Solid-State ^47/49Ti NMR of Titanocene Chlorides

Magic angle spinning (MAS) and static ^47/49Ti solid-state NMR (SSNMR) spectra of the cyclopentadienyl (Cp) titanium chloride compounds, Cp₂TiCl₂ (<b>1</b>), Cp*₂TiCl₂ (<b>2</b>) [Cp* = C₅Me₅], CpTiCl₃ (<b>3</b>), and Cp*TiCl₃ (<b>4</b>) have been acquired at magnetic field strengths of 21.1 and 9.4 T. From these spectra, it is possible to measure anisotropic ^47/49Ti NMR interaction parameters, which are extremely sensitive to differences in molecular structure. At a magnetic field of 21.1 T, the ^47/49Ti spectra can be acquired efficiently with standard echo pulse sequences. Quantum chemical calculations of ^47/49Ti electric field gradient (EFG) and chemical shielding (CS) tensor parameters are presented. The theoretical EFG and CS tensor orientations are utilized to relate the anisotropic NMR tensor parameters to the molecular and electronic structures of the complexes

<i>In Situ</i> NMR Tracks Real-Time Li Ion Movement in Hybrid Supercapacitor–Battery Device

Lithium ion capacitors (LICs) are emerging as promising energy storage devices; thus, understanding their electrochemistry is of great interest. Here we report the study of a novel LIC by employing <i>in situ</i> nuclear magnetic resonance spectroscopy (NMR) as a nondestructive tool, revealing the sequence of electrochemical processes in it. We have performed <i>in situ</i> ⁷Li NMR experiments on the LIC by simultaneously cycling the LIC pouch cell in the voltage range 2.0–4.0 V. NMR spectra recorded for multiple cycles reveal the ⁷Li NMR signals arising from different parts of the capacitor. By employing a combination of <i>in situ</i> ⁷Li NMR, component isolation, and Gaussian–Lorentzian peak fitting, we investigate the resonances arising from the Li metal from stabilized lithium metal powder (SLMP), free ions in electrolyte, the solid electrolyte interface layer (SEI), intercalated lithium in carbon anode, and the Li ions in the electric double layer on cathode. The recorded <i>in situ</i> ⁷Li NMR spectra showed that the charge and discharge processes caused electrochemical reactions, resulting in considerable repetitive changes in peak intensities and chemical shifts over multiple cycles. Further cycle experiments revealed contributions from individual electrodes. This series of experiments aid in the visualization of the Li ion transfer mechanisms in LICs

Isotropic High Field NMR Spectra of Li-Ion Battery Materials with Anisotropy >1 MHz

The use of a magic-angle turning and phase-adjusted spinning sideband NMR experiment to resolve and quantify the individual local environments in the high field ⁷Li and ³¹P NMR spectra of paramagnetic lithium-ion battery materials is demonstrated. The use of short radio frequency pulses provides an excitation bandwidth that is sufficient to cover shift anisotropy of >1 MHz in breadth, allowing isotropic and anisotropic components to be resolved

A Quadrupole-Central-Transition ¹⁷O NMR Study of Nicotinamide: Experimental Evidence of Cross-Correlation between Second-Order Quadrupolar Interaction and Magnetic Shielding Anisotropy

We have examined the ¹⁷O quadrupole-central-transition (QCT) NMR signal from [¹⁷O]­nicotinamide (vitamin B3) dissolved in glycerol. Measurements were performed at five magnetic fields ranging from 9.4 to 35.2 T between 243 and 363 K. We found that, in the ultraslow motion regime, cross-correlation between the second-order quadrupole interaction and magnetic shielding anisotropy is an important contributor to the transverse relaxation process for the ¹⁷O QCT signal of [¹⁷O]­nicotinamide. While such a cross-correlation effect has generally been predicted by relaxation theory, we report here the first experimental evidence for this phenomenon in solution-state NMR for quadrupolar nuclei. We have discussed the various factors that determine the ultimate resolution limit in QCT NMR spectroscopy. The present study also highlights the advantages of performing QCT NMR experiments at very high magnetic fields (e.g., 35.2 T)

Solid-State ¹⁷O NMR of Pharmaceutical Compounds: Salicylic Acid and Aspirin

We report solid-state NMR characterization of the ¹⁷O quadrupole coupling (QC) and chemical shift (CS) tensors in five site-specifically ¹⁷O-labeled samples of salicylic acid and <i>o</i>-acetylsalicylic acid (Aspirin). High-quality ¹⁷O NMR spectra were obtained for these important pharmaceutical compounds under both static and magic angle spinning (MAS) conditions at two magnetic fields, 14.0 and 21.1 T. A total of 14 ¹⁷O QC and CS tensors were experimentally determined for the seven oxygen sites in salicylic acid and Aspirin. Although both salicylic acid and Aspirin form hydrogen bonded cyclic dimers in the solid state, we found that the potential curves for the concerted double proton transfer in these two compounds are significantly different. In particular, while the double-well potential curve in Aspirin is nearly symmetrical, it is highly asymmetrical in salicylic acid. This difference results in quite different temperature dependencies in ¹⁷O MAS spectra of the two compounds. A careful analysis of variable-temperature ¹⁷O MAS NMR spectra of Aspirin allowed us to obtain the energy asymmetry (Δ<i>E</i>) of the double-well potential, Δ<i>E</i> = 3.0 ± 0.5 kJ/mol. We were also able to determine a lower limit of Δ<i>E</i> for salicylic acid, Δ<i>E</i> > 10 kJ/mol. These asymmetrical features in potential energy curves were confirmed by plane-wave DFT computations, which yielded Δ<i>E</i> = 3.7 and 17.8 kJ/mol for Aspirin and salicylic acid, respectively. To complement the solid-state ¹⁷O NMR data, we also obtained solid-state ¹H and ¹³C NMR spectra for salicylic acid and Aspirin. Using experimental NMR parameters obtained for all magnetic nuclei present in salicylic acid and Aspirin, we found that plane-wave DFT computations can produce highly accurate NMR parameters in well-defined crystalline organic compounds

Structural and Topological Control on Physical Properties of Arsenic Selenide Glasses

The structures of Ge-doped arsenic selenide glasses with Se contents varying between 25 and 90 at. % are studied using a combination of high-resolution, two-dimensional ⁷⁷Se nuclear magnetic resonance (NMR) and Raman spectroscopy. The results indicate that, in contrast to the conventional wisdom, the compositional evolution of the structural connectivity in Se-excess glasses does not follow the chain-crossing model, and chemical order is likely violated with the formation of a small but significant fraction of As–As bonds. The addition of As to Se results in a nearly random cross-linking of Se chains by AsSe₃ pyramids, and a highly chemically ordered network consisting primarily of corner-shared AsSe₃ pyramids is formed at the stoichiometric composition. Further increase in As content, up to 40 at. % Se, results in the formation of a significant fraction of As₄Se₃ molecules with As–As homopolar bonds, and consequently the connectivity and packing efficiency of the network decrease and anharmonic interactions increase. Finally, in the highly As-rich region with <40 at. % Se, the relative concentration of the As₄Se₃ molecules decreases rapidly and large clusters of As atoms connected via Se–<b>Se</b>–As and As–<b>Se</b>–As linkages dominate. These three composition regions with distinct structural characteristics and the corresponding mixing entropy of the Se environments are reflected in the appearance of multiple extrema in the compositional variation of a wide range of physical properties of these glasses, including density, glass transition temperature, thermal expansivity, and fragility

Magic Angle Spinning and Oriented Sample Solid-State NMR Structural Restraints Combine for Influenza A M2 Protein Functional Insights

As a small tetrameric helical membrane protein, the M2 proton channel structure is highly sensitive to its environment. As a result, structural data from a lipid bilayer environment have proven to be essential for describing the conductance mechanism. While oriented sample solid-state NMR has provided a high-resolution backbone structure in lipid bilayers, quaternary packing of the helices and many of the side-chain conformations have been poorly restrained. Furthermore, the quaternary structural stability has remained a mystery. Here, the isotropic chemical shift data and interhelical cross peaks from magic angle spinning solid-state NMR of a liposomal preparation strongly support the quaternary structure of the transmembrane helical bundle as a dimer-of-dimers structure. The data also explain how the tetrameric stability is enhanced once two charges are absorbed by the His37 tetrad prior to activation of this proton channel. The combination of these two solid-state NMR techniques appears to be a powerful approach for characterizing helical membrane protein structure

Structural Changes Associated with Transthyretin Misfolding and Amyloid Formation Revealed by Solution and Solid-State NMR

Elucidation of structural changes involved in protein misfolding and amyloid formation is crucial for unraveling the molecular basis of amyloid formation. Here we report structural analyses of the amyloidogenic intermediate and amyloid aggregates of transthyretin using solution and solid-state nuclear magnetic resonance (NMR) spectroscopy. Our solution NMR results show that one of the two main β-sheet structures (CBEF β-sheet) is maintained in the aggregation-competent intermediate, while the other DAGH β-sheet is more flexible on millisecond time scales. Magic-angle-spinning solid-state NMR revealed that AB loop regions interacting with strand A in the DAGH β-sheet undergo conformational changes, leading to the destabilized DAGH β-sheet

Dynamic Solid-State NMR Experiments Reveal Structural Changes for a Methyl Silicate Nanostructure on Deuterium Substitution

Structural characterizations of three different solid–gas reaction products, recently obtained from abraded solid-state silicate free radicals reacting with two isotopically enriched methane gases, ¹³CH₄ and CD₄ (a possible sink for methane on MARS) and with ¹³CO₂, are derived from various dynamic solid-state NMR experiments. These include cross-polarization/depolarization zero-cross times (ZCTs), variable temperature (VT) NMR to study 3-site jump CH₃/CD₃ activation energies (<i>E</i>_a), and ¹³CO₂/¹³CH₃ molecular species as a spy to determine the approximate diameters for the channel structures for some of these structures. Literature <i>E</i>_a data indicate that l-alanine and 4-CH₃-phenanthrene exhibit the highest known <i>E</i>_a values (= 20–22.6 kJ/mol) for CH₃ 3-site jump motions. The ZCTs for these two compounds are 120 and 162 μs, respectively, indicative of the high <i>E</i>_a values for CH₃/CD₃ groups. Determination of <i>E</i>_a for 4-CD₃-phenanthrene by low-temperature ²H MAS NMR experiments supplemented the previously reported liquid-state <i>E</i>_a value (<i>E</i>_a = 21 kJ/mol) for 4-CH₃-phenanthrene. Finally, such experiments also revealed the structural difference for the free-radical reaction products with ¹³CH₄ and CD₄, i.e, a change from helical to chain structure

Differential Binding of Rimantadine Enantiomers to Influenza A M2 Proton Channel

Rimantadine hydrochloride (α-methyl-1-adamantane-methalamine hydrochloride) is a chiral compound which exerts antiviral activity against the influenza A virus by inhibiting proton conductance of the M2 ion channel. In complex with M2, rimantadine has always been characterized as a racemic mixture. Here, we report the novel enantioselective synthesis of deuterium-labeled (<i>R</i>)- and (<i>S</i>)-rimantadine and the characterization of their protein–ligand interactions using solid-state NMR. Isotropic chemical shift changes strongly support differential binding of the enantiomers to the proton channel. Position restrained simulations satisfying distance restraints from ¹³C–²H rotational-echo double-resonance NMR show marked differences in the hydrogen-bonding pattern of the two enantiomers at the binding site. Together these results suggest a complex set of interactions between (<i>R</i>)-rimantadine and the M2 proton channel, leading to a higher stability for this enantiomer of the drug in the channel pore