Electron Paramagnetic Resonance Properties Research Articles

Combined TL and EPR study to elucidate the origin of luminescent emission from La-doped CaF2 phosphor

This paper presents the results of the combined study of the thermoluminescence (TL) and electron paramagnetic resonance (EPR) properties of La-doped CaF2 (CaF2: La) phosphor. X-ray diffraction (XRD) results confirmed that CaF2 has been successfully synthesized. Scanning electron microscopy (SEM) results and energy dispersive spectrometer (EDS) spectra show that the synthesized phosphor is pure and has a good overall morphology. The glow curve of CaF2: La phosphor shows an emission band between 230 and 500 nm centered at 340 nm. The Tm-Tstop method and the deconvolution technique were used to find the number of overlapping TL peaks in the 50–300 °C region and their kinetic parameters. A linear dose-response curve was obtained for the range 1–9 Gy, with the onset of a slight supralinear behavior for doses higher than 9 Gy. Gamma-irradiated CaF2: La phosphor has been studied using the technique of EPR to identify the defect centers and also to infer the centers involved in the TL process. Thermal annealing studies indicate the presence of three defect centers. One of the centers (center I) is identified as the O2− ion and is characterized by an axial g-tensor with principal values g∥ = 2.013 and g⊥ = 2.0053. This center correlates with the TL peak at 200 °C. Center II with an isotropic g-value of 2.0005 is attributed to an F center and also relates to the 200 °C TL peak. The third center (center III) is assigned to an F center.

The role of the Terminal Benzene Derivatives in Triphenylamine‐Based Oxime Esters for Free Radical Photopolymerization

AbstractIn this study, we investigate the effect of terminal benzene derivatives in triphenylamine‐based oxime esters on their photoreactivity. Five novel oxime esters (Miko series) that containing different benzene derivatives as terminal groups, were synthesized. The electronic properties of the substituents on the benzene ring range from electron‐donating to electron‐withdrawing. Specifically, a methoxy benzene for Miko‐MOB, tert‐butyl benezene for Miko‐t‐Bu, chloro benzene for Miko‐CB, trifluoromethyl benzene for Miko‐TFM, and nitro benzene for Miko‐NB. Additionally, a compound with benzene only as the terminal substituent (TP‐1) was selected for comparision. All the new compounds exhibited higher molar extinction coefficients than TP‐1. Additionally, the steady‐state photolysis and electron spin resonance (ESR) properties would decrease from the electron‐donating substitutes to electron‐withdrawing substitutes. Finally, these oxime esters are utilized in Type I photoinitiating systems for the free radical polymerization of trimethylolpropane triacrylate (TMPTA) under UV or LED@405 nm light irradiation conditions. Among all formulations, Miko‐MOB demonstrated the highest double bond conversion efficiency under both UV and LED@405 nm light irradiation. Thus, terminal benzene derivatives in oxime esters play an important role in tuning the optical and photochemical reaction performance, offering significant value for the design strategy of such photoinitiators.

Exploring the impact of annealing temperature on morphological, structural, vibrational and electron paramagnetic resonance properties of starch-mediated spinel CoAl2O4: Experimental and DFT study

In the present work, CoAl2O4 nanoparticles were successfully synthesized using the simple auto-combustion sol-gel method employing starch as an organic fuel source. The effect of the annealing temperature on the structural, vibrational, morphological, and magnetic properties is studied. The obtained spherical nanoparticles have grain sizes ranging from 23.54 nm to 46.50 nm. X-ray diffraction (XRD) patterns reveal the formation of a face centered cubic (fcc) spinel phase with the Fd-3m space group for all prepared samples. The calculated average crystallite sizes exhibit an increase from 24.65 nm to 56.27 nm, consistent with the observed grain sizes. Density Functional Theory (DFT) calculations are applied to reach the optimized cell parameters as well as to confirm the structural properties of the synthetized CoAl2O4 nanoparticles. Geometry optimization of the CoAl2O4 spinel oxide is performed using the CASTEP module of Materials Studio software. The optimized unit cell, lattice parameter and unit cell volume were obtained from this simulation. The combined experimental and density functional theory study of structural properties of CoAl2O4 revealed that an increase in the calcination temperature leads to an increase in the volume of the unit cell and hence to an increase in the particles size. Fourier-transform infrared spectroscopy (FTIR) further corroborated these findings by demonstrating subtle shifts in characteristic CoAl2O4 absorption peaks with varying annealing temperatures. These peak shifts suggest a potential redistribution of Al³⁺ and Co2⁺ ions between tetrahedral and octahedral sites within the spinel structure. Electron paramagnetic resonance (EPR) spectroscopy revealed a magnetic state transition as a function of both the annealing temperature and the resulting crystallite size. Occupancy fluctuations of Al³⁺ and Co2⁺ ions at specific sites have also impacted on the nanoparticles color since the hue of the material depends on the valence state and the cationic coordination environment in the CoAl2O4 crystal lattice. Correlations between structural, vibrational, magnetic and color properties are presented to understanding how synthesis parameters influence CoAl2O4 characteristics which is essential for tailoring materials for specific applications.

The Active-Site [4Fe-4S] Cluster in the Isoprenoid Biosynthesis Enzyme IspH Adopts Unexpected Redox States during Ligand Binding and Catalysis.

(E)-4-Hydroxy-3-methylbut-2-enyl diphosphate reductase, or IspH (formerly known as LytB), catalyzes the terminal step of the bacterial methylerythritol phosphate (MEP) pathway for isoprene synthesis. This step converts (E)-4-hydroxy-3-methylbut-2-enyl diphosphate (HMBPP) into one of two possible isomeric products, either isopentenyl diphosphate (IPP) or dimethylallyl diphosphate (DMAPP). This reaction involves the removal of the C4 hydroxyl group of HMBPP and addition of two electrons. IspH contains a [4Fe-4S] cluster in its active site, and multiple cluster-based paramagnetic species of uncertain redox and ligation states can be detected after incubation with reductant, addition of a ligand, or during catalysis. To characterize the clusters in these species, 57Fe-labeled samples of IspH were prepared and studied by electron paramagnetic resonance (EPR), 57Fe electron-nuclear double resonance (ENDOR), and Mössbauer spectroscopies. Notably, this ENDOR study provides a rarely reported, complete determination of the 57Fe hyperfine tensors for all four Fe ions in a [4Fe-4S] cluster. The resting state of the enzyme (Ox) has a diamagnetic [4Fe-4S]2+ cluster. Reduction generates [4Fe-4S]+ (Red) with both S = 1/2 and S = 3/2 spin ground states. When the reduced enzyme is incubated with substrate, a transient paramagnetic reaction intermediate is detected (Int) which is thought to contain a cluster-bound substrate-derived species. The EPR properties of Int are indicative of a 3+ iron-sulfur cluster oxidation state, and the Mössbauer spectra presented here confirm this. Incubation of reduced enzyme with the product IPP induced yet another paramagnetic [4Fe-4S]+ species (Red+P) with S = 1/2. However, the g-tensor of this state is commonly associated with a 3+ oxidation state, while Mössbauer parameters show features typical for 2+ clusters. Implications of these complicated results are discussed.

Electron Paramagnetic Resonance, Electronic Ground State, and Electron Spin Relaxation of Seven Lanthanide Ions Bound to Lanmodulin and the Bioinspired Chelator, 3,4,3-LI(1,2-HOPO).

The electron paramagnetic resonance (EPR) spectra of lanthanide(III) ions besides Gd3+ , bound to small-molecule and protein chelators, are uncharacterized. Here, the EPR properties of 7 lanthanide(III) ions bound to the natural lanthanide-binding protein, lanmodulin (LanM), and the synthetic small-molecule chelator, 3,4,3-LI(1,2-HOPO) ("HOPO"), were systematically investigated. Echo-detected pulsed EPR spectra reveal intense signals from ions for which the normal continuous-wave first-derivative spectra are negligibly different from zero. Spectra of Kramers lanthanide ions Ce3+ , Nd3+ , Sm3+ , Er3+ , and Yb3+ , and non-Kramers Tb3+ and Tm3+ , bound to LanM are more similar to the ions in dilute aqueous:ethanol solution than to those coordinated with HOPO. Lanmodulins from two bacteria, with distinct metal-binding sites, had similar spectra for Tb3+ but different spectra for Nd3+ . Spin echo dephasing rates (1/Tm ) are faster for lanthanides than for most transition metals and limited detection of echoes to temperatures below ~6 to 12 K. Dephasing rates were environment dependent and decreased in the order water:ethanol>LanM>HOPO, which is attributed to decreasing librational motion. These results demonstrate that the EPR spectra and relaxation times of lanthanide(III) ions are sensitive to coordination environment, motivating wider application of these methods for characterization of both small-molecule and biomolecule interactions with lanthanides.

Electron paramagnetic resonance and luminescence properties of Eu2+ in oxyfluoride barium borate glasses

The Eu-doped xBaO-(100-x)B2O3 and xBaF2-(100-x)B2O3 glasses were prepared by melting method under a reducing atmosphere, exhibiting broadband luminescence peaks of 4f65 d1→4f7 in the excitation range of 300–400 nm. PL, EPR, and XPS were used to analyze the effects of the Ba2+, F−, and Eu ion content on the Eu2+/Eu3+ ratio and Eu2+ emission wavelength and intensity. An excess of Ba2+ ions with a high optical basicity reduced the Eu2+/Eu3+ ratio and enhanced the ligand field strength surrounding the Eu2+ ions, decreasing the Eu2+ luminescence intensity and red-shifting the luminescence peak. The addition of F− had the effect of diminishing the optical basicity, decreasing the OH groups content and eliminating Eu2+ clusters, which not only mitigated the effects of the Ba2+ ions, but also significantly enhanced the luminescence intensity of Eu2+ ions. An increase in the Eu ion concentration led to a red-shift in the Eu2+ fluorescence, which was attributed to the nephelauxetic effect. Therefore, the luminescence color of these glass samples could be manipulated by adjusting the doping concentrations of Ba2+, F−, and Eu2+.

Phaffia rhodozyma biorefinery: A sustainable pathway to obtain natural pigments and production of methane biogas as renewable fuel

As the world strives to create a more sustainable and healthier lives, natural pigments are coming to the fore in an attempt to minimize the use of synthetics counterparts and shift the focus towards microbial sources. In this in mind, the yeast Phaffia rhodozyma was used in this study, in a biorefinery context, as biomass feedstock for the intracellular astaxanthin production and reuse of biomass to produce biogas (cf., methane). A sustainable extraction procedure using bio-based solvents was established to directly recover astaxanthin-rich extract from P. rhodozyma dry biomass. The molecular influence of the solutes and solvents involved in the solid–liquid extraction procedures was investigated. Envisioning the full sustainability of the process, and to estimate if the biosolvents used to the process is astaxanthin-friendly, the evaluation of antioxidant activity was confirmed by determining the antioxidant activity using Electron Paramagnetic Resonance (EPR) and toxicological (mutagenicity and ecotoxicity) properties of astaxanthin-rich extract, and the recyclability of solvent was efficiently accomplished. Furthermore, to demonstrate the circularity of the process, the remaining biomass (waste biomass) obtained after astaxanthin-rich extract recovery was effectively used in methanogenic process to produce biogas. The anaerobic digestion used to evaluate methane production from the waste biomass demonstrated the potential to generate 86.88 kWh/year, 225,774.95 kcal/year and 10,287.53 kg/year of electricity, heat recovery and savings on waste management in an P. rhodozyma plant/biorefinery, respectively.

Reconstructing dust provenance from quartz optically stimulated luminescence (OSL) and electron spin resonance (ESR) signals: Preliminary results on loess from around the world

Quantitative provenance analysis studies are instrumental in understanding the tectonic and climatic processes that shape the earth's landscape. Although the most abundant mineral in the sedimentary system is quartz, almost all studies in provenance analysis investigate accessory minerals. Quartz crystals contain a vast number of point defects, intrinsic or due to impurities. For a signal to be an accurate indicator of provenance one needs to show that it is either dose independent or reaches a quantifiable steady state characteristic of the source rock. For signals used by trapped charge dating methods (optically stimulated luminescence (OSL) and electron spin resonance (ESR)), the latter option is the feasible one. By using quartz samples collected from the Chinese Loess Plateau (Luochuan loess-paleosol section), we show that the laboratory and natural dose response curves of E'1 and peroxy electron spin resonance signals of quartz (as defined later) overlap and reach a steady state for doses over about 1000 Gy. For E′1 signals we attribute this steady state to reaching an equilibrium state between diamagnetic oxygen vacancies (the oxygen deficiency centre (ODC), Si=Si) and paramagnetic oxygen vacancies (E′1). For sedimentary quartz irradiated naturally or artificially in this dose range we show a strong linear relationship with zero intercept between E′1 and peroxy signals for samples worldwide, supporting the hypothesis that these defects are Frenkel pairs. Further, we show significant correlations between the optically stimulated (OSL) sensitivity and the above two mentioned ESR signals. The very strong correlations (Pearson's r >0.9) between E′1, peroxy and OSL sensitivity remain valid after the samples have been heated for 15 min to 350 °C for E′1 to reach its maximum value, believed to be a result of the conversion of diamagnetic oxygen vacancies to E′1, clearly suggesting a relationship between OSL sensitivity and oxygen vacancies in general. Samples collected from different loess sites around the world can be distinguished based on both these OSL and ESR properties. An empirical increase in OSL sensitivity as well as oxygen related defect concentrations is observed in areas where the source material has components with older detrital zircon U–Pb ages, inferring a positive correlation between OSL sensitivity, as well as the signal intensity for E1’ and peroxy defects and the age of the source rocks.

Computational engineering of previously crystallized pyruvate formate-lyase activating enzyme reveals insights into SAM binding and reductive cleavage

Radical S-adenosyl-l-methionine (SAM) enzymes are ubiquitous in nature and carry out a broad variety of difficult chemical transformations initiated by hydrogen atom abstraction. Although numerous radical SAM (RS) enzymes have been structurally characterized, many prove recalcitrant to crystallization needed for atomic-level structure determination using X-ray crystallography, and even those that have been crystallized for an initial study can be difficult to recrystallize for further structural work. We present here a method for computationally engineering previously observed crystallographic contacts and employ it to obtain more reproducible crystallization of the RS enzyme pyruvate formate-lyase activating enzyme (PFL-AE). We show that the computationally engineered variant binds a typical RS [4Fe-4S]2+/+ cluster that binds SAM, with electron paramagnetic resonance properties indistinguishable from the native PFL-AE. The variant also retains the typical PFL-AE catalytic activity, as evidenced by the characteristic glycyl radical electron paramagnetic resonance signal observed upon incubation of the PFL-AE variant with reducing agent, SAM, and PFL. The PFL-AE variant was also crystallized in the [4Fe-4S]2+ state with SAM bound, providing a new high-resolution structure of the SAM complex in the absence of substrate. Finally, by incubating such a crystal in a solution of sodium dithionite, the reductive cleavage of SAM is triggered, providing us with a structure in which the SAM cleavage products 5'-deoxyadenosine and methionine are bound in the active site. We propose that the methods described herein may be useful in the structural characterization of other difficult-to-resolve proteins.

Structural, BET and EPR properties of mixed zinc-manganese spinel ferrites nanoparticles for energy storage applications

X-ray diffraction data (XRD) validated the characteristic crystalline spinel cubic structure and a Field-emission scanning electron microscope/energy dispersive spectroscopy (FE-SEM/EDS) validated the proper stoichiometric element ratio of the prepared materials. By using X-ray photoelectron spectroscopy (XPS), the oxidation state of the elements in the prepared materials was verified. An electron paramagnetic resonance (EPR) spectrometer confirmed that Lande factor (g) values decreased as Zn2+ concentration in MnFe2O4 increased. The electrochemical (EC) characteristics of the materials were verified during production using a three electrode instrument. The EC performances confirm the EC behavior of the electrode materials. We investigated the EC properties of the electrodes using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) methods. The Mn0.85Zn0.15Fe2O4 (code as: MZF1) electrode material has a specific capacitance (Cs) of 62.96 F/g compared to Mn0.75Zn0.25Fe2O4 (code as: MZF2) and Mn0.65Zn0.35Fe2O4 (code as: MZF3) electrode at a current density (CD) of 0.5 A/g. According to recent research, electrode materials should be made with the necessary form and size for great act supercapacitor (SC) applications.

Electron spin resonance on a 2D van der Waals CrBr3 uniaxial ferromagnet

We report on the electron spin resonance (ESR) properties of a van der Waals bulk CrBr3 single crystal. Angular- and temperature-dependent studies are performed in the range of the critical temperature for ferromagnetic order. The angular-dependent data enable the determination of a strong uniaxial magnetic anisotropy constant, K, with the easy axis of magnetization being parallel to the crystallographic c axis. The peak-to-peak ESR linewidth below the Curie temperature shows contributions from the term (3 cos2 θ−1)2, thus suggesting the occurrence of long-wavelength (q → 0) modes of spin fluctuations typical for 2D systems. Finally, we analyze the temperature dependence of the double integrated ESR intensity, resonance field, and linewidth for the direction of the magnetic field parallel and perpendicular to the c easy axis. We determine Weiss's constant and the thermal dependence of K and observe the crossover region from linewidth narrowing to broadening approaching the critical temperature. Finally, we discuss our findings in the view of the potential application of CrBr3 in perpendicular magnetic tunneling junctions.

Methods of Modeling of Strongly Correlated Electron Systems.

The discovery of high-Tc superconductivity in cuprates in 1986 moved strongly correlated systems from exotic worlds interesting only for pure theorists to the focus of solid-state research. In recent decades, the majority of hot topics in condensed matter physics (high-Tc superconductivity, colossal magnetoresistance, multiferroicity, ferromagnetism in diluted magnetic semiconductors, etc.) have been related to strongly correlated transition metal compounds. The highly successful electronic structure calculations based on density functional theory lose their predictive power when applied to such compounds. It is necessary to go beyond the mean field approximation and use the many-body theory. The methods and models that were developed for the description of strongly correlated systems are reviewed together with the examples of response function calculations that are needed for the interpretation of experimental information (inelastic neutron scattering, optical conductivity, resonant inelastic X-ray scattering, electron energy loss spectroscopy, angle-resolved photoemission, electron spin resonance, and magnetic and magnetoelectric properties). The peculiarities of (quasi-) 0-, 1-, 2-, and 3- dimensional systems are discussed.

Structural, Morphological, and Magnetic Characterization of Iron Oxide Nanoparticles Synthesized at Different Reaction Times via Thermal Decomposition Method

Background: Superparamagnetic Iron Oxide Nanoparticles (SPIONs) were synthesized by the thermal decomposition method. Methods: In this work, the properties of the nanoparticles synthesized at different reaction times were investigated. Fourier Transformed Infrared Spectroscopy (FTIR) and thermal analysis were carried out to characterize oleate adsorbed on the surface of nanoparticles. Results: The oleate-coated surfaces were obtained for all samples, and the amount of oleate on the surfaces of the particles changed with the change in reaction time. The size, size distribution, and shape of SPIONs were determined by X-ray Diffraction (XRD), transmission electron microscopy (SEM), and Dynamic Light Scattering (DLS). It was seen that changing the reaction time affected the shape of the nanoparticles, but almost the same sized nanoparticles were obtained with the increase of reaction time. The sample's crystallite size of 12.5-14.2 nm achieved with XRD is in good agreement with the mean size of 15-16.4 that was obtained by TEM results. Maximum magnetic saturation of the sample was achieved at 3 h reaction time. Conclusion: The magnetic properties of iron oxide nanoparticles were characterized by Electron- Spin Resonance (ESR) and Physical Properties Measurement System (PPMS). All samples showed superparamagnetic behaviors at room temperature.

Triplet states in the reaction center of Photosystem II.

In oxygenic photosynthesis sunlight is harvested and funneled as excitation energy into the reaction center (RC) of Photosystem II (PSII), the site of primary charge separation that initiates the photosynthetic electron transfer chain. The chlorophyll ChlD1 pigment of the RC is the primary electron donor, forming a charge-separated radical pair with the vicinal pheophytin PheoD1 (ChlD1+PheoD1-). To avert charge recombination, the electron is further transferred to plastoquinone QA, whereas the hole relaxes to a central pair of chlorophylls (PD1PD2), subsequently driving water oxidation. Spin-triplet states can form within the RC when forward electron transfer is inhibited or back reactions are favored. This can lead to formation of singlet dioxygen, with potential deleterious effects. Here we investigate the nature and properties of triplet states within the PSII RC using a multiscale quantum-mechanics/molecular-mechanics (QM/MM) approach. The low-energy spectrum of excited singlet and triplet states, of both local and charge-transfer nature, is compared using range-separated time-dependent density functional theory (TD-DFT). We further compute electron paramagnetic resonance properties (zero-field splitting parameters and hyperfine coupling constants) of relaxed triplet states and compare them with available experimental data. Moreover, the electrostatic modulation of excited state energetics and redox properties of RC pigments by the semiquinone QA- is described. The results provide a detailed electronic-level understanding of triplet states within the PSII RC and form a refined basis for discussing primary and secondary electron transfer, charge recombination pathways, and possible photoprotection mechanisms in PSII.

Current density functional framework for spin–orbit coupling

Relativistic two-component density functional calculations are carried out in a non-collinear formalism to describe spin-orbit interactions, where the exchange-correlation functional is constructed as a generalization of the non-relativistic density functional approximation. Contrary to non-relativistic density functional theory (DFT), spin-orbit coupling, however, leads to a non-vanishing paramagnetic current density. Density functionals depending on the kinetic energy density, such as meta-generalized gradient approximations, should therefore be constructed in the framework of current DFT (CDFT). The latter has previously exclusively been used in the regime of strong magnetic fields. Herein, we present a consistent CDFT approach for relativistic DFT, including spin-orbit coupling. Furthermore, we assess the importance of the current density terms for ground-state energies, excitation energies, nuclear magnetic resonance shielding, and spin-spin coupling constants, as well as hyperfine coupling constants, Δg-shifts, and the nuclear quadrupole interaction tensor in electron paramagnetic resonance (EPR) spectroscopy. The most notable changes are found for EPR properties. The impact of the current-dependent terms rises with the number of unpaired electrons, and consequently, the EPR properties are more sensitive toward CDFT. Considerable changes are observed for the strongly constrained and appropriately normed functionals, as well as the B97M family and TASK. The current density terms are less important when exact exchange is incorporated. At the same time, the current-dependent kernel ensures the stability of response calculations in all cases. We, therefore, strongly recommend to use the framework of CDFT for self-consistent spin-orbit calculations.

Revealing 2D Magnetism in a Bulk CrSBr Single Crystal by Electron Spin Resonance

Abstract2D magnets represent material systems in which magnetic order and topological phase transitions can be observed. Based on these phenomena, novel types of computing architectures and magnetoelectronic devices can be envisaged. Unlike conventional magnetic films, their magnetism is independent of the substrate and interface qualities, and 2D magnetic properties manifest even in formally bulk single crystals. However, 2D magnetism in layered materials is rarely reported often due to weak exchange interactions and magnetic anisotropy, and low magnetic transition temperatures. Here, the electron spin resonance (ESR) properties of a layered antiferromagnetic CrSBr single crystal are reported. The W‐like shape angular dependence of the ESR linewidth provides a signature for room temperature spin–spin correlations and for the XY spin model. By approaching the Néel temperature the arising of competing intralayer ferromagnetic and interlayer antiferromagnetic interactions might lead to the formation of vortex and antivortex pairs. This argument is inferred by modeling the temperature dependence of the ESR linewidth with the topological Berezinskii‐Kosterlitz‐Thouless phase transition. These findings together with the chemical stability and semiconducting properties, make CrSBr a promising layered magnet for future magneto‐ and topological‐electronics.

Synthesis, Electron Spin Resonance and Photoluminescence properties of Sm3+ ion doped Zn-Mn nanoferrites synthesized by glycol-thermal method

Nanocrystalline Zn0.5Mn0.5SmxFe2-xO4 (0 ≤ x ≤ 0.05) with average crystallite sizes varying between 12 and 17 nm were synthesized by the glycol-thermal process. X-ray diffraction (XRD) analysis confirmed a single-phase cubic spinel structure in all the compounds. Substituting a smaller Fe3+ ion with a larger rare-earth (RE) Sm3+ ion has affected the distribution of metal ions on tetrahedral (A) and octahedral (B) sites. In this work, a comprehensive study has been carried out on nanocrystalline particles of RE Sm3+ ion doped into Zn-Mn ferrite to estimate intrinsic strain through Williamson-Hall (W–H) analysis. The results were compared with those obtained from the Debye Scherrer equation. Tensile microstrain within the nanocrystals were detected from W-H plots for x = 0, 0.01,0.02 and 0,05 samples, while compressive microstrain was observed for x = 0.02 and 0.05 samples. A scanning electron microscope (SEM) has been used to study the morphology of nanoparticles and it revealed spherical-shaped nanoparticles. Energy dispersive X-rays (EDX) confirmed the phase purity and the elemental composition with no impurity elements were observed. Crystallite sizes affect electron spin resonance (ESR) signal intensity and the line width. The g-values fluctuated ranging from 2.31 to 2.51 due to fluctuating crystallite sizes. Photoluminescence (PL) displayed a blue-violet emission with maxima centred at 418 nm and 438 nm under the excitation wavelength of 320 nm. The electron spin resonance and photoluminescence results show that the Sm doped Zn-Mn ferrites are promising candidates for applications in blue LEDs.

Pulsed Multifrequency Electron Paramagnetic Resonance Spectroscopy Reveals Key Branch Points for One- vs Two-Electron Reactivity in Mn/Fe Proteins.

Traditionally, the ferritin-like superfamily of proteins was thought to exclusively use a diiron active site in catalyzing a diverse array of oxygen-dependent reactions. In recent years, novel redox-active cofactors featuring heterobimetallic Mn/Fe active sites have been discovered in both the radical-generating R2 subunit of class Ic (R2c) ribonucleotide reductases (RNRs) and the related R2-like ligand-binding oxidases (R2lox). However, the protein-specific factors that differentiate the radical reactivity of R2c from the C-H activation reactions of R2lox remain unknown. In this work, multifrequency pulsed electron paramagnetic resonance (EPR) spectroscopy and ligand hyperfine techniques in conjunction with broken-symmetry density functional theory calculations are used to characterize the molecular and electronic structures of two EPR-active intermediates trapped during aerobic assembly of the R2lox Mn/Fe cofactor. A MnIII(μ-O)(μ-OH)FeIII species is identified as the first EPR-active species and represents a common state between the two classes of redox-active Mn/Fe proteins. The species downstream from the MnIII(μ-O)(μ-OH)FeIII state exhibits unique EPR properties, including unprecedented spectral breadth and isotope-dependent g-tensors, which are attributed to a weakly coupled, hydrogen-bonded MnIII(μ-OH)FeIII species. This final intermediate precedes formation of the MnIII/FeIII resting state and is suggested to be relevant to understanding the endogenous reactivity of R2lox.

Electron paramagnetic resonance and optical properties of green-emitting Mn(II)-doped hardystonite phosphor prepared by sol-gel method

A series of Mn(II)-doped hardystonite phosphors was obtained by using a sol-gel technique. Structural, morphological, and optical properties of the prepared samples were investigated by X-ray diffraction, Fourier transform infrared, scanning electron microscopy and diffuse reflectance spectrum. The photoluminescence excitation and emission spectra of the prepared phosphors exhibited a strong band at ~ 254 nm and an intensified emission band at ~ 525 nm, respectively. Of all the prepared phosphors, 0.03Mn(II) doped hardystonite phosphor possessed a highly intense emission band, which demonstrated that 0.03 mol was the optimized concentration of Mn(II) ions. All the prepared phosphors emitted a bright green color, as confirmed by the Commission International de I′Eclairage co-ordinates. Electron paramagnetic resonance study suggested the existence of Mn(II) ions at sites with octahedral symmetry (g value ~2) and the dominance of covalent bonding in the prepared phosphors.

EPR Spectroscopy of Cu(II) Complexes: Prediction of g-Tensors Using Double-Hybrid Density Functional Theory

Computational electron paramagnetic resonance (EPR) spectroscopy is an important field of applied quantum chemistry that contributes greatly to connecting spectroscopic observations with the fundamental description of electronic structure for open-shell molecules. However, not all EPR parameters can be predicted accurately and reliably for all chemical systems. Among transition metal ions, Cu(II) centers in inorganic chemistry and biology, and their associated EPR properties such as hyperfine coupling and g-tensors, pose exceptional difficulties for all levels of quantum chemistry. In the present work, we approach the problem of Cu(II) g-tensor calculations using double-hybrid density functional theory (DHDFT). Using a reference set of 18 structurally and spectroscopically characterized Cu(II) complexes, we evaluate a wide range of modern double-hybrid density functionals (DHDFs) that have not been applied previously to this problem. Our results suggest that the current generation of DHDFs consistently and systematically outperform other computational approaches. The B2GP-PLYP and PBE0-DH functionals are singled out as the best DHDFs on average for the prediction of Cu(II) g-tensors. The performance of the different functionals is discussed and suggestions are made for practical applications and future methodological developments.