Number Of Paramagnetic Centers Research Articles

A DFT study of electronic structure, magnetic properties and cyclization reaction of [5]helicene derivatives

A series of 1,14-dimethyl-[5]helicene derivatives comprising radicals ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl, 1,2,3,5-dithiadiazolyl, 1,5-dimethyl-6-oxo-verdazyl and 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide) was investigated by quantum-chemical modeling of their electronic structure, exchange interactions and cyclization reaction. The performed DFT study of 1,14-dimethyl-[5]helicene, supported by DFT/CASSCF and DFT/NEVPT2 calculations, have shown that cyclic form of this compound represents a singlet biradical. The energy barriers of transition between open and cyclic forms of 1,14-dimethyl-[5]helicene radical derivatives are comparable to the values characteristic of enantiomerization reaction, which indicates a high probability of formation of cyclic isomers under thermal conditions. The TD DFT calculations of electronic transitions predicted the presence of non-overlapping bands in the studied isomers that favors switching between them by irradiation. The variation in a number of paramagnetic centers as a result of interconversion of open and cyclic forms, and the presence of structures bearing strong ferromagnetic exchange interactions, make the considered [5]helicene derivatives a suitable basis for molecular electronics devices.

A nitroxide-containing cathode material for organic radical batteries studied with pulsed EPR spectroscopy

An electron spin echo in a nitroxide-containing polymer cathode film for organic radical batteries is observed for various states of charge at cryogenic temperatures. The EPR-detected state of charge (ESOC), as inferred from the number of paramagnetic centers in the film, is compared to the results of Coulomb counting based on galvanostatic charging. Spin concentration, longitudinal relaxation times T1 and phase memory times Tm strongly correlate with the ESOC. In the discharged film, the spin concentration reaches 5±3×1020 cm−3, causing a phase memory time Tm≪ 100 ns (shorter than the resonator ring-down time) that hinders the detection of the spin echo. In the charged film, the decreased spin concentration results in a longer Tm between 100 ns and 300 ns that enables spin-echo detection, yet limits the length of the microwave pulse sequence. The short, broad-band pulses cause instantaneous diffusion in the unoxidized domains across the oxidized film, affecting the relative peak intensities in the pulsed EPR spectrum. By simulating the spectral distortion caused by instantaneous diffusion, we obtain information on the local spin concentration, which complements the information on the ‘bulk’ spin concentration determined by electrochemistry and continuous-wave EPR spectroscopy.

Electronic structure of carbon nanotube network

Covalently cross-linked carbon nanotube network has been synthesized using spark plasma sintering followed by nitric acid treatment. EPR investigation of its electronic structure in comparison with pristine carbon nanotubes has revealed that the covalent cross-linking leads to a decrease in the number of paramagnetic centers, while the oxidation results in an increase in their number. The oxidation affects the cross-linked and pristine materials in a different manner

Local electronic structure of carbon nanotubes consolidated by spark plasma sintering

Local electronic structure of carbon nanotubes (CNTs) consolidated by spark plasma sintering and further oxidized by nitric acid was studied by electron paramagnetic resonance (EPR), transmission electron microscopy, X-ray photoelectron and Raman spectroscopy. It was found that long time oxidation perforated and fragmented sintered CNTs and increased the oxygen content in the material causing an increase in the number of paramagnetic centers. Two components in the absorption EPR spectra were distinguished: A narrow and a broad line with different [Formula: see text]-factors, which testifies the different surrounding of paramagnetic centers associated with the lines. The results are of great importance for understanding the mechanism of nanotubes oxidation and development new types of catalysts, electromagnetic shielding materials and absorbents.

Application of the electronic paramagnetic resonance method for detecting radiation-processed food products

This paper presents the results of work on studying the capabilities of EPR spectroscopy for identifying certain types of food products treated with ionizing radiation at various doses, determining the dose and time elapsed since the product was treated with ionizing radiation. The samples were examined for 270 days. For the prepared samples, the post-effect of irradiation with ionizing radiation was studied, which manifested itself in a change in the number of paramagnetic centers in the sample.

Rice Husk Research: From Environmental Pollutant to a Promising Source of Organo-Mineral Raw Materials.

Rice husk is a large-tonnage waste left from rice production. It is not subject to humification and therefore becomes a serious environmental pollutant. Due to the presence of two essential elements—carbon and silicon—in its composition, rice husk is a promising organo-mineral raw material. The known methods for processing of rice husk are associated with the formation of even more aggressive waste. The creation of a waste-free technology for processing this plant material requires a detailed study. Rice husk of Kyzylorda oblast was studied using IR, SEM, TA, TPD-MS, EPR, and TEM methods. It was determined that under a temperature up to 500 °C, the ligno-carbohydrate component of rice husk decomposes almost completely. Three main peaks are recorded during the decomposition: hemicellulose at 200 °C, cellulose at 265 °C, and lignin at 350–360 °C. This process is endothermic. However, above of 300 °C the exothermic reactions associated with the formation of new substances and condensation processes in the solid residue begin to prevail. This explains the increase in the concentration of paramagnetic centers (PMCs) in products of rice husk carbonization in the range of up to 450 °C. Further increase in temperature leads to a decrease in the number of PMCs as a result of carbon graphite-like structures formation. The silicon–carbon product of rice husk carbonization (nanocomposite) is formed by interconnected nanoscale particles of carbon and silicon dioxide, the modification of which depends on the temperature of carbonization. The obtained data allow management of the rice husk utilization process while manufacturing products in demand based on ecofriendly technologies.

Development of an NMR Relaxometer for Determining Magnetization Dynamics of Water Protons in Living Tissues and Its Use for Evaluating Age-Related Changes

A desktop NMR relaxometer was created, which allows determining spin–spin relaxation times of water protons in living tissues. Spin relaxation times of protons of water reflect changes in the mean number of paramagnetic centers, since the presence of these centers increases the magnetization recovery rate of water protons in living tissues tens of times. In particular, this method allowed observing age-related changes (sarcopenia) related to substitution of connective tissue for muscle fibers. Relaxometers of this type are comfortable to use and are promising devices for analysis and diagnostics of changes and disorders of metabolic processes.

Schottky Anomalies in the Low-Temperature Specific Heat of YBa2Cu3Oy HTSC

The evolution of the Schottky anomalies in the temperature dependence of the specific heat (in the temperature range T = 2–10 K and the magnetic field range H = 0–9 T) of polycrystalline YBa2Cu3Oy HTSCs is studied when the oxygen content in coarse-crystalline samples changes and when the degree of structural disordering in optimally doped fine-crystalline samples of the same compound changes. The number of paramagnetic centers with uncompensated spin moments, which generate the Schottky anomalies, and the splitting of the lower energy levels are estimated. A correlation between these parameters and both the number of oxygen vacancies in chain planes and the method of oxygen ordering in chains is found. The experimental data are compared with the results of studying the magnetic susceptibility at T > Tc in the same samples. Based on this comparison, we can conclude that the nature of the Schottky centers is related to the formation of paramagnetic centers at breaks in the Cu1–O4 copper–oxygen chains in the basal planes of the crystal structure.

Change of parameters in molecular structure of Donbas coals under the influence of external factors

The molecular structure of coal is estimated by 13 parameters of infrared spectra, electron paramagnetic resonance, volatile yield and ash content. A new criterion of hydrophobicity is applied, which has shown its informativeness in analyzing the molecular structure of coals of various ranks. It is more sensitive to changes in coal rank than the standard index of volatile yield. The coalification process leads to a significant increase in this index due to the water release and other hydroxyl-containing compounds from the substance of coal. Two-way analysis of variance showed that the influence of the metamorphism factor is significant for 9 molecular parameters. The strongest effects are manifested in the sorption capacity, the criterion of hydrophobicity and the number of paramagnetic centers. The stratigraphic factor does not have a significant impact on one of the molecular parameters. Factor analysis by the method of principal components of the molecular structure of coal showed that the most significant independent factors are metamorphism and sedimentation conditions, the latter ones include independent processes: accumulation of mineral components, decomposition of biomass and geochemical environment during the sedimentation period.

Study of transplanted lichens with the use of physical methods: the case of Xanthoria parietina (L.) Th. Fr.

Electron paramagnetic resonance spectroscopy and inductively coupled plasma optical emission spectrometry were used to study the epiphytic lichen Xanthoria parietina (L.) Th. Fr. transplanted to natural and urbanized zones in the Moscow region. The combination of physical and lichenindication methods allowed us to determine periods as well as the magnitude of changes in the biophysical characteristics of the transplanted lichen in different environmental conditions. Electron paramagnetic resonance spectroscopy demonstrated that the number of paramagnetic centers in lichens transplanted to the natural zone did not change in comparison with initial value (2.6 ± 0.4)·1017 spin/mg) and remained stable after four weeks after transplantation. The number of paramagnetic centers in lichens transplanted to the urbanized zone increased by 1.1·1017 spin/mg by the end of the third week as compared with its initial value ((2.6 ± 0.4)·1017 spin/mg). Inductively coupled plasma optical emission spectrometry was used to measure concentration of iron, copper, manganese, magnesium, and zinc. We detected substantial changes in concentration of these elements during first four weeks after transplantation. For instance, concentration of copper in lichens transplanted to the urbanized zone increased from 7.5 ± 2.7 μg/g to 56.4 ± 8.6 μg/g by the end of the third week of the experiment.

Modern treatment technologies of carbon fibre for ensuring the high strength carbon fibre reinforced plastic production

The investigation results of physical and mechanical properties of carbon fibre and carbon fibre-reinforced plastics are presented. It is shown that the number of paramagnetic centres, which were determined by electron paramagnetic resonance method, were used as an index to define the chemical activity of the carbon fibre relative to the matrix. The number of paramagnetic centres in the carbon fibres and CFRP were analysed. The interlayers shear strength of CFRP based on epoxy resin was investigated.

Copper Sorption by Coal Substances from Aqueous Solutions

<p>The copper cations sorption by brown coal, humic acids and residual coal samples after the extraction of humic acid was studied by using the methods of spectroscopy FT-IR, EPR, Solid State CP/MAS <sup>13</sup>C-NMR. Samples of lignite (brown coal) of the Tisulsky deposit (TL) of Kansk-Achinsk brown coal basin (Russia), its naturaloxidized form (NOLF), as well as samples of the humic acids (HA) extracted from them, and the residual coal after the extraction of humic acids (RC) were used as sorbents. The copper cations sorption process by the investigated samples depends on contribution of several mechanisms in varying degrees. It may be an ion exchange; complexation with the functional groups of the electron donor; complexation to form the bond Me<sup>z+</sup> – HA due to the presence of the free π-electrons on the surface. The quantity of adsorbed copper cations depends on the functional and structural composition of the investigated samples. The copper cations sorption by the coals, humic acids and residual coals reduces the number of paramagnetic centers in the samples and the intensity of the NMR spectra. The copper cations sorption by HA from TL is determined by the significant contribution of the ion exchange process. HA from NOLF; initial TL and NOLF; RC TL and RC NOLF sorb copper cations mainly due to the formation of complexes with individual functional groups and the negatively charged surface portions. The data of <sup>13</sup>C-NMR and EPR studies have shown that the aromatic structures make the main contribution to the complexation.</p>

Application of Electron Paramagnetic Resonance to Study of Gallstones

We present the results of an electron paramagnetic resonance (EPR) study of mixed cholesterol gallstones. We have established that free radicals are distributed nonuniformly within the interior of the stone. The type and number of paramagnetic centers depend on the pigment content in the selected layer. We show that the parameters of the sextet lines in the EPR spectrum of the pigment are close to the parameters of lines in the spectrum of a brown pigment stone.

Biomonitoring of indicator species of lichen by EPR spectroscopy

EPR spectroscopy is beginning to occupy a key position in the field of quality control environment. The analysis of the number of paramagnetic centers in the thallus of indicator species of lichen (in a tolerant and sensitive species) was carried out. The number of paramagnetic centers in the lichen thallus was found to depend on the intensity of human impact on habitats of the species studied and their taxonomic affiliation. Possible mechanisms of resistance to atmospheric pollution for tolerant species df lichens were discussed.

DEER Distance Measurements on Proteins

Distance distributions between paramagnetic centers in the range of 1.8 to 6 nm in membrane proteins and up to 10 nm in deuterated soluble proteins can be measured by the DEER technique. The number of paramagnetic centers and their relative orientation can be characterized. DEER does not require crystallization and is not limited with respect to the size of the protein or protein complex. Diamagnetic proteins are accessible by site-directed spin labeling. To characterize structure or structural changes, experimental protocols were optimized and techniques for artifact suppression were introduced. Data analysis programs were developed, and it was realized that interpretation of the distance distributions must take into account the conformational distribution of spin labels. First methods have appeared for deriving structural models from a small number of distance constraints. The present scope and limitations of the technique are illustrated.

EPR spectroscopic studies in (30 − x) Li2O–xK2O–10CdO–59B2O3–1MnO2 multi-component glass system

EPR studies were carried out in (30–x) Li2O–xK2O–10CdO–59B2O3–1MnO2 multi-component glass system to understand the effect of the variation in the alkali ratios on the EPR parameters. The observed EPR spectra of Mn2+ ion exhibits resonances at g=2.0, 3.3 and 4.3. The resonance at g=2.0 is due to Mn2+ ions in an environment close to the octahedral symmetry, where as the resonances at g=3.3 & 4.3 are due to the rhombic surroundings of Mn2+ ions. Hyperfine splitting constant values at g=2.0 and number of paramagnetic centers & paramagnetic susceptibility at different observed resonances were evaluated. These parameters show non linear variation with progressive substitution of Li+ ion with K+ ions may be due to the changes in cation field strengths and local structural variation due to the variation in mixed alkali ion ratios.

Optical absorption and EPR spectroscopic studies of (30 − x)Li 2O– xK 2O–10CdO–59B 2O 3–1Fe 2O 3: An evidence for mixed alkali effect

Optical absorption and EPR spectroscopic studies were carried on (30 − x)Li 2O– xK 2O–10CdO–59B 2O 3–1Fe 2O 3 ( x = 0–30) glass system to understand the effect of progressive doping of Li + ion with K + ion. Optical absorption results show typical spectra of Fe 3+ ions and the various optical parameters such as, optical band gap, Urbach energy, oxide ion polarizability, optical basicity and interaction parameter were evaluated from the experimental data. The observed optical band gap and Urbach energy values show large deviation from the linearity where as the other parameters show small deviation from the linearity with the progressive substitution of Li + ions with K + ions. The observed EPR spectra are representative of Fe 3+ ion in octahedral and axial fields in the glass network. The number of paramagnetic centers and paramagnetic susceptibility values were evaluated at different resonance lines for all the specimens and these parameters show non-additive nature with the progressive substitution of Li + ions with K + ions in the glass network. This is first ever observation of mixed alkali effect (MAE) in EPR and optical parameters of mixed alkali borate glasses.

Evolution of the optical, magnetic and morphological properties of PVA films filled with CuSO 4

Polyvinylalcohol (PVA) films filled with different concentrations of CuSO 4 were prepared by casting method. The optical, magnetic and morphological properties of these films were intensively investigated in this study. The optical absorption spectra were performed. The assignment of the main absorption peaks was done. The optical parameters such as the absorption coefficient, the electronic band structure, the band tail and the energy gap were estimated. The characteristic features of the electron paramagnetic resonance (EPR) spectra were discussed. The dependence of the g values, the hyperfine coupling constant, the peak to peak linewidth, the number of paramagnetic centres and the asymmetry ratio on filling were studied. The morphology of the polymeric films demonstrated structural modifications with filling. A correlation between the optical, magnetic and morphological properties was accomplished. Three types of Cu 2+ were depicted in this study and were accompanied by three regions of filling, low and high FLs as well as an intermediate one, of interesting physical properties. The studied samples revealed significant changes of the physical properties with filling. This indicated the high sensitivity of these samples to filling and suggested their applicability in magnetic and optical devices.

EPR and optical absorption studies of Cu 2+ and VO 2+ impurities in diammonium d-tartarate single crystal grown in ammonium dihydrogen phosphate buffer

The study is extended by growing the Cu 2+ and VO 2+ doped diammmonium d-tartarate single crystal in buffered solution with different pH values and the results are discussed referring to the color center forming property by means of transition metal impurities which are stable point defects. XRD study shows that the crystals grown in ammonium dihydrogen phosphate buffer with pH value of 5.9 which is smaller than that of water, the monoclinic symmetry of the crystal changes to orthorhombic and the number of paramagnetic centers is doubled. The spin-Hamiltonian parameters are determined and the ground state wave functions of Cu 2+ ions are constructed. The optical absorption spectrum of both Cu 2+ and VO 2+ doped single crystals show three absorption bands corresponding to the splitting of d orbitals of metal ions in octahedral environment with tetragonal distorsion. Two of the bands lie in visible range and third one lies in infrared region.

Modification of nitroxyl contrast agents with multiple spins and their proton T1 relaxivity

The purpose of this study is to test the performance of multispin nitroxyl contrast agents in improving the sensitivity of MR detection for nitroxyl contrast agents. The relation between T(1) relaxivity and the number of paramagnetic centers in a molecule was investigated. Compound 1 is a single molecule of methoxycarbonyl-PROXYL (MC-PROXYL). Two and three MC-PROXYL molecules were chemically coupled to obtain Compounds 2 and 3, which have two and three nitroxyl spins in the molecule, respectively. A good linear relation, the slope of which increased depending on the number of nitroxyl spins in the molecule, was obtained between T(1)-weighted (fast low-angle shot) MR image contrast enhancement at 7 T and the concentration of nitroxyl contrast agents. T(1)-weighted MR image contrast enhancement and T(1) relaxivity levels of nitroxyl contrast agents were increased depending on the number of nitroxyl spins in the molecule. Multicoupling nitroxyl molecules can enhance the T(1)-weighted contrast effect while maintaining the quantitative behavior of the molecule for up to three spins.