Fundamental Concepts of NMR in Paramagnetic Systems Part I: The Isotropic Shift

Isostructural bridging diferrous chalcogenide cores [FeII(μ-E)FeII] (E = O, S, Se, Te) with decreasing antiferromagnetic coupling down the chalcogenide series

Iron compounds containing a bridging oxo or sulfido moiety are ubiquitous in biological systems, but substitution with the heavier chalcogenides selenium and tellurium, however, is much rarer, with only a few examples reported to date. Here we show that treatment of the ferrous starting material [(^tBupyrpyrr₂)Fe(OEt₂)] (1-OEt₂) (^tBupyrpyrr₂ = 3,5-^tBu₂-bis(pyrrolyl)pyridine) with phosphine chalcogenide reagents E = PR₃ results in the neutral phosphine chalcogenide adduct series [(^tBupyrpyrr₂)Fe(EPR₃)] (E = O, S, Se; R = Ph; E = Te; R = ^tBu) (1-E) without any electron transfer, whereas treatment of the anionic starting material [K]₂[(^tBupyrpyrr₂)Fe₂(μ-N₂)] (2-N₂) with the appropriate chalcogenide transfer source yields cleanly the isostructural ferrous bridging mono-chalcogenide ate complexes [K]₂[(^tBupyrpyrr₂)Fe₂(μ-E)] (2-E) (E = O, S, Se, and Te) having significant deviation in the Fe-E-Fe bridge from linear in the case of E = O to more acute for the heaviest chalcogenide. All bridging chalcogenide complexes were analyzed using a variety of spectroscopic techniques, including ¹H NMR, UV-Vis electronic absorbtion, and ⁵⁷Fe Mössbauer. The spin-state and degree of communication between the two ferrous ions were probed via SQUID magnetometry, where it was found that all iron centers were high-spin (S = 2) Fe^II, with magnetic exchange coupling between the Fe^II ions. Magnetic studies established that antiferromagnetic coupling between the ferrous ions decreases as the identity of the chalcogen is tuned from O to the heaviest congener Te.

Surface Engineering Strategy Using Urea To Improve the Rate Performance of Na2 Ti3 O7 in Na-Ion Batteries

Na2 Ti3 O7 (NTO) is considered a promising anode material for Na-ion batteries due to its layered structure with an open framework and low and safe average operating voltage of 0.3 V vs. Na+ /Na. However, its poor electronic conductivity needs to be addressed to make this material attractive for practical applications among other anode choices. Here, we report a safe, controllable and affordable method using urea that significantly improves the rate performance of NTO by producing surface defects such as oxygen vacancies and hydroxyl groups, and the secondary phase Na2 Ti6 O13 . The enhanced electrochemical performance agrees with the higher Na+ ion diffusion coefficient, higher charge carrier density and reduced bandgap observed in these samples, without the need of nanosizing and/or complex synthetic strategies. A comprehensive study using a combination of diffraction, microscopic, spectroscopic and electrochemical techniques supported by computational studies based on DFT calculations, was carried out to understand the effects of this treatment on the surface, chemistry and electronic and charge storage properties of NTO. This study underscores the benefits of using urea as a strategy for enhancing the charge storage properties of NTO and thus, unfolding the potential of this material in practical energy storage applications.

Syntheses, spectroscopic, redox, and structural properties of homoleptic Iron(III/II) dithione complexes

Two sets of FeIII/II dithione complexes [FeII( i Pr2Dt0)3][PF6]2 ([1][PF6]2), [FeII(Me2Dt0)3][PF6]2 ([2][PF6]2), and [FeIII( i Pr2Dt0)3][PF6]3 ([3][PF6]3), [FeIII(Me2Dt0)3][PF6]3 ([4][PF6]3), and compound [FeIII( i Pr2Dt0)3][FeCl4][PF]2 ([3][FeCl4][PF6]2) were synthesized from N,N'-diisopropyl piperazine-2,3-dithione ( i Pr2Dt0) and N,N'-dimethyl piperazine-2,3-dithione (Me2Dt0) ligands. Complexes [1][PF6]2-[4][PF6]3 have been characterized by NMR, IR, and UV-visible spectroscopies, and by electrochemistry. The molecular structures of [2][PF6]2 and [3][FeCl4][PF6]2 have been determined by X-ray crystallography. Complexes [2][PF6]2 and [3][FeCl4][PF6]2 both crystallized in the monoclinic space group P21/n. Both complexes exhibit distorted octahedral geometry and the three coordinated ligands in each complex exhibit different dithione folding. Complexes [1][PF6]2-[4][PF6]3 exhibit a single FeIII/II based couple and three quasi-reversible ligand-based redox couples. The electronic spectra of [1][PF6]2-[4][PF6]3 show intense MLCT bands that indicate strong mixing between metal and ligand orbitals. DFT calculations were used to provide a framework for understanding the electronic origin of their redox chemistry and spectroscopic features.

A brief introduction to the basics of NMR spectroscopy and selected examples of its applications to materials characterization

Nuclear magnetic resonance (NMR) spectroscopy is an analytical technique that gives information on the local magnetic field around atomic nuclei. Since the local magnetic field of the nucleus is directly influenced by such features of the molecular structure as constitution, configuration, conformation, intermolecular interactions, etc., NMR can provide exhaustive information on the chemical structure, which is unrivaled by any other analytical method. Starting from the 1950s, NMR spectroscopy first revolutionized organic chemistry and became an indispensable tool for the structure elucidation of small, soluble molecules. As the technique evolved, NMR rapidly conquered other disciplines of chemical sciences. When the analysis of macromolecules and solids also became feasible, the technique turned into a staple in materials characterization, too. All aspects of NMR spectroscopy, including technical and technological development, as well as its applications in natural sciences, have been growing exponentially since its birth. Hence, it would be impossible to cover, or even touch on, all topics of importance related to this versatile analytical tool. In this tutorial, we aim to introduce the reader to the basic principles of NMR spectroscopy, instrumentation, historical development and currently available brands, practical cost aspects, sample preparation, and spectrum interpretation. We show a number of advanced techniques relevant to materials characterization. Through a limited number of examples from different fields of materials science, we illustrate the immense scope of the technique in the analysis of materials. Beyond our inherently limited introduction, an ample list of references should help the reader to navigate further in the field of NMR spectroscopy.

A New Class of Lanthanide Complexes with Three Ligand Centered Radicals: NMR Evaluation of Ligand Field Energy Splitting and Magnetic Coupling

Combination of three radical anionic Ph-BIAN ligands (Ph-BIAN=bis-(phenylimino)-acenaphthenequinone) with lanthanoid ions leads to a series of homoleptic, six-coordinate complexes of the type Ln(Ph-BIAN)₃ . Magnetic coupling data were measured by paramagnetic solution NMR spectroscopy. Combining ¹ H NMR with ² H NMR of partially deuterated compounds allowed a detailed study of the magnetic susceptibility anisotropies over a large temperature range. The observed chemical shifts were separated into ligand- and metal-centered contributions by comparison with the Y analogue (diamagnetic at the metal). The metal-centered contributions of the complexes with the paramagnetic ions could then be separated into pseudocontact and Fermi contact shifts. The latter is large within the Ph-BIAN scaffold, which shows that magnetic coupling is significant between the lanthanide ion and the radical ligand. Pseudocontact shifts were further correlated to structural data obtained from X-ray diffraction experiments. Ligand-field parameters were determined by fitting the temperature dependence of the observed magnetic susceptibility anisotropies. The electronic structure determined by this approach shows, that the Er and Tm analogues are candidates for single molecule magnets (SMM). These results demonstrate the possibilities for the application of NMR spectroscopy in investigations of paramagnetic systems in general and single molecule magnets in particular.

Morphology of fast-tumbling bicelles: a small angle neutron scattering and NMR study

Bilayered micelles, or bicelles, which consist of a mixture of long- and short-chain phospholipids, are a popular model membrane system. Depending on composition, concentration, and temperature, bicelle mixtures may adopt an isotropic phase or form an aligned phase in magnetic fields. Well-resolved (1)H NMR spectra are observed in the isotropic or so-called fast-tumbling bicelle phase, over the range of temperatures investigated (10-40 degrees C), for molar ratios of long-chain lipid to short-chain lipid between 0.20 and 1.0. Small angle neutron scattering data of this phase are consistent with the model in which bicelles were proposed to be disk-shaped. The experimentally determined dimensions are roughly consistent with the predictions of R.R. Vold and R.S. Prosser (J. Magn. Reson. B 113 (1996)). Differential paramagnetic shifts of head group resonances of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC), induced by the addition of Eu(3+), are also consistent with the bicelle model in which DHPC is believed to be primarily sequestered to bicelle rims. Selective irradiation of the DHPC aliphatic methyl resonances results in no detectable magnetization transfer to the corresponding DMPC methyl resonances (and vice versa) in bicelles, which also suggests that DHPC and DMPC are largely sequestered in the bicelle. Finally, (1)H spectra of the antibacterial peptide indolicidin (ILPWKWPWWPWRR-NH(2)) are compared, in a DPC micellar phase and the above fast-tumbling bicellar phases for a variety of compositions. The spectra exhibit adequate resolution and improved dispersion of amide and aromatic resonances in certain bicelle mixtures.

Determination of membrane immersion depth with O(2): a high-pressure (19)F NMR study

Oxygen is known to partition with an increasing concentration gradient toward the hydrophobic membrane interior. At partial pressures (P(O2)) of 100 Atm or more, this concentration gradient is sufficient to induce paramagnetic effects that depend sensitively on membrane immersion depth. This effect is demonstrated for the fluorine nucleus by depth-dependent paramagnetic shifts and spin-lattice relaxation rates, using a fluorinated detergent, CF3(CF(2))(5)C(2)H(4)-O-maltose (TFOM), reconstituted into a lipid bilayer model membrane system. To interpret the spin-lattice relaxation rates (R) in terms of a precise immersion depth, two specifically fluorinated cholesterol species (6-fluorocholesterol and 25-fluorocholesterol), whose membrane immersion depths were independently estimated, were studied by (19)F NMR. The paramagnetic relaxation rates, R, of the cholesterol species were then used to parameterize a Gaussian profile that directly relates R to immersion depth z. This same Gaussian curve could then be used to determine the membrane immersion depth of all six fluorinated chain positions of TFOM and of two adjacent residues of specifically fluorinated analogs of the antibacterial peptide indolicidin. The potential of this method for determination of immersion depth and topology of membrane proteins is discussed.

The effect of the Asn82-->Asp mutation in yeast cytochrome c peroxidase studied by proton NMR spectroscopy

Proton NMR studies of the mutant of baker's yeast cytochrome c peroxidase-cyanide with the Asn 82-->Asp mutation ([N82D]cytochrome c peroxidase-CN) are presented and compared to the wild-type enzyme. This mutation alters an amino acid that forms a hydrogen bond to His52, the distal histidine residue that interacts in the heme pocket with heme-bound ligands. His52 is an important participant in the initial hydrogen peroxide decomposition step of cytochrome c peroxidase. In wild-type cytochrome c peroxidase-CN, His52 hydrogen bonds to the neighboring Asn82 peptide carbonyl group and to heme-coordinated cyanide. His52 thus manifests itself as an extensively hydrogen bonded histidinium moiety. The principal result from this study is the observation that three hyperfine-shifted resonances disappear from the spectrum of [N82D] cytochrome c peroxidase-CN compared to the wild-type enzyme. All three absent resonances in [N82D]cytochrome c peroxidase-CN belong to His52 and this leads to the conclusion that the result of the mutation has been elimination of the His52-Asn82 and His52-heme-coordinated cyanide hydrogen bonds.