Paramagnetic Centers Research Articles

Film-electrochemical EPR spectroscopy to investigate electron transfer in membrane proteins in their native environment.

Film-electrochemical electron paramagnetic resonance spectroscopy (FE-EPR) enables simultaneous electrochemical and spectroscopic characterisation of paramagnetic electron-transfer centres, including in soluble proteins. We now report a modified set-up FE-EPR with tuneable macroporous working electrodes and demonstrate the feasibility to investigate electron transfer in membrane proteins in their native membrane environment.

Effects of exchange-covalent bonds on the orbital moment of paramagnetic centers with unfilled 3d-shells

Effects of exchange-covalent bonds on the orbital moment of paramagnetic centers with unfilled 3d-shells

Binucleating Jäger-type {(N2O2)2}4- ligands: magnetic and electronic interactions of Fe(II), Ni(II) and Cu(II) across an in-plane TTF-bridge.

The simultaneous presence of different electrophores provides an interesting playground for responsive materials. Herein, we present the incorporation of a twice-reversibly oxidizable tetrathiafulvalene (TTF) unit into a binucleating ligand, bridging two metal centers in a fully conjugated plane. A two-step synthesis scheme gave the D2h symmetric Schiff base-like ligand H4L in moderate yields from which the corresponding copper(II) [Cu2L], nickel(II) [Ni2L], [Ni2L(py)4] and iron(II) complexes [Fe2L(py)4], [Fe2L(dmap)4] and [Fe2L(bpee)2]·1 Tol could be obtained. Characterization was performed through 1H-NMR, IR, UV-vis and 57Fe-Mössbauer spectroscopy, SQUID magnetometry and cyclic voltammetry, supported by density functional theory (DFT) calculations. Single crystal X-ray analysis of [Ni2L(py)4] revealed six-coordinate paramagnetic centers, whereas [Ni2L] underwent gradual coordination induced spin state switching (CISSS) in solution. The magnetic independence of both metal centers is echoed by close-to-ideal Curie-plots of the [Cu2L] system and the gradual spin crossover of all iron(II) compounds. By contrast, cyclic voltammetry measurements in solution indicated oxidation-dependent TTF-metal interactions, as well as metal-metal interactions. The reversible TTF-borne events in H4L and [Ni2L] are overlaid with metal-borne events in the case of [Fe2L(py)4], as is corroborated by an analysis of the frontier orbital landscapes and through diagnostic spectral features upon chemical oxidation.

Exploring spin multiplicity in MoS2.

The study of native point-defect-induced spin centres and the synergy of their origin and dynamics are key factors for developing the next-generation spintronics and quantum technologies using quasi-2D transition-metal dichalcogenides (TMDCs). With the help of low-temperature electron paramagnetic resonance (EPR) measurements and first-principles calculations within density functional theory (DFT), herein we report for the very first time the presence of high-spin paramagnetic centres Mo3+ and Mo2+ in sulfur-deficient hexagonal molybdenum disulfide (2H-MoS2-x) nanocrystals. This in fact opposes the established notion of spin S = 1/2 mediated by Mo5+ centres reported so far. The intrinsic lattice strain generated in the nanostructure was found to play a crucial role for such spin localization in this layered material. By performing spin-echo measurements, we find that molybdenum interstitials (S = 3/2) possess the shortest spin-lattice relaxation time (T1) as compared to the sulfur (S = 3/2) and oxygen vacancies (S = 1/2). Moreover, the temperature-dependent T1 measurements revealed a direct process for the spin-lattice relaxation of interstitial defects and a Raman process for the vacancy sites.

Modulation of single-chain magnet behaviour in a heterometallic Fe2Co cyanide-bridged 2D sheet.

A cyanide-bridged Fe2Co 2D sheet exhibiting electron transfer coupled spin transition (ETCST) with co-existence of magnetic ordering below 50 K is reported. The complex exhibits single-chain magnet behaviour where the uncoordinated water molecules act as an exchange-breaking impurity by allowing only a fraction of the molecule to undergo a spin state change. The paramagnetic centres prevail throughout the chain on desolvation, thereby increasing the number of correlated units in the chain.

Relating Paramagnetic Properties to Molecular Parameters of Humic Acids Isolated from Permafrost Peatlands in the European Arctic.

Free radicals (FRs) are intermediate participants in the transformation process of soil organic matter, and free radical activity is a fundamental property of humic substances. The aim of this work was to conduct a comparative study of the paramagnetic properties of humic acids (HAs) isolated from Histosols by electron paramagnetic resonance (EPR) spectroscopy. The studied Histosols are found in permafrost peatlands in four natural geographic subzones of the European Arctic (from forest tundra to northern tundra). The results obtained showed that in anaerobic conditions on the peatlands in the tundra zone, the formation of semiquinone-type radicals occurs through the reduction of quinone fragments of HAs and leads to an increase in the concentration of paramagnetic centres within HAs. PCA analysis allowed us to reveal relationships between the properties of the initial raw peat samples, the molecular composition of the isolated HAs, and their paramagnetic parameters. It was found that FR localization occurs predominantly on aromatic fragments of lignin nature, which are confined to the low molecular weight fraction of HAs. The g-factor values of the EPR spectra of HAs indicate the presence of carbon- and oxygen-centred FRs in the HA structure, with a predominance of the latter.

EPR of LiNaGe4O9:Mn crystal

EPR spectra of manganese ions were studied in lithium-sodium tetragermanate LiNaGe4O9 crystal. The observed spectrum was typical for doping ions in tetravalent Mn4+ state (3d 3, S=3/2) and for main orientations of static magnetic field along the crystal axes (T=296 K) consisted of hyperfine sextuplet from electronic transition Ms= +1/2 ↔ – 1/2. Transitions Ms= ±3/2 ↔±1/2 were not detected owing to strong interaction of the spin system with crystal field as compared with electron Zeeman interaction in the X frequency range of EPR registration. The angular variations of the spectrum were measured for static field B rotated in three main crystal planes. Since only central transition (Ms= +1/2 ↔ – 1/2) was detected and the splitting of the fine structure remained unknown, the spectrum was described in the approximation of effective spin Seff=1/2. The parameters of the spin Hamiltonian, including electron Zeeman and hyperfine interactions, were calculated. The magnetic multiplicity of the spectrum (km=2) evidenced that paramagnetic centers occupied the sites with monoclinic C2 point symmetry. Taking into consideration the positional symmetry as well ionic charges and radii of the guest ion and the hosts, it was concluded that paramagnetic Mn4+ centers substituted for Ge4+ within oxygen octahedral complexes in the crystal lattice of LiNaGe4O9.

Energy state of the electronic subsystem of porous carbon material caused by laser irradiation

The changes in the electronic subsystem of a porous carbon material caused by its doping with Mn, Cr, and Fe and laser irradiation were studied by the method of electron paramagnetic resonance. It has been found that Mn doping leads to the appearance of several paramagnetic centers, and laser irradiation facilitates the redistribution of electrons between different states so that their mobility increases due to the transition from the g = 3.9 to the g = 6.0 state. The Mössbauer spectroscopy of porous Fe-doped carbon material indicates the presence of oxygen ligands for iron ions, in particular the formation of octa- and tetra-complexes with redistribution of electron density between iron and oxygen nuclei, and, accordingly, changes in the degree of covalence of the chemical bond from Fe3+ to Fe2+.

Investigation of Surface Nanoclusters and Paramagnetic Centers of ZnO/Por-Si Structures as the Basis of Sensory Properties

The detection of particles with uncompensated charge and the determination of the features of their interaction during the formation of nanocrystals on substrates with a developed surface are an interesting area of research. The porous surface formed via the electrochemical etching of silicon acquired fractal properties as a result of the deposition of zinc oxide layers. Microscopy methods using different resolutions revealed a hierarchical structure of the surface, where each of the three consecutive levels contains uniformly distributed formations. The deposition of 20 layers of ZnO maximizes the concentration of nanocrystals at the pore boundaries, while the deposition of 25 layers leads to the formation of a continuous layer. The increase in photoluminescence intensity with an increase in the number of deposited layers is due to the saturation of surface nanostructures with electrons through several mechanisms. Electron paramagnetic resonance (EPR) studies have shown that the main mechanism of radiation recombination is the capture of electrons on oxygen vacancies. The different nature of the EPR saturation of the signal of interconnected paramagnetic centers revealed the formation of zinc oxide particles at the boundaries of pores with different sizes. The results of these studies of surface-active structures effectively complement the knowledge about sensory materials.

HTHP treatment of diamonds containing N1 centers. Neutral state of the N1 center as a limiting step in the aggregation of substitutional nitrogen into A form

The step-by-step annealing of natural plastically deformed type IaAB diamond revealed new information on the N1 center, which is two nitrogen atoms separated by one carbon in an ionized state (S = 1/2). A slight increase in the intensity of the N1 electron paramagnetic resonance (EPR) spectrum after annealing at 1000 °C (10h) was observed, and the complete disappearance was detected at a temperature of 1300 °C (4 h). This behavior of the N1 paramagnetic center was associated with the annealing of electron acceptors, and we analyzed the conditions for the formation of complex vacancy defects. The temperature of 1000 °C corresponds to the formation of three vacancy chains from single vacancies and 1300 °C corresponds to their disappearance. We experimented with electron irradiation of the crystals (energy of 3 MeV and a fluence of 1018 e/cm2) followed by annealing at 1000 °C (for 10 h) and the N1 EPR spectrum appeared back with an intensity five times higher than in the initial crystal. The next heat treatment at 1400 °C (2 h) resulted in the expected vanish of the N1 EPR spectrum. In this case, electron irradiation followed by annealing at 1000 °C led to the restoration of a very weak EPR spectrum. The presence of the “EPR-silent” N1 centers in a neutral state in plastically deformed diamond prompted us to investigate their existence in crystals that were not subjected to plastic deformation. We generated acceptor centers in different diamonds by the above method and found N1 centers in the type IaAB natural and type IaA synthetic diamonds. The results obtained can suggest that the rate-limiting step in the aggregation of two impurity nitrogen atoms into close pair is the neutral N1 defect. In the ionized state of N1, the Coulomb repulsion between the unpaired extra electrons of nitrogen is eliminated, reducing the energy barrier in the final stage of transformation into the A center.

Magnetic Resonance-Based Analytical Tools to Study Polyvinylpyrrolidone-Hydroxyapatite Composites.

The synthesis of biocompatible and bioresorbable composite materials, such as a "polymer matrix-mineral constituent," stimulating the natural growth of living tissues and the restoration of damaged parts of the body, is one of the challenging problems in regenerative medicine and materials science. Composite films of bioresorbable polymer of polyvinylpyrrolidone (PVP) and hydroxyapatite (HA) were obtained. HA was synthesized in situ in the polymer solution. We applied electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) approaches to study the composite films' properties. The application of EPR in two frequency ranges allowed us to derive spectroscopic parameters of the nitrogen-based light and radiation-induced paramagnetic centers in HA, PVP and PVP-HA with high accuracy. It was shown that PVP did not significantly affect the EPR spectral and relaxation parameters of the radiation-induced paramagnetic centers in HA, while light-induced centers were detected only in PVP. Magic angle spinning (MAS) 1H NMR showed the presence of two signals at 4.7 ppm and -2.15 ppm, attributed to "free" water and hydroxyl groups, while the single line was attributed to 31P. NMR relaxation measurements for 1H and 31P showed that the relaxation decays were multicomponent processes that can be described by three components of the transverse relaxation times. The obtained results demonstrated that the applied magnetic resonance methods can be used for the quality control of PVP-HA composites and, potentially, for the development of analytical tools to follow the processes of sample treatment, resorption, and degradation.

Origin of Weak Magnetic Coupling in a Dimanganese(II) Complex Bridged by the Tetrathiafulvalene-Tetrathiolate Radical.

Magnetic exchange coupling (J) between different spin centers plays a crucial role in molecule-based magnetic materials. Direct exchange coupling between an organic radical and a metal is frequently stronger than superexchange through diamagnetic ligands, and the strategy of using organic radicals to engender desirable magnetic properties has been an area of active investigation. Despite significant advances and exciting bulk properties, the magnitude of J for radical linkers bridging paramagnetic centers is still difficult to rationally predict. It is thus important to elucidate the features of organic radicals that govern this parameter. Here, we measure J for the tetrathiafulvalene-tetrathiolate radical (TTFtt3-•) in a dinuclear Mn(II) complex. Magnetometry studies show that the antiferromagnetic coupling in this complex is much weaker than that in related Mn(II)-radical compounds, in contrast to what might be expected for the S-based chelating donor atoms of TTFtt. Experimental and computational analyses suggest that this small J coupling may be attributed to poor overlap between Mn- and TTFtt-based magnetic orbitals coupled with insignificant spin density on the coordinating S-atoms. These factors override any expected increase in J from the comparatively strong S-donors. This work elucidates the magnetic coupling properties of the TTFtt3-• radical for the first time and also demonstrates how multiple competing factors must be considered in rationally designing organic radical ligands for molecular-based magnetic compounds.

Composition and Electronic Structure of La2O3/CNFs@C Core-Shell Nanoparticles with Variable Oxygen Content.

La2O3 nanoparticles stabilized on carbon nanoflake (CNF) matrix were synthesized and graphitized to produce core-shell structures La2O3/CNFs@C. Further oxidation of these structures by nitric acid vapors for 1, 3 or 6 h was performed, and surface-oxidized particles La2O3/CNFs@C_x (x = 1, 3, 6) were produced. Bulk and surface compositions of La2O3/CNFs@C and La2O3/CNFs@C_x were investigated by thermogravimetric analysis and X-ray photoelectron spectroscopy. With increasing the duration of oxidation, the oxygen and La2O3 content in the La2O3/CNFs@C_x samples increased. The electronic structures of samples were assessed by electron paramagnetic resonance. Two paramagnetic centers were associated with unpaired localized and mobile electrons and were registered in all samples. The correlation between bulk and surface compositions of the samples and their electronic structures was investigated for the first time. The impact of the ratio between sp2- and sp3-hybridized C atoms, the number and nature of oxygen-containing groups on the surface and the presence and proportion of coordinated La atoms on the EPR spectra was demonstrated.

Size matters: Computational insight into magnetic properties of extended acenes

A DFT study of a series of extended acenes comprising from 7 to 35 conjugated six-membered rings (n) predicted their polyradical character and ascertained correlation between the length and magnetic properties of these compounds. A regularity was revealed indicating that increasing the chain length by every seven rings bring into the formation of two additional paramagnetic centers, leading to the presence of ten unpaired electrons in the acene, for which n = 35. The computational results obtained allow one to expect increasing the number of unpaired electrons with a further (n > 35) expanding the acene length.

Theoretical studies and structural analysis of the Nd3+ centers with trigonal symmetry in congruent lithium niobate single crystals

In this paper, the superposition model (SM) and complete diagonalization procedure of energy matrix (CDM method) were used for a few selected Nd3+ (4f3) paramagnetic centers with trigonal symmetry in congruent lithium niobate (LN, LiNbO3) single crystals. Additionally, the g-shift method was applied to these systems. In general, the calculated g-factors (parallel and perpendicular) match the measured values very well. The displacements of the Nd3+ ions calculated by g-shift method are also fit very well to the experimental values. A detailed structural analysis of paramagnetic Nd3+ centers with trigonal symmetry in congruent LN single crystals was also performed. The charge compensation mechanism and its role in establishing Nd3+ centers in congruent LN are also debated.

Individual and combinative effect of NaCl and γ-radiation on NADPH-generating enzymes activity in corn (Zea mays L.) sprouts

Being a universal reducing agent nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) plays an important role in the cellular metabolism and the implementation of anti-stress reactions in plants. There are only a few enzymes that ensure the NADPH pool formation in cells. Among them, the most important are glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49), malate dehydrogenase decarboxylating (DMDH, malic enzyme, EC 1.1.1.40) and NADP-isocitrate dehydrogenase (NADP-IDH, EC 1.1.1.42). The presented investigation is devoted to studying the influence of the individual and combinative effects of NaCl and γ-radiation as abiotic stress factors on biometric indicators and activity of these NADPH-generating enzymes, on organic content, and the formation of paramagnetic centers as defense reaction in corn (Zagatala-68 genotype) sprouts. It was found that 100 mM NaCl had an inhibitory effect on the development of sprouts. Relatively lower doses (50 Gy and 100 Gy) of γ-radiation had a positive, but its higher doses (150 Gy and 200 Gy) had a negative effect on this process. 500 Gy was a lethal dose (LD) for the corn sprouts. Combinative stress in all cases considerably delayed the development of sprouts. G6PDH showed the highest activity in the first, whereas, NADP-IDH showed the same activity in the last days of the experiment. All three enzymes, especially the G6PDH, have been activated in both root and stem tissues under the influence of stress factors (either radiation or salt). Combinative stress (γ-radiation + salt) also led to an induction of these activities which was necessary to neutralize the negative consequences of stress factors. Stress factors in all cases also had a negative effect on the content of organic matter in seedlings. Ionizing gamma radiation, which resulted in the formation of new paramagnetic centers as an anti-stress defense reaction in many cases was observed in wheat seedlings, but not in corn sprouts, which clearly shows that there are some differences in the protective mechanisms of these C3- and C4-types of plants to γ-radiation.

Reduction of the Spin–Phonon Coupling of Quadrupole Nuclei in NaF Crystals under Magnetic Saturation

The rate of nuclear spin-lattice relaxation is determined by the efficiency of interaction between thermal phonons and nuclear spins. The results on reducing the efficiency of spin–phonon coupling by suppressing the contribution from paramagnetic centers to quadrupole nucleus relaxation are presented. The suppression has been performed by stationary magnetic effect at the Larmor frequency. It is shown that, as in the presence of an acoustic field, the rate of spin-lattice relaxation of 23Na nuclei in a sodium fluoride crystal at magnetic saturation of the NMR signal does not change in the region of a negative average spin temperature.In the region of positive spin temperature, the rate of relaxation of 23Na spins significantly decreases and nuclear magnetization recovery with time is described by the sum of two exponentials. The contribution from nuclear spins with a lower efficiency of spin–phonon coupling, corresponding to the exponential with a long relaxation time, increases with increasing saturating field intensity. It is demonstrated that the efficiency of spin–phonon coupling for 19F nuclei without quadrupole moment does not change under the saturation conditions.The results obtained can be used for analyzing the structure of real crystals.

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.

Paramagnetic NMR to study iron sulfur proteins: 13C detected experiments illuminate the vicinity of the metal center.

The robustness of NMR coherence transfer in proximity of a paramagnetic center depends on the relaxation properties of the nuclei involved. In the case of Iron-Sulfur Proteins, different pulse schemes or different parameter sets often provide complementary results. Tailored versions of HCACO and CACO experiments significantly increase the number of observed Cα/C' connectivities in highly paramagnetic systems, by recovering many resonances that were lost due to paramagnetic relaxation. Optimized 13C direct detected experiments can significantly extend the available assignments, improving the overall knowledge of these systems. The different relaxation properties of Cα and C' nuclei are exploited in CACO vs COCA experiments and the complementarity of the two experiments is used to obtain structural information. The two [Fe2S2]+ clusters containing NEET protein CISD3 and the one [Fe4S4]2+ cluster containing HiPIP protein PioC have been taken as model systems. We show that tailored experiments contribute to decrease the blind sphere around the cluster, to extend resonance assignment of cluster bound cysteine residues and to retrieve details on the topology of the iron-bound ligand residues.

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.