Types Of Paramagnetic Centers Research Articles

Acene-Linked Zethrenes and Bisphenalenyls: A DFT Search for Organic Tetraradicals.

Polycyclic aromatic hydrocarbons are of special interest due to their promising nonlinear optical and magnetic properties. A series of acene-linked zethrenes and bisphenalenyls comprising from five to nine benzene rings in the linker group have been computationally studied by the DFT UB3LYP/6-311++G(d,p) quantum-chemical modeling of their electronic structure, possible spin states, and exchange interactions. The zethrenes with octacene and nonacene linkers as well as bisphenalenyls comprising heptacene, octacene, and nonacene linker groups have been revealed to possess tetraradicaloid nature, which makes them promising building blocks for organic optoelectronic and spintronic devices. The results obtained open a way of constructing tetraradicaloid organic molecules characterized by the presence of two types of paramagnetic centers.

Paramagnetism and super paramagnetism of nanocrystalline titanium dioxide powders

Studies of magnetization of undoped and Mn-doped nanosized titanium dioxide powders showed that its dependence on the magnetic field can be approximated by the function that is a linear combination of two Langevin functions with different parameters. The linear part along with the diamagnetic contribution typical for metal oxides is due to the contribution of two types of paramagnetic centers. The first one is present in both undoped and doped powders. As follows from the published models, the centers are complex lattice defects Ti+3 ions + O− vacancies. The second is the vast majority of the manganese ions (and related oxide vacancies) in the doped powders that are uniformly distributed over all nanoparticles. The nonlinear part, even present in the undoped samples, is caused by the contribution of a small number of regions with a high magnetic moment (ferromagnetic clusters). These regions do not interact magnetically, and the powder as a whole exhibits superparamagnetic behavior. According to our estimates, the introduction of Mn into the TiO2 lattice mainly leads to the predominance of the paramagnetic contribution to the magnetization over the diamagnetic contribution characteristic of the TiO2 lattice and a small increase in the number of the ferromagnetic clusters.

ИЗУЧЕНИЕ МЕТОДОМ ЭЛЕКТРОННОГО ПАРАМАГНИТНОГО РЕЗОНАНСА ВЛИЯНИЯ ФИЗИКО-ХИМИЧЕСКИХ И СТРУКТУРНЫХ ХАРАКТЕРИСТИК КОСТНОЙ ТКАНИ НА ПРОЦЕССЫ ОСТЕОИНТЕГРАЦИИ ДЕНТАЛЬНЫХ ИМПЛАНТАТОВ С РАЗЛИЧНОЙ МИКРОСТРУКТУРОЙ ПОВЕРХНОСТИ

Aim. Study by the method of electronic paramagnetic resonance (EPR) physical- chemical and structural features of bone tissue and their effect on the processes of osseointegration during dental implantation using implants with different macro-microstructure of the surface.
 Material and methods. The experimental study was conducted on 7 Vietnamese visobryu-pig mini-pigs. Under anesthesia, implants with different macro-microstructures of the surface (SLA, RBM, ) were installed under the methods of direct and delayed implantation. At different dates of the experiment, after the appropriate anesthesia, the gentle method was to remove implants with small fragments of periemplant bone tissue. Macro-drugs of bone tissue of the jaws after sawing on blocks and extraction of implants, as well as appropriate preparation, were investigated by the method of stationary electronic paramagnetic resonance.
 Results. As a result of the study it was revealed that the insertation of the implant into the jaw bone leads to structural changes in the surrounding bone implant. After prior exposure, the EPR spectrums, caused by different types of paramagnetic centers, were observed in the samples studied. Two types of centers, , have been identified. and F is the center. The spectrum of the second type reflects the presence in hydroxiapatites of lattice defects associated with isomorphism, and, . The findings suggest that one of the reasons for the higher content of free radicals (SR) in bone tissue derived from radiation, after dental implantation is the weakening of chemical bonds, greater mobility of fragments that make up its structure. It must be assumed that the high content of SR in the bone in the postoperative period is nothing but a result of the disruption of the mineralization process, which is accompanied by the replacement of phosphate groups with carbonate ions that are free-radicalized. It has also been revealed that the structural integrity of the bone depends to a large extent on the degree of orderliness of microcrystals of hydroxyapatite. According to EPR, the most favorable bone recovery (the process of osseointegration) occurs when using implants with a surface, .
 Conclusions. EPR research of native and carbonate radicals is a unique tool for studying the physical and chemical and structural features of bone tissue and their role in the process of osseointegration of implants. The speed of osseointegration for implants with different microstructures of the surface varies. EPR can be used as an additional method to control the integration of implants into bone tissue.

X-Ray Diffraction and Multifrequency EPR Study of Radiation-Induced Room Temperature Stable Radicals in Octacalcium Phosphate.

Octacalcium phosphate (OCP) {Ca8H2(PO4)6×5H2O] has attracted increasing attention over the last decade as a transient intermediate to the biogenic apatite for bone engineering and in studies involving the processes of pathological calcification. In this work, OCP powders obtained by hydrolysis of dicalcium phosphate dehydrate were subjected to X- and γ-ray irradiation and studied by means of stationary and pulsed electron paramagnetic resonance at 9, 36 and 94 GHz microwave frequencies. Several types of paramagnetic centers were observed in the investigated samples. Their spectroscopic parameters (components of the g and hyperfine tensors) were determined. Based on the extracted parameters, the induced centers were ascribed to H0, CO33-, CO2- and nitrogen-centered (presumably NO32-) radicals. The spectroscopic parameters of the nitrogen-centered stable radical in OCP powders were found to be markedly different from those in hydroxyapatite. According to X-ray diffraction data, γ-ray irradiation allowed the phase composition of calcium phosphates to change; all minor phases with the exception of OCP and hydroxyapatite disappeared, while the OCP crystal lattice parameters changed after irradiation. The obtained results could be used for the tracing of mineralization processes from their initiation to completion of the final product, identification of the OCP phase, and to follow the influence of radiation processes on phase composition of calcium phosphates.

Formation of different types of paramagnetic centers in the alanine dosimeters exposed to alpha and gamma radiation - Study by EPR spectroscopy

EPR spectra of alanine dosimeters irradiated by gamma-rays and by alpha-particles were measured at different microwave power. The spectra were decomposed into three components corresponding to different stable radiation-induced radicals. Different ratio of the components was obtained for gamma- and alpha-irradiated alanine. The observed effect can be used for discrimination of the contribution of alpha- and gamma-radiation in mixed radiation field. That may be useful for control of dose at phantom experiments with neutron boron capture therapy, performed in mixed neutron-gamma field in which alpha-particles are emitted through neutrons capture reaction with boron.

Measuring transverse relaxation in highly paramagnetic systems

The enhancement of nuclear relaxation rates due to the interaction with a paramagnetic center (known as Paramagnetic Relaxation Enhancement) is a powerful source of structural and dynamics information, widely used in structural biology. However, many signals affected by the hyperfine interaction relax faster than the evolution periods of common NMR experiments and therefore they are broadened beyond detection. This gives rise to a so-called blind sphere around the paramagnetic center, which is a major limitation in the use of PREs. Reducing the blind sphere is extremely important in paramagnetic metalloproteins. The identification, characterization, and proper structural restraining of the first coordination sphere of the metal ion(s) and its immediate neighboring regions is key to understand their biological function. The novel HSQC scheme we propose here, that we termed R2-weighted, HSQC-AP, achieves this aim by detecting signals that escaped detection in a conventional HSQC experiment and provides fully reliable R2 values in the range of 1H R2 rates ca. 50–400 s−1. Independently on the type of paramagnetic center and on the size of the molecule, this experiment decreases the radius of the blind sphere and increases the number of detectable PREs. Here, we report the validation of this approach for the case of PioC, a small protein containing a high potential 4Fe-4S cluster in the reduced [Fe4S4]2+ form. The blind sphere was contracted to a minimal extent, enabling the measurement of R2 rates for the cluster coordinating residues.

EPR Evidence for Dynamic Rearrangements of Vanadium Paramagnetic Centers on the Surface of V-Doped Titanium Dioxide

Photocatalytic activity of vanadium modified Hombikat UV 100 TiO2 was investigated in of the Rhodamine B photodegradation in a wide range of vanadium content. Structure and properties of VO2+ paramagnetic centers (PCs) formed on TiO2 surface were studied by X-band electron paramagnetic resonance (EPR). Two types of PCs, aggregated and isolated ones, were revealed and described quantitatively. EPR results in comparison with photocatalytic data allowed conclude that the most probable catalytically active centers are the isolated vanadium centers on the V-doped TiO2 surface.

EPR Spectroscopic Examination of Different Types of Paramagnetic Centers in the Blood in the Course of Burn Healing.

The multicomponent electron paramagnetic resonance spectra of the blood during healing of skin burned wounds treated with a new generation biodegradable dressings containing poly(lactide-co-glycolide) were analysed. The evolution of different types of paramagnetic centers in the blood with time of healing was determined. The EPR spectra of the blood samples at 230 K temperature were measured at 1, 10, and 21 days after burning of the pig skin. The EPR lines of the following paramagnetic centers: the high-spin Fe3+ in methemoglobin (line I), high-spin Fe3+ in transferrin (line II), and Cu2+ in ceruloplasmin and free radicals (line III) were observed in the X-band (9.3 GHz) spectra of the blood. The multicomponent structure of the EPR spectra of the tested blood samples depended on the time of the healing of the burned wounds. The amount of the high-spin Fe3+ in methemoglobin (line I) in the blood decreased after 21 days of the healing of the burned wounds. The amount of the high-spin Fe3+ in transferrin (line II) slightly increased after 21 days of therapy with the basis. The amount of Cu2+ in ceruloplasmin and free radicals (line III) in the blood was very high after 10 days of therapy. At the first day of the healing of the burned wounds, the highest amount of the high-spin Fe3+ in methemoglobin (line I), the relatively lower amounts of the high-spin Fe3+ in transferrin (line II), and Cu2+ in ceruloplasmin and free radicals (line III) existed in the blood. In the medium phase (after 10 days) of the healing of the burned wounds, the extremely higher amounts of Cu2+ in ceruloplasmin and free radicals (line III) appeared in the blood. In the last phase (after 21 days), only the low differences between the amounts of the high-spin Fe3+ in methemoglobin (line I), the high-spin Fe3+ in transferrin (line II), and Cu2+ in ceruloplasmin and free radicals (line III) were observed. The present study may serve as a starting point for the development of a new technique for monitoring molecular complexes containing iron Fe3+ (methemoglobin, transferrin) or copper Cu2+ ions (ceruloplasmin) and free radicals in the blood during wound healing.

Synthesis of TiO2/ZrO2/SiO2 powders and their structural, optical and photocatalytic properties

Ternary oxide powders based on titania, zirconia and silica have been synthesized by the sol–gel method. The characterization of the powders was performed using UV–Vis spectroscopy, EPR, X-ray diffraction and Raman spectroscopy. The XRD study indicated the formation of TiO2 nanocrystals in anatase phase and titanium zirconate as srilankite phase. The band gap energy values of ternary powders are in the range of 3.2–3.3 eV. It is found that different types of paramagnetic centers were formed depending on silica sources after high-energy irradiation. As shown by the electron paramagnetic resonance study, the stability of SiO2 against β-irradiation is observed only for TiO2/ZrO2/SiO2 powder obtained from tetraethoxysilane as well as the formation of oxygen vacancies in TiO2 structure. Photocatalytic activity of TiO2/ZrO2/SiO2 powders is investigated in the process of photoreduction of Cr(VI) ions to Cr(III). As shown, the efficiency of photocatalyst depended on the nature of defect sites of the nanocomposites’ surface. It is found that the TiO2/ZrO2/SiO2 system obtained using tetraethoxysilane exhibited the highest photocatalytic activity as a result of the certain defect sites formation after high-energy irradiation.

Investigation of Photoelectron Properties of Polymer Films with Silicon Nanoparticles

Hybrid samples consisting of polymer poly-3(hexylthiophene) (P3HT) and silicon nanoparticles were prepared. It was found that the obtained samples were polymer matrixes with conglomerates of silicon nanoparticles of different sizes (10–104 nm). It was found that, under illumination, the process of nonequilibrium charge carrier separation between the silicon nanoparticles and P3HT with subsequent localization of the hole in the polymer can be successfully detected using electron paramagnetic resonance (EPR) spectroscopy. It was established that the main type of paramagnetic centers in P3HT/silicon nanoparticles are positive polarons in P3HT. For comparison, samples consisting only of polymer and silicon nanoparticles were also investigated by the EPR technique. The polarons in the P3HT and Pb centers in the silicon nanoparticles were observed. The possibility of the conversion of solar energy into electric energy is shown using structures consisting of P3HT polymer and silicon nanoparticles prepared by different methods, including the electrochemical etching of a silicon single crystal in hydrofluoric acid solution and the laser ablation of single-crystal silicon in organic solvents. The results can be useful for solar cell development.

Pulsed EPR Methods to Study Biomolecular Interactions.

Orthogonal site-directed spin labelling in combination with pulsed EPR spectroscopy is a powerful approach to study biomolecular interactions on a molecular level. Following a surge in pulse EPR method development, it is now possible to access distance distributions in the nanometre range in systems of complex composition. In this article we briefly outline the necessary considerations for measurements of distance distributions in macromolecular systems labelled with two or more different types of paramagnetic centres. We illustrate the approach with two examples: an application of the Double Electron-Electron Resonance (DEER) method on a triple spin-labelled protein dimer labelled with nitroxide and Gd(<small>III</small>), and an optimisation study of the Relaxation Induced Dipolar Modulation Enhancement (RIDME) experiment for the orthogonal spin pair Cu(<small>II</small>)-nitroxide.

Kinetics of the thermal disappearance of radicals formed during the radiolysis of L-α-anhydrous asparagine

An EPR study of paramagnetic centers formed by irradiation of polycrystalline L-α-anhydrous asparagine (L-Asn) was performed. The EPR spectra of gamma irradiated samples at room temperature, shown the presence of three types of paramagnetic centers. A possible mechanisms of formation for the three radical species is suggested, based also on literature data. The kinetics of the disappearance of radicals during thermal annealing indicated a complex mechanism.

Identification of a Slowly Relaxing Paramagnetic Center in Graphene Oxide

We demonstrate the existence of two types of paramagnetic centers in a pure graphene oxide. Saturation features of the electron paramagnetic resonance (EPR) spectrum in the temperature range of 4.2–300 K reveal that one of these centers has the spin–lattice relaxation time longer than 1.6 μs at room temperature. The spectrum of these centers consists of a central line and two satellite lines. The satellite lines result from the forbidden transitions in the hyperfine structure induced by protons in the vicinity of the slowly relaxing centers. The observability of the satellites can be used as a characteristic signature of graphene oxide purity. Since the number of slowly relaxing centers is reduced during chemical reduction of graphene oxide, this type of centers can be attributed to the isolated unfunctionalized carbons in the highly functionalized regions of GO. The second type of paramagnetic centers has shorter relaxation times and dominates in the reduced graphene oxide.

Reply to 'Comment on "Angstrom-scale probing of paramagnetic centers location in nanodiamonds by 3He NMR at low temperatures"' by A. Shames, V. Osipov and A. Panich, Phys. Chem. Chem. Phys. 2018, 20, DOI.

Shames et al. made a comment on our article (DOI: 10.1039/C7CP05898E) stating that their experience in EPR studies of detonation nanodiamonds suggests the existence of two main types of paramagnetic center in detonation nanodiamonds which questions our results. In this reply we provide insights into why there is only one main type of paramagnetic centers detected in nanodiamonds used in this work, which validates the correctness of the proposed original method to determine the distances between paramagnetic centers and nanoparticle surfaces by 3He NMR.

EPR and optical spectroscopy of Yb3+ ions in hexagonal perovskite RbMgF3 crystals

EPR and optical spectroscopy of trigonal Yb3+ centers in RbMgF3 are reported. The results of these experiments make it possible to conclude that Yb3+ ions replace Mg2+ host cations at two different sites forming three different types of paramagnetic centers. The empirical schemes of the energy levels for trigonal paramagnetic centers of Yb3+ ions have been established and parameters of the corresponding crystal fields have been determined. Structural models of the observed centers are proposed.

Enhancement of room temperature ferromagnetism in tin oxide nanocrystal using organic solvents

The effect of organic solvents (ethanol & ethylene glycol) on the room temperature ferromagnetism in nanocrystalline tin oxide has been studied. The samples were synthesized using sol-gel method with the mixture of water & organic liquid as solvent. It is found that pristine SnO2 nanocrystal contain two different types of paramagnetic centres over their surface:(i) surface chemisorbed oxygen species and (ii) Sn interstitial & oxygen vacancy defect pair. The magnetic moment induced in the as-prepared samples is mainly contributed by the alignment of local spin moments resulting from these defects. These surface defect states are highly activated by the usage of ethylene glycol solvent rather than ethylene in tin oxide nanostructure synthesis. Powder X-ray diffraction, transmission electron microscope imaging, energy dispersive spectrometry, Fourier transformed infrared spectroscopy, UV–vis absorption spectroscopy, photoluminescence spectroscopy, vibrating sample magnetometer measurement and electron spin resonance spectroscopy were employed to characterize the nanostructured tin oxide materials.

EPR study of ordered Al2O3-based aerogel

Samples of oriented aerogel based on aluminum oxide are studied by the electron paramagnetic resonance (EPR) technique under steady-state and pulse conditions. At least two types of paramagnetic centers interacting with Al nuclei on the surface of aerogel are revealed. Their spectra are characterized by the presence or absence of superhyperfine structure in the EPR spectra, respectively. The X-ray irradiation at room temperature gives rise to the formation of additional long-lived paramagnetic centers of the second kind. Their characteristic decay times for the “fast” and “slow” processes are determined. The interaction of induced paramagnetic centers with protons located on the surface of aerogel is revealed.

Analysis of paramagnetic centers for threevalent iron in aluminosilicates

We present the results of investigation of the defects in fluorine aluminosilicates from the Volyn-field Al2 [SiO4][F,OH]2 by the Electron Paramagnetic Resonance (EPR) method. The studies were carried out on the spectrometer Bruker ER 220D. Three types of EPR spectra of single centers were obtained. Their angular dependence was also investigated. The obtained EPR spectra correspond to the paramagnetic ion Fe3+ in the high-spin state S = 5/2. Three types of paramagnetic centers were found: one with cubic-symmetry and two with orthorhombic- symmetries.

Recharge processes of paramagnetic centers during illumination in nitrogen-doped nanocrystalline titanium dioxide

Nitrogen-doped titanium dioxide (N-TiO2) has been investigated by the EPR-technique. Two types of paramagnetic centers – N• and NO•-radicals – were detected in the samples. Both N• and NO•-related centers are recharged during illumination. Band diagrams of TiO2 with N• and NO• radicals energy level position are proposed.

Electronic structure, magnetic and optical properties of the Ba7(BO3)4−xF2+3x crystal

Large size Ba7(BO3)4−xF2+3x (x=0.49) single crystals of optical quality, up to 20mm in size, were grown. Crystals exhibit a wide transparent spectral range from 0.215 to 6.6μm with the indirect energy band gaps of 4.97 and 5.09eV at 300 and 80K, respectively. X-ray irradiation can induce strong absorption in crystals in the UV to near IR regions, mainly from four types of paramagnetic centers: two hole-type centers where oxygen near some defect is ionized (OX1- or O1-) and two electron-type centers where fluorine vacancies capture electrons (e6- and e4-). The electron spin resonance analysis and first-principles studies identified the detailed optical transitions in these centers. The induced absorption can be removed by illumination with photon energy 2.5<hν<4.5eV or after heating to 450K. The electronic structure calculations reveal that in Ba7(BO3)4−xF2+3x both the top of valence bands and the bottom of conduction bands are composed of B 2p and O 2p orbitals. The linear and nonlinear optical properties are dominantly determined by the electron transition within (BO3)3− groups. Nonlinear susceptibility and birefringence are predicted to be d31=0.14pm/V and 0.016 at x=0.25 and decrease to 0.01 and 0.011, respectively, as x grows to 0.75, due to the increase of F content in Ba7(BO3)4−xF2+3x.