Home – Home – Jornals – Journal of Structural Chemistry 2013 number Приложение 1

Electron Spin Echo Envelope Modulation (ESEEM) spectroscopy in conjunction with biochemical techniques of site-directed spin labeling and deuterium substitution in molecules or parts of molecules can be used to gain insights into supramolecular structure of biological membranes. The review covers the applications of this approach to study water penetration into the hydrophobic part of model phospholipid membranes and into the ion channels of biological membranes, to determine the localization and orientation of peptide antibiotics in membranes, and to investigate the conformations of enzymes and membrane proteins.

EPR and Mössbauer spectroscopy is used to study a new liquid crystal complex of iron(III) with a Schiff base: 4,4'-dodecyloxybenzoyloxybenzoyl-4-oxysalicylidene-2-aminopyridine with a PF^-₆ counterion. It is shown that Fe(III) ions exist only in the high-spin (HS, S = 5/2) state. It is found that under the influence of temperature the system demonstrates the stepwise behavior of the product of the integrated intensity of EPR lines (I) and temperature (proportional to χТ, where χ is the magnetic susceptibility) with an inflection point at ~80 K. Above 80 K a new EPR spectrum is detected due to the excited S = 2 state and the formation of dimeric molecules (through oxygen bridges) with a strong intramolecular antiferromagnetic exchange interaction J₁ = 162.1 cm^–1. Below 80 K iron(III) complexes are organized in 1D chains where the exchange value J₂ = 2.1 cm^–1. At 80 K there is a structural phase transition in the system: the transition from a 1D chain organization of HS Fe(III) centers to dimeric molecules. Based on quantum chemical calculations a model of the binuclear iron(III) complex is proposed.

Results of the ⁷Li, ¹⁹F, and ²³Na NMR studies of ionic mobility in bismuth fluoride glasses in the systems BiF₃–LiF and BiF₃–MF–ZrF₄ (M = Li, Na, K, Cs) are summarized. Analysis of the ⁷Li, ¹⁹F, and ²³Na NMR spectra made it possible to reveal changes in the nature of ion motions in the fluoride, lithium and sodium sublattices of glasses upon temperature variation and to determine their types. The temperature ranges were found where main types of ion motions in the tested glasses are represented by diffusion of lithium ions, reorientations of fluorine-containing groups constituting the glass network, and diffusion of fluorine ions. The role of alkali cations in the formation of ionic mobility in bismuth fluorozirconate glasses is considered.

The review considers the main propositions of PELDOR theory. It is shown how from the analysis of PELDOR time traces it is possible to find the parameters of a spin system such as the distance and the distance distribution (spectrum), number of spins in aggregates and complexes, exchange integral and how to separate for the following analysis the inter- and intramolecular contributions to the general dipole interaction. Examples of PELDOR application in the studies of the spatial distribution of nitroxyl radicals, the charge effect of dipolar interacting nitroxyls on their spatial distribution are given and the results of the determination of distances and the spectrum of distances for nitroxyl bi-, tri-, and tetraradicals are presented. The works on nitroxyl radicals in which the orientation selectivity effect, spin exchange, and conformational properties of the radicals are examined by the PELDOR method are analyzed. The studies of the structure of paramagnetic ion–nitroxyl radical pairs and the PELDOR data on nitroxyls at high frequencies (high fields) are considered. The last section of the review is devoted to the works examining the properties such as the molecular flexibility of oligomers and supramolecules contains nitroxyl radicals.

The X-band PELDOR spectroscopy was used to investigate the magnetic dipole-dipole interactions in glassy solutions of nitroxide mono-labeled tylopeptin B and heptaibin peptaibiotics at 77 K. Specifically, a study was performed of the tylopeptin B peptides labeled at either position 3, 8 or 13, denoted as Т3, Т8 and Т13, respectively. The heptaibin analogs labeled at either position 2 or 14, denoted as H2 and H14, respectively, were also investigated. It was shown that in frozen glassy peptide solutions in methanol, the spin labels are randomly distributed over the solvent volume. This result points to the absence of specific dipolar interactions between the peptides under these conditions. However, peptide aggregation was detected in weakly polar methanol/toluene environments. To study the properties of the resulting aggregates, we examined the depth of modulation for the PELDOR traces as a function of the concentration of the peptides in solution and the distances between the spin labels in the aggregates. Based on the concentration dependencies, the number of peptide molecules in the aggregates was estimated. We find that this value ranges from 2 to 3, depending on the position of the spin label in the peptide sequence. The combined analysis of the distance spectra and the number of peptide molecules in the aggregates allows us to suggest that dimer formation is the prevailing mode of self-association. In the case of spin-labeled tylopeptin B, the molecules in the dimer are head-to-head oriented. In addition, the distance spectra of the aggregates show that the C-termini of the molecules in the tylopeptin B dimer are more mobile than the N-termini. This phenomenon leads to an increase in the spread of the distances between the nitroxides as the label position is approaching the peptide C-terminus. For heptaibin, we show that two forms of dimerization (head-to-head and head-to-tail) occur. Finally, in addition to dimers, aggregates containing 3 or 4 peptide molecules, which give broad lines in the distance spectra, are seen in solution.

The effect is studied of electron and X-ray irradiation on phosphorous centers in synthetic diamonds grown in the P–C medium by the Bars technology. After exposure to X-ray irradiation, a new paramagnetic phosphorus-containing center NP6, in addition to the phosphorous centers NP4 and NP5, is observed in diamonds annealed at a temperature of 2300°С and pressure of 7.5 GPa. The spectrum of NP6 is simulated to give the following parameters: A₁ = 29.42 G, A₂ = 23.28 G, A₃ = 75.85 G, g₁ = 2.00085, g₂ = 2.00083, and g₃ = 2.00083. The NP4–NP6 centers are assumed to be genetically related to the three nitrogen–phosphorous centers NP1–NP3 and be formed as a result of the transformation of the tetrahedral environment around the phosphorous atom into an octahedral environment at an annealing temperature of 2300°C. The synthetic diamonds annealed at 2300°С were successively exposed to irradiation with electrons with energies of 3.5 MeV (5×10¹⁷ e/cm²) and annealing at temperatures of 500°C and 700°С. The EPR method is used to find that annealing of the electron-irradiated crystals at 700°С leads to the formation of a new paramagnetic center with spin S = 1 and hyperfine structure (HFS) from one phosphorus atom with the parameters: g = 2.0012, D = 19.7 G, and А(Р) = 3.6 G. The center is likely to have an eight-vacancy chain structure with a phosphorus atom located at the center.

The transformation is studied of the compounds {[Ni(cyclam)]@CB[8]}Cl₂·16H₂O and {cis-[Ni(cyclen)(H₂O)Cl]@CB[8]}Cl×12H₂O during thermal annealing in a temperature range of 20 °C to 340 °C. Using EPR spectroscopy, {cis-[Ni(cyclen)(H₂O)Cl]@CB[8]}Cl×12H₂O, Ni is found to reduce to the 3d⁹ state during annealing in a hydrogen atmosphere yet oxidize to the 3d ⁷ state in the air at 20°C. In the case of {[Ni(cyclam)]@CB[8]}Cl₂×16H₂O, nickel ions remain in the 3d⁸ state after annealing in a hydrogen atmosphere and make a transition to the 3d⁷ state after air pumping. In both cases, annealing in a hydrogen atmosphere leads to the decomposition of the nickel complexes; simultaneously, a single line with g = 2.003 appears in the EPR spectra, which is likely to be due to the resulting metal nickel clusters.

Electrochemical reduction in DMF is used to produce the radical anion of 2,4,6-trimethoxynitrobenzene with an EPR spectrum that shows hyperfine splitting for ¹⁵N nuclei of the nitro group and ¹³С nuclei of the benzene ring given the natural abundance of the isotopes. According to UB3LYP/6-31+G* quantum chemical calculations considering solvation in the PCM model, the nitro group in the equilibrium conformation of the RA of 2,4,6-trimethoxynitrobenzene is rotated relative to the benzene ring plane by an angle close to 90° and has a pyramidal structure. The calculated isotropic hyperfine coupling constants for this conformation are closest to the experimental results.

An internal-field ⁵⁹Co NMR study of cobalt-containing Fischer–Tropsch synthesis catalysts supported on different alumina modifications was reported. The Co/δ-Al₂O₃ sample was shown to contain single-domain fcc packing and stacking faults, whereas Co/γ-Al₂O₃ gave signals from the fcc domain walls, hcp and stacking faults, thus indicating differences in the particle size of the studied samples. T₂ relaxation times were measured; their distribution in a spectrum is non-uniform, which allows signals to be distinguished by their relaxation times. Quantitative measurements of the relative atoms content in different packings revealed that the catalysts have mostly a defect structure. A brief historical background was presented to characterize the internal-field ⁵⁹Co NMR technique, the related problems, and different approaches to acquired data interpretation.

A comparative analysis of ^6,7Li NMR spectra is performed for the samples of monoclinic lithium titanate obtained at different synthesis temperatures. In the ⁷Li NMR spectra three lines are found, which differ in quadrupole splitting frequencies ν_Q and according to ab initio EFG calculations are assigned to three crystallographic sites of lithium: Li1 (ν_Q ~ 27 kHz); Li2 (ν_Q ~ 59 kHz); Li3 (ν_Q ~ 6 kHz). The dynamics of lithium ions is studied in a wide temperature range from 300 K to 900 K. It is found that the narrowing of ⁷Li NMR spectra as a result of thermally activated diffusion of lithium ions in the low-temperature Li₂TiO₃ sample is observed at a higher temperature in comparison with a sample of high-temperature lithium titanate. Based on the analysis of ⁶Li NMR spectra it is assumed that there is mixed occupancy of lithium and titanium sites in the corresponding layers of the crystal structure of low-temperature lithium titanate, which hinders lithium ion transfer over regular crystallographic sites.

The structure of zeolite water in single crystals of natural zeolites represented by clinoptilolite Na₂K₂Ca[Al₆Si₃₀O₇₂]×22H₂O and heulandite Са₃Mg[Al₈Si₂₈O₇₂]×24H₂O is studied with ¹H NMR. Below 170 K the distribution of H₂O over the structural positions is shown to be fixed but different for the two minerals. Above 290 K translational and orientational diffusion of zeolite water molecules is observed and the structure of water is almost identical in both heulandite and clinoptilolite. Diffusion mechanism may be associated with the interaction between librational modes of H₂O and high-frequency oscillations of aluminosilicate framework. The microwave absorption is shown to be caused in certain conditions by this type of interaction.

⁵¹V NMR, DTA, and thermogravimetric analysis are used to study the thermal behavior of a manganese-nickel pyrovanadate complex Mn_2–2xNi_2xV₂O₇. It is shown that the nature of ⁵¹V hyperfine interactions in manganese pyrovanadate and solid solutions based on its polymorphic modifications is specified by the structurally activated oxygen exchange with the gaseous phase.

The ¹¹B NMR method is used to study the crystals of trigonal alumina borates Ho_1–xY_xAl ₃(BO₃)₄ (x = 0, 0.5, 1). The temperature-field evolution of quadrupole and hyperfine interactions in these compounds is studied and described.

Ionic mobility and phase transitions in heptafluorodiantimonates with homo- and heteroatomic cations in the systems MSb₂F₇ (M = K, Cs, NH₄) and CsM'Sb₂F₇ (M' = K, NH₄) have been studied using the ¹⁹F, ¹H NMR and DSC methods. Analysis of the ¹⁹F and ¹H NMR spectra made it possible to reveal the nature of ion motions in the fluoride and proton sublattices upon temperature variation and to determine the types and temperature ranges of such motions. It was found that diffusion of fluorine ions becomes the dominant form of ion motions in the high-temperature modifications, the amount of diffusing ammonium ions depending on the composition of a sample. The observed phase transitions in cesium–potassium and cesium–ammonium fluoroantimonates(III) leading to the formation of high-temperature modifications are the transitions to superionic state. According to preliminary results of electrophysical studies, high-temperature phases Cs_1–x(NH₄)_xSb₂F₇ (0.05 ≤ x ≤ 0.6) are superionic, their conductivity reaching ~10 ^–3—10 ^–4 S/cm at 463-483 K.

The dynamic structure of hydrogen sublattice in lawsonite, CaAl₂[Si₂O₇](OH)₂×H₂O is studied by the solid-state proton magnetic resonance (¹H NMR) spectroscopy of single crystal at room temperature. It is shown that both encapsulated water molecules, and hydroxyl OH-groups undergoes the rocking librations of the amplitudes of ~20° for H₂O, and ~40° for hydroxyls.

¹H, ²³Na and ²⁹Si solid-state NMR spectroscopy was used to investigate the formation of fiberglass zirconium-silicate supports and catalysts (glass → fiberglass → leached fiberglass → catalyst). The main emphasis was made on OH–H₂O domains in fiberglass and their role in catalyst preparation. The size of OH–H₂O domains and their accessibility to H–D exchange were determined. Platinum was shown to be anchored on the bridging hydroxyl groups. The quantum-chemical calculations demonstrated the activity of Pt⁰ clusters forming in fiberglass to depend on their nuclearity.

Ionic mobility of Li₄ZrF₈ (I) and Li_3.72Mg_0.14ZrF₈ (II) compounds has been studied over the temperature range of 300-420 K using ⁷Li, ¹⁹F broad-line NMR and NMR relaxation techniques. The fluorine sublattice in these compounds was found to be rigid, whereas in the lithium sublattice there occurs a diffusion with the activation energy of 0.2-0.3 eV. NMR data indicate the formation of lithium vacancies in solid solution II. MAS NMR of fluorine was used to determine the isotropic chemical shifts of its magnetically nonequivalent positions and to correlate these shifts with the crystal structure of Li₄ZrF₈.

NMR spectroscopy is used to study the structural features of polymer composites produced by the simultaneous pyrolysis of a polytetrafluoroethylene and ammonium fluorosilicate mixture. The effect is discussed of technological factors on the composition and structure of fluoropolymer composites.

Chemical shifts in ¹⁹F and ¹³C NMR spectra of substituted pentafluorobenzenes are calculated by Hartree-Fock and density functional theory methods. The calculated values are compared with the experimental data known from the literature. It is shown that chemical shifts in non-polar solvents can be predicted sufficiently accurately by the GIAO-DFT(PBE/L22) method. This method is used to predict the ¹⁹F and ¹³C chemical shifts of a heptafluorobenzyl cation in the SbF₅ medium. The best agreement between the calculated and experimental values is achieved when the counterion effect is taken into account.