phosphatidylserine membranes induced a gain in the α-helix structure along with an immersion of K19 into the phospholipid bilayer as indicated by a reduction of the lipid chain 2H NMR order parameter. Our results provide structural insights into the membrane-bound state of tau K19 and support a potential role of phospholipid membranes in mediating the physiological and pathological functions of tau.

Direct Investigation of Covalently Bound Chlorine in Organic Compounds by Solid‐State 35Cl NMR Spectroscopy and Exact Spectral Line‐Shape Simulations

35/37Cl NMR spectroscopy studies of organic systems are very rare, with only a few neat liquids having been studied.1 The lack of chlorine NMR spectroscopy data may be explained by the fact that 35Cl and 37Cl are quadrupolar (spin I=3/2) and low-frequency isotopes. The quadrupole moments of the chlorine nuclei couple with the electric field gradient (EFG) tensor at the nuclei; this phenomenon is known as the quadrupolar interaction (QI). The quadrupolar coupling constant, CQ, and the quadrupolar asymmetry parameter, ηQ, describe the magnitude and asymmetry of the QI. In solution, one of the consequences of the QI is fast relaxation, which means that the 35/37Cl NMR signals for covalently bound chlorines are very broad and are of low intensity.1 For these reasons, chemically distinct chlorine sites are very difficult to distinguish with solution NMR spectroscopy. However, in the solid state, nuclear spin relaxation is typically slower, thus enabling higher quality 35Cl NMR spectra to be collected, at least in principle. Unfortunately, the magnitude of the QI for covalently bound chlorines is very large because of the substantial, anisotropic EFG at the Cl atom, owing mainly to its electronic configuration when it forms a chlorine–carbon bond. Conventional wisdom is that such chlorine sites cannot be studied in powders by solid-state NMR spectroscopy as the central transition (CT; mI=1/2↔−1/2) can span tens of megahertz in typical commercially available magnetic fields. For this reason, only ionic chlorides2 and inorganic chlorides3 have been studied, as the EFG at these chlorides is often an order of magnitude smaller than at covalently bound chlorine atoms in organic molecules. The bonding environments for these types of chlorine atoms are substantially different from the environments in those chloride-containing molecules that have been studied previously.2, 3 A partial 35Cl NMR spectrum for hexachlorophene has been briefly mentioned in the literature.4 On the other hand, most of the interesting chlorine chemistry occurs when Cl is covalently bound to a carbon atom, where the chlorine atom often acts as a leaving group. Chlorine atoms are also important in many organic pharmaceuticals as well as in crystal design applications where they can form halogen bonds.5 Recent studies show that covalently bound chlorine is also important in biological chemistry where, for example, the tryptophan 7-halogenase was found to selectively chlorinate tryptophan moieties.6

An empirical model to calculate 19F isotropic chemical shifts in alkali-hexafluoroaluminates

As magic angle spinning solid-state NMR (MAS NMR) has become a powerful method in solid-state sciences, also models for the prediction of chemical shift values have become important topics in recent research. In the present study, an empirical model to describe the cationic contribution to the 19F isotropic chemical shift values in hexafluoroaluminates with isolated AlF 6-octahedra is presented. Using the corresponding alkali cryolites M I 3AlF 6 (M I = Li-Cs) as the basis of the calculation, this model is able to provide mean δ iso-values for the whole series of Al-elpasolites used here in order to test the model. The achieved results are compared to experimentally derived values and values calculated using the superposition model developed by Bureau et al. [B. Bureau, G. Silly, J.Y. Buzaré, J. Emery, Chem. Phys. 249 (1999) 89–104] to demonstrate the quality of this new approach. It is shown that a proper choice of the calculation basis leads to values very close to the experiment. As this model can be extended to other systems with isolated AlF 6-octahedra, it can be regarded as an additional possibility to the methods used so far.