Paramagnetic Centers Research Articles

Morphology and defect-impurity composition of the Zapolyarnaya pipe diamonds

A representative collection of Zapolyarnaya pipe diamonds of the first batch of industrial production was studied by methods of morphological analysis, IR, FL, and EPR. Diamonds belong to the medium-low-nitrogen, they are the closest to the Daldyn-Alakit area diamonds in terms of their composition and ratios of impurity centers. Typomorphic features for diamonds of Zapolyarnaya pipe are the predominance of curved dodecahedroids, a significant degree of development of the processes of dissolution, etching and plastic deformation, close to the maximum content of IR defects B2, the minimum content of hydrogen, as well as a wide spread of paramagnetic centers W7.

EPR characteristics of radiation-induced defects in Ge-rich α-quartz

Electron paramagnetic resonance (EPR) in a synthetic Si0.975Ge0.025O2 single crystal of α–quartz structure has been studied. After fast electron irradiation previously studied and new Ge-related paramagnetic centers (PCs) have been recognized using computer simulation of the observed EPR spectra. Increasing radiation doses allow to distinguish four paramagnetic [GeO4/M+]0 centers AGeLi, CGeLi, DGeLi, and AGeNa as well as four oxygen-vacancy electron centers Ge Eˊ1, Ge Eˊ2, Ge(IV) and a new Ge(V) center. The selective UV bleaching and distinct thermal stability of different centers allow us to determine the spin Hamiltonian parameters for the Ge(V) center.

Recent Developments in Electron Paramagnetic Resonance for Spectroscopic Applications

This review investigated the developments of Electron paramagnetic resonance such as SDSL EPR, pulsed/CW ENDOR, high frequency/ high-field EPR, and ESEEM in biological systems such as photosynthesis, metalloproteins, radical enzymes, and phospholipid membranes. EPR spectroscopy act as a powerful tool to calculate the dynamics and structure of the biological systems from the paramagnetic centers. Applying the EPR measurements on the biomolecules like calmodulin, pyruvate kinase, nitrogenase, bromoperoxidase can also help understand their intermolecular behavior. The better insight into the detailed kinetics of the protein in the radical state can be evaluated quickly with the EPR spectroscopy than that of the XRD or NMR techniques. Hence, the specific potentials, principles, technicality, developments, and limitations, of EPR spectroscopy are highlighted here to understand its applications in the research and development of bio-medicine.

Microwave-Free Dynamic Nuclear Polarization via Sudden Thermal Jumps.

Dynamic nuclear polarization (DNP) presently stands as the preferred strategy to enhance the sensitivity of nuclear magnetic resonance measurements, but its application relies on the use of high-frequency microwave to manipulate electron spins, an increasingly demanding task as the applied magnetic field grows. Here we investigate the dynamics of a system hosting a polarizing agent formed by two distinct paramagnetic centers near a level anticrossing. We theoretically show that nuclear spins polarize efficiently under a cyclic protocol that combines alternating thermal jumps and radio-frequency pulses connecting hybrid states with opposite nuclear and electronic spin alignment. Central to this process is the difference between the spin-lattice relaxation times of either electron spin species, transiently driving the electronic spin bath out of equilibrium after each thermal jump. Without the need for microwave excitation, this route to enhanced nuclear polarization may prove convenient, particularly if the polarizing agent is designed to feature electronic level anticrossings at high magnetic fields.

Invariants in the paramagnetic resonance spectra of impurity-doped crystals

We show that in cubic crystals with anisotropic impurity centers the sum of squares of the magnetic resonance (EPR) frequencies is invariant with respect to the magnetic field direction. The connection between such an invariant and the g-tensor components of the impurity is derived for different types of centers. The established regularity is confirmed experimentally for the spin-noise spectra of a CaF2-Nd3+ crystal. We show how this property of the EPR spectra can be efficiently used for the assignment of paramagnetic centers in cubic crystals.

Pseudocontact Shifts in Biomolecular NMR Spectroscopy.

Paramagnetic centers in biomolecules, such as specific metal ions that are bound to a protein, affect the nuclei in their surrounding in various ways. One of these effects is the pseudocontact shift (PCS), which leads to strong chemical shift perturbations of nuclear spins, with a remarkably long range of 50 Å and beyond. The PCS in solution NMR is an effect originating from the anisotropic part of the dipole-dipole interaction between the magnetic momentum of unpaired electrons and nuclear spins. The PCS contains spatial information that can be exploited in multiple ways to characterize structure, function, and dynamics of biomacromolecules. It can be used to refine structures, magnify effects of dynamics, help resonance assignments, allows for an intermolecular positioning system, and gives structural information in sensitivity-limited situations where all other methods fail. Here, we review applications of the PCS in biomolecular solution NMR spectroscopy, starting from early works on natural metalloproteins, following the development of non-natural tags to chelate and attach lanthanoid ions to any biomolecular target to advanced applications on large biomolecular complexes and inside living cells. We thus hope to not only highlight past applications but also shed light on the tremendous potential the PCS has in structural biology.

Application of solid-state NMR techniques for structural characterization of metal-organic frameworks

Solid-state NMR can afford the structural information about the chemical composition, local environment, and spatial coordination at the atomic level, which has been extensively applied to characterize the detailed structure and host-guest interactions in metal-organic frameworks (MOFs). In this review, recent advances for the structural characterizations of MOFs using versatile solid-state NMR techniques were briefly introduced. High-field sensitivity-enhanced solid-state NMR method enabled the direct observation of metal centers in MOFs containing low-γ nuclei. Two-dimensional (2D) homo- and hetero-nuclear correlation MAS NMR experiments provided the spatial proximity among linkers, metal clusters and the introduced guest molecules. Moreover, quantitative measurement of inter-nuclear distances using solid-state NMR provided valuable structural information about the connectivity geometry as well as the host-guest interactions within MOFs. Furthermore, solid-state NMR has exhibited great potential for unraveling the structure property of MOFs containing paramagnetic metal centers.

Structure and Magnetism of Few-Layer Nanographene Clusters in Carbon Microspheres

The solid-phase pyrolysis method was used to synthesize carbon microspheres, consisting of clusters of few-layer nanographene and amorphous carbon. Powders of metal-free phthalocyanine and polyethylene served as precursors of the synthesized carbon microspheres. The pyrolysis products of metal-free phthalocyanine samples SPc(700) and SPc(900) contained 4 and 1 atom % nitrogen, respectively, replacing carbon in the graphene lattice in pyrrolic and pyridinic coordination. There are no impurity nitrogen atoms in the products of the pyrolysis of polyethylene. The SPc(700) sample showed strong paramagnetism with a concentration of paramagnetic centers of ∼5 × 1019 spin g–1 and a temperature-independent diamagnetism susceptibility of χDia = −1 × 10–6 emu g–1 Oe–1. In a temperature range of 5–300 K, ferromagnetism was also revealed with a temperature dependence similar to that of ferromagnetic cluster spin glasses, with maximum saturation magnetization, MSFM = 3 × 10–2 emu g–1, and coercive force, Hc = 400 Oe, at Tsg = 25 K. It was shown that the ferromagnetism in the SPc(700) sample is due to π(p)-electrons of zigzag-type edge states as well as nitrogen impurity atoms. The experimental results are interpreted based on the temperature dependence of the spin correlation length. It was revealed that the temperature dependence of the integral of the magnetic resonance absorption intensity closely resembles the temperature behavior of the saturation magnetization of the ferromagnetic component.

Сверхизлучение и синтез наночастиц металла в полистирольном композите с многоспиновыми комплексами Eu при быстром одноосном сдавливании

Composites of heterospin molecular magnet [EuIII(SQ)3bipy] in a polystyrene (PS) matrix have been synthesized. This complex [EuIII(SQ)3bipy] contains four paramagnetic centers - the Eu3+ ion and three SQ ligands (SQ—3,6-di-tert-butyl benzoquinone radical anion); bipy (bipyridyl) is diamagnetic. It has been established that intensive mechanical activation of [EuIII(SQ)3bipy]/PS samples leads to an reological explosion, as a result of which radio frequency superradiance, the appearance of free electrons and the formation of Eu metal nanoparticles are observed. The duration of this process is 10 ns.

PHYSICO-CHEMICAL CHARACTERISTICS OF MATERIALS BASED ON CERIUM AND CHROMIUM LANTHANOIDES

Some physical and chemical characteristics of hexa(thiocyanate-N)chromate complexes of cerium lanthanides with 3-pyridine-3-carboxylic acid are studied. It was shown by IR spectroscopy for displacement of the main absorption bands that the compounds are N-thiocyanate, the organic ligand is bidentate due to the proton transfer of the carboxyl group to atom nitrogen with the formation of a conjugate system: imino-amine tautomerism of organic ligand leads to the occurrence of pyridinium-3-carboxylic ion. It was established by X-ray diffraction analysis of monocrystals that substances have an isle structure, crystallize according the same structural type, and their crystallographic parameters change regularly in accordance with the change in the radii of Ln3+ ions. The obtained substances of the isle type are isostructural. Iintermolecular hydrogen bonds between crystallization water molecules and N–H groups of coordinated molecules of organic ligands are found. The compositions of the substances differ only in the content of crystallization water in the molecules. Studies of the magnetic properties of the samples were carried out taking into account their stability in the range of 80-360 K. The decomposition sequence is established and composition of thermolysis products. The experimental and calculated values effective magnetic moments is in agreement. Values effective magnetic moments characterize the antiferromagnetic nature of the interaction between paramagnetic centers. At temperatures below 100 K, discrepancies between the experimental and calculated values are observed, which may be due to the transition of compounds from the paramagnetic to antiferromagnetic state. The thermal behavior of substances in an inert atmosphere and oxidizing atmospheres to interval of tempetures 293-1273 K are presented. Compounds obtained determine the promising magnetic and thermal properties of substances as precursors for the low-temperature production of heterometallic oxide ceramic materials, as well as molecular magnets and thermoindicators for visual control of temperatures and temperature fields.

Paramagnetic centers in chromium-doped scandium orthosilicate

The EPR spectrum of single crystals of scandium orthosilicate doped with chromium was studied in the X-band. In the crystallographic and local coordinate systems, the parameters of the spin Hamiltonians of two Cr3+ centers that have replaced scandium in the nonequivalent triclinic positions and demonstrate orthorhombic symmetry in the principal axes are determined. The spectrum of the Gd3+ center localized in one of scandium position was discovered and studied, and the parameters of the fine structure of its ground state were determined. In the studied crystals, EPR signals of unidentified paramagnetic centers also were observed. Keywords: scandium orthosilicate, impurity ions, paramagnetic resonance.

Wideband EPR spectroscopy of Y-=SUB=-3-=/SUB=-Al-=SUB=-5-=/SUB=-O-=SUB=-12-=/SUB=- : Ho-=SUP=-3+-=/SUP=- crystals

EPR spectra of Ho3+ impurity ions were recorded in single crystals of yttrium aluminum garnet (Y3Al5O12) in the frequency range of 114-170 GHz, at a temperature of 4.2 K. The measurement results indicate the substitution of Ho3+ ions for Y3+ ions in the dodecahedral position with D2 local symmetry. The magnitude of the g-factor, the hyperfine structure constant, and the energy interval between the main and the first excited nondegenerate sublevels of the main multiplet 5I8 are determined. Weak satellite signals have also been registered, the origin of which is associated with the formation of antisite defects in the environment of Ho3+ paramagnetic centers. Keywords: hyperfine interaction, rare earth ions, spin Hamiltonian, non-Kramers ion.

Широкополосная ЭПР-спектроскопия кристалла Y-=SUB=-3-=/SUB=-Al-=SUB=-5-=/SUB=-O-=SUB=-12-=/SUB=-:Ho-=SUP=-3+-=/SUP=-

EPR spectra of Ho3+ impurity ions were recorded in single crystals of yttrium aluminum garnet (Y3Al5O12) in the frequency range of 114-170 GHz, at a temperature of 4.2 K. The measurement results indicate the substitution of Ho3+ ions for Y3+ ions in the dodecahedral position with D2 local symmetry. The magnitude of the g-factor, the hyperfine structure constant, and the energy interval between the main and the first excited nondegenerate sublevels of the main multiplet 5I8 are determined. Weak satellite signals have also been registered, the origin of which is associated with the formation of antisite defects in the environment of Ho3+ paramagnetic centers.

Light-induced pulsed dipolar EPR spectroscopy for distance and orientation analysis.

Measuring distances in biology at the molecular level is of great importance for understanding the structure and function of proteins, nucleic acids and other biological molecules and their complexes. Pulsed Dipolar Spectroscopy (PDS) offers advantages with respect to other methods as it is uniquely sensitive and specific to electronic spin centers and allows measurements in near-native conditions, comprising the in-cell environment. PDS methods measure the electron spin-spin dipolar interaction, therefore they require the presence of at least two paramagnetic centers, which are often stable radicals. Recent developments have introduced transient triplet states, photo-activated by a laser pulse, as spin labels and probes, thereby establishing a new family of techniques-Light-induced PDS (LiPDS). In this chapter, an overview of these methods is provided, looking at the chromophores that can be used for LiPDS and some of the technical aspects of the experiments. A guide to the choice of technique that can yield the best results, depending on the type of system studied and the information required, is provided. Examples of previous LiPDS studies of model systems and proteins are given. Characterization data for the chromophores used in these studies is tabulated to help selection of appropriate triplet state probes in future studies.

Paramagnetic centers in chromium-doped Na-=SUB=-5-=/SUB=-AlF-=SUB=-2-=/SUB=-(PO-=SUB=-4-=/SUB=-)-=SUB=-2-=/SUB=- single crystal

The paramagnetic resonance of Na5AlF2(PO4)2 single crystals with chromium impurity was studied. The Cr3+ centers have been found to replace the triclinic Al3+ positions. In the local coordinate system, the spectra of these centers were described by the spin Hamiltonian of rhombic symmetry. In addition, vanadium centers and unidentified triclinic symmetry centers have been observed. Keywords: impurity ions, aluminum fluoride phosphate, paramagnetic resonance.

High-frequency EPR spectroscopy of paramagnetic manganese centers in GaAs : Mn crystals

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs : Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+-SH complex, which is a manganese ion with spin S=5/2, which replaces gallium (MnGa2+) and an ionized acceptor (A-) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J=3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2- replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the <111> axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+-SH complex. Keywords: high-frequency EPR, GaAs crystal, Mn acceptor, shallow hole, exchange interaction.

High-frequency EPR spectroscopy of paramagnetic manganese centers in GaAs : Mn crystals

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs : Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+-SH complex, which is a manganese ion with spin S=5/2, which replaces gallium (MnGa2+) and an ionized acceptor (A-) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J=3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2- replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the <111> axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+-SH complex. Keywords: high-frequency EPR, GaAs crystal, Mn acceptor, shallow hole, exchange interaction.

Turn On/off Fluorescence in the Presence of Zn(ii) and Tb(iii) Metal Ions in The Schiff Base Complex

Zinc is one of the most essential trace elements in periodic table which is directly involved in several enzymatic and biological activities in human body. Research has mainly focused in the catalytic and biological applications of Zinc complex; however the fluorescence property and its co-relation with the 3d and 4f metal ions are hitherto unexplored. In the last decades several fluorescence active Schiff base (H2 L) was synthesised using o-vanillin and ethylene diamine in room temperature stirring. Herein, metal complexes of Zn(II) and Zn(II)-Tb(III) were synthesised and characterized. Photo physical properties of all these complexes were studied using fluorescence spectroscopy. Interestingly it has been found that the Zn(II) complex shows enhanced fluorescence intensity than the other complexes. However, the fluorescence intensity of Zn(II)-Tb(III) complex decreases compare to the Zn(II) complex. The chelation enhanced fluorescence plays the major role for the enhanced fluorescence intensity of the Zn(II) complex however the presence of paramagnetic metal centre in Zn(II)-Tb(III) complex decreases the overall fluorescence intensity as compare to the Zn complex.

Controlling and exploiting intrinsic unpaired electrons in metalloproteins.

Electron paramagnetic resonance spectroscopy encompasses a versatile set of techniques that allow detailed insight into intrinsically occurring paramagnetic centers in metalloproteins and enzymes that undergo oxidation-reduction reactions. In this chapter, we discuss the process from isolating the protein to acquiring and analyzing pulse EPR spectra, adopting a practical perspective. We start with considerations when preparing the protein sample, explain techniques and procedures available for determining the reduction potential of the redox-active center of interest and provide details on methodologies to trap a given paramagnetic state for detailed pulse EPR studies, with an emphasis on biochemical and spectroscopic tools available when multiple EPR-active species are present. We elaborate on some of the most commonly used pulse EPR techniques and the choices the user has to make, considering advantages and disadvantages and how to avoid pitfalls. Examples are provided throughout.

The impact of geological factors on the sorption capacity of gas and fat ranks of coal in the krasnoarmeyskiy district

At present, based on the research of the conditions and factors of outburst hazard of coal beds, various predicting methods have been developed grounded on contemporary views concerning the nature and mechanism of gas-dynamic phenomena. One of the main factors affecting the outburst hazard of coals is gas saturation, which is determined by the sorption capacity of coals. The sorption properties of coal matter are significantly determined by the degree of coal transformation (metamorphism). Therefore, the analysis of the data obtained for a coal matter of the same rank makes it possible to reveal the role and importance of factors that have formed the parameters of the signals observed by EPR (electron paramagnetic resonance) spectroscopy. These signals define the features of the sorption coal-gas interaction for a particular sample. It is established that the average values of the concentration of paramagnetic centers are (2.29 ± 0.36) and of contingency - (0.476 ± 0.02), and the calculated value of the sorption capacity of coals (24.3 ± 0.29) can be considered basic for gas and fat ranks of coals in the Krasnoarmeyskiy district. Minor deviations from the basic parameters can be caused by various geological and technological factors. Significant deviations of the average values for the sorption capacity of coals can be caused by tectonic processes. The purposeis to research the impact of geological factors on the sorption properties of gas and fat ranks of coals determined by the EPR method in the mines of the Krasnoarmeyskiy district. During the research, the following parameters were defined: 1) the integrated intensity of the EPR spectrum - the concentration of paramagnetic centers (Na) in the matter under research, which characterizes the conjoint impact of external factors on the coal matter, and 2) the contingency coefficient (Ksc), i.e. the number and status of the conjugate system in the molecular structure of a matter, which characterizes the transmitting depth of structural transformations (a degree of structuredness) of fossil organics at the nanolevel. As a result, it is found that various factors acting in different directions affect the calculated value of the sorption methane capacity of coals. The average sorption capacity of gas and fat ranks of coals from the Krasnoarmeyskiy district is 24.3 ± 0.29 and mainly depends on the deformation conditions. The average concentrations of paramagnetic centers are (2.29 ± 0.36), and of contingency (0.476 ± 0.02). The calculated value of the sorption capacity of coals (24.3 ± 0.29) can be considered as the base one for the gas and fat ranks of coals in the Krasnoarmeyskiy district. Minor deviations from the basic parameters can be caused by various geological and technological factors. Significant deviations in the average values of the coals sorption capacity can be caused by tectonic processes.