Electron Spin Resonance Studies Research Articles

ROS scavenging of SOD/CAT mimics probed by EPR and reduction of lipid peroxidation in S. cerevisiae and mouse liver, under severe hydroxyl radical stress condition

The interaction between CuII, FeIII and MnII complexes, derived from the ligands 1-[bis(pyridine-2-ylmethyl)amino]-3-chloropropan-2-ol (hpclnol) and bis(pyridine-2-ylmethyl)amine (bpma), and the free radical 2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl (DPPH) and reactive oxygen species (ROS), was investigated by colorimetric and EPR (Electron Paramagnetic Resonance) techniques. A comparison between these results and those reported to [Mn(salen)Cl] or EUK-8 was also addressed. EPR studies allowed us the identification of intermediates species such as superoxide‑copper(I) and superoxide‑copper(II), a mixed-valence FeIIIFeII species and a 16-line feature attributed to MnIII-oxo-MnIV species. The biomarker malondialdehyde (MDA) was determined by TBARS assay in S. cerevisiae cells, and the determination of the IC50 indicate that the antioxidant activity shown dependence on the metal center (CuII ≈ FeIII > MnII ≈ [Mn(salen)Cl]. The lipid peroxidation attenuation was also investigated in liver homogenates obtained from Swiss mice and the IC50 values were in the nanomolar concentrations. We demonstrated here that all the complexes interact with the free radical DPPH and with ROS (H2O2, O2•- and hydroxyl radical), enhancing the cellular protection against oxidative stress generated by hydroxyl radical, employing two experimental model systems, S. cerevisiae (in vivo) and mouse liver (ex vivo).

Synthesis of Epitaxial Structures with Two-Dimensional Si Layers Embedded in a CaF2 Dielectric Matrix

The possibility of fabricating two-dimensional Si layers on a CaF2/Si(111) film by molecular beam epitaxy is studied. The growth conditions, under which the regions of two-dimensional Si layers are formed, are found. Raman spectroscopy, transmission electron microscopy, photoluminescence, and electron paramagnetic resonance (EPR) studies have shown that regions of two-dimensional Si layers are formed in epitaxial structures prepared by the deposition of one to three biatomic Si layers on the CaF2/Si(111) film surface at a temperature of 550°С. The Raman spectra of these structures exhibit a narrow peak at 418 cm–1, which is due to light scattering on vibrations of Si atoms in the plane of a two-dimensional Si layer intercalated with calcium. The EPR spectra of multilayer structures with regions of two-dimensional Si layers embedded in CaF2 demonstrate an isotropic signal with an asymmetric Dyson shape and the g-factor g = 1.9992 under illumination. Consequently, this signal can be attributed to photoinduced conduction electrons in extended two-dimensional Si islands. These results may be useful for understanding the mechanisms of the formation of two-dimensional materials on CaF2/Si(111) substrates.

Interactions of cellulose- and lignin-derived radicals during pyrolysis: An in-situ Electron Paramagnetic Resonance (EPR) study

Lignocellulosic biomass can be converted to biochar, bio-oil and gas through reactions of non-volatile stabilised free radicals (SFRs) and volatile radicals during thermal conversion. In the process, the interactions between radicals from different biomass components play an essential role. Figuring out the interactions of radicals during pyrolysis holds the key to revealing the interaction mechanism of biomass components. In this study, the evolution of SFRs during the co-pyrolysis of cellulose and lignin was explored using in-situ electron paramagnetic resonance (EPR) detection. The interactions of cellulose- and lignin-derived radicals were analysed based on the variation of SFRs, thermogravimetric behavior and Raman spectrum measurement. The results indicate that the interactions of radicals influence char yield, char structure and the SFRs evolution during pyrolysis. The interaction of volatile radicals and SFRs increases the char yield and promotes the condensation of aromatic structure in biochar. At temperatures above 400 °C, the interaction between cellulose- and lignin-derived SFRs leads to the coupling of radicals, which forms an unstable structure in the hot nascent biochar. In the subsequent cooling process, the weak bonds from interactions may break due to the structure shrinkage, which promotes radical reactions and further changes the char structure.

Cu-HAB/GO composite with superior active surface area/sites and conductivity properties for the electrocatalytic degradation of tetracycline hydrochloride

Conventional metal–organic frameworks (MOFs) used for electrocatalytic applications have the drawbacks of poor conductivity and instability. Herein, a Cu–1,2,3,4,5,6-benzenehexamine (Cu-HAB)/graphene oxide (GO) composite was prepared for the electrocatalytic degradation of tetracycline hydrochloride (TCH). Our characterization results show that Cu-HAB/GO has more active sites (ID/IG = 1.0030), larger surface area (SBET = 395.3441 m2/g), and faster electron transport (Rct = 20.69) than Cu-HAB. Simultaneously, the improvement in the number of active sites, surface area and electron transport leads to a superior TCH electrocatalytic degradation efficiency. The reaction rate constant (0.70575 min−1) of the composite was higher with a 51.81% improvement observed when compared to Cu-HAB under the optimum conditions of 1 g/L NaCl, 20 mA, and 20 °C. In addition, 100% of the degradation efficiency was maintained after five cycles. A detailed study of the mechanism revealed that 1O2 and active chlorine were the critical active species for the degradation of TCH via quenching and electron paramagnetic resonance (EPR) studies. The possible TCH degradation pathway has been proposed. This study provides a novel and feasible method for the preparation of MOF-based electrocatalysts to expand their application to environmental fields.

Impact of Methylaluminoxane Oxidation on Ethylene Polymerization Using Ni Catalysts

The effect of oxidation of methylaluminoxane (MAO) on ethylene polymerization was investigated to see the effect of aging and the possibility of using monomer gas that is not purified enough. In the polymerization using phenoxyimine-ligated titanium catalyst, almost no changes in polymerization behavior were observed until 35% of O2 was added to MAO at enough Al/Ti ratio. However, Brookhart-type nickel-diimine catalysts activated with oxidized MAO gave polyethylenes with improved molecular weight and linearity. NMR, UV–vis, and electron paramagnetic resonance studies combined with theoretical prediction revealed that the electronic property on nickel was consistent, regardless of MAO modification. The effect on the polymerization behavior can thus be explained by the change of steric effect of counteranions derived from MAO via modification with oxygen.

(Invited) Operando EPR Study on Radicals in Anion-Exchange Membrane Fuel Cells

In the rapidly developing modern society, there is an urgent need for the wide-ranging availability of advanced and eco-friendly energy sources. One of the possible alternatives is the application of anion exchange membrane fuel cells (AEMFCs) with a catalyst reducing dioxygen efficiently. These promising devices can revolutionize the energy sector since they practically produce no pollution. However, to make them more widely used, several obstacles must be overcome. As a result, vast-ranging investigations focus on improving the properties of conductive polymer membranes [1] and catalysts for the oxygen reduction reaction (ORR) [2]. The durability of the membrane electrode assembly (MEA) is one of the critical requirements for the successful commercialization of anion exchange membrane fuel cells (AEMFCs). Despite significant impacts of nucleophilic degradation on ion-exchange capability and the anionic conductivity of investigated membranes, it is believed to affect only cationic sites of membrane polymers and thus cannot explain the reported loss in the mechanical strength of degraded AEMs. Such a phenomenon might be related to polymer backbone degradation caused by free radicals. This was widely described in the literature in the case of fuel cells using proton-conducting membranes [3] but barely mentioned for AEMFCs [4]. Since the oxidative degradation of hydrocarbon polymers is very well known, we aimed to comprehensively investigate the formation of the short-lived species generated during the operation of AEMFCs as well as stable radicals present in the polymer membranes.We investigated the LDPE-base membranes with Pt black, Pd black, PdAg, and Ag as the ORR catalysts, whereas for HOR the Pt black, Pd black, and NiFe catalysts were used. The in-situ measurements are performed with a micro-AEMFC inserted into a resonator of an electron paramagnetic resonance (EPR) spectrometer, which enables separate monitoring of radicals formed on the anode and cathode sides. The creation of radicals was monitored by the EPR spin trapping technique. In Figure 1 the EPR spectra of DMPO spin adducts trapped during operation of micro-fuel cell placed in EPR spectrometer cavity are presented. In this experiment, the LDPE-base membrane with platinum catalysts on both sides was used. The main detected adducts during the operation of the micro-AEMFC were DMPO-OOH and DMPO-OH on the cathode side and DMPO-H on the anode side. Additionally, we clearly show the formation and presence of stable radicals in AEMs during and after long-term AEMFC operation [5]. Preliminary results suggest that the creation of the short-living radicals during AEMFCs operation is independent of the used membrane. However, the applied catalysts determine the number of detected radicals. The EPR investigations indicate that, in addition to the known chemical degradation mechanisms of the cationic ammonium groups of the membrane, oxidative degradation, including radical reactions, has to be taken into account when the stability of an anion conductive polymer for AEMFCs is investigated. The formation of stable radicals in AEMs was proven for the first time in this study. All short-living radicals formed during the AEMFC operation were fully identified. The presence of radicals in the AEM after AEMFC testing indicates that reactive oxygen species may play a very important role in the degradation mechanism of the anion conducting polymers. Results from this study shed light on the understanding of radical formation and presence in the membranes during AEMFC tests, which in turn may help to solve the challenge of anion exchange membrane stability. Acknowledgments. This work was supported by the Polish National Science Centre (NCN) project OPUS-14, No. 2017/27/B/ST5/01004.

Continuous wave and pulsed EPR study of Cd1-xMnxTe crystals with different Mn content

The cadmium manganese telluride (Cd1–xMnxTe) crystals (x < 0.001 and x = 0.02, 0.04, 0.1) grown using the Bridgman method were studied by applying continuous wave and pulsed electron paramagnetic resonance (EPR) spectroscopy in the wide temperature range. The Cd1–xMnxTe crystals with x < 0.001 revealed the EPR spectrum from isolated Mn2+ with g⊥ = g∥ = 2.0074(3), |A⊥| = |A∥| = 56.97 ⋅ 10–4 cm–1, |a| = 30.02⋅10–4 cm–1, while Cd1–xMnxTe crystals with x = 0.02…0.04 are characterized by two single broad isotropic EPR lines of Lorentzian shape (g ~ 2.009 and g ~ 1.99) due to Mn clusters of different sizes. The EPR spectrum of Cd1–xMnxTe crystals with x = 0.01 consists of the single broad line at g ~ 2.01 due to higher level of homogeneity inherent to these crystals. The temperature dependence of spin relaxation times for the isolated Mn2+ center in the Cd1–xMnxTe crystals with x < 0.001 has been described using the conceptions of Orbach process for TM–1 and two-phonon Raman process for T1–1.

Understanding the Sensing Mechanism of ZnO Nanoparticles through Identifying Intrinsic Defects

Progress in crystal growth made ZnO applicable as the next generation of inorganic light-emitting diodes and lasers, which has attracted much attention. Our previous work is a proof that Zn vacancies are occupied by three protons to form VZnH3 complexes in high-quality ZnO single crystals. In this work, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) studies on high-quality ZnO single crystals further confirm that three proton clusters or cations with three positive charges are the intrinsic defects and/or impurities related to doping in the host-lattice of high-quality ZnO single crystals. Unintentionally doped hydrogen (H) clusters might be relating to ZnO’s sensing capabilities. The sensor response of ZnO semiconductors is promoted by a decreasing size; however, the nature of the size effects has not been elucidated satisfactorily. In this work, broccoli-like ZnO nanoarrays, which can detect H2S gas in the parts per billion (ppb) region, has been synthesized under a hydrothermal method. High-resolution transmission electron microscopy (HRTEM) experimental results demonstrate that there are multiple polar faces exposed on the surfaces of ZnO nano crystals. The high catalytic active of oxygen species adsorbed on the multiple polar surfaces should respond to the high sensing performance and the high selectivity of ZnO toward reducing H2S gas. However, the enormous variations in electric current cannot be produced by releasing electrical charges from O– adsorbates into semiconductors. EPR results show that Zn+ interstitials have been produced after the reaction between H2S and O– adsorbates on ZnO. In contrast to the unintentionally doped hydrogen (H) clusters, our experimental results show that the Zn+ interstitials acting as shallow donors is what really offer the extra free-electron carriers.

Optical Properties and Light-Induced Charge Transfer in Selected Aromatic C60 Fullerene Derivatives and in Their Bulk Heterojunctions with Poly(3-Hexylthiophene).

Fullerene derivatives offer great scope for modification of the basic molecule, often called a buckyball. In recent years, they have been the subject of numerous studies, in particular in terms of their applications, including in solar cells. Here, the properties of four recently synthesized fullerene C60 derivatives were examined regarding their optical properties and the efficiency of the charge transfer process, both in fullerene derivatives themselves and in their heterojunctions with poly (3-hexylthiophene). Optical absorption, electron spin resonance (ESR), and time-resolved photoluminescence (TRPL) techniques were applied to study the synthesized molecules. It was shown that the absorption processes in fullerene derivatives are dominated by absorption of the fullerene cage and do not significantly depend on the type of the derivative. It was also found by ESR and TRPL studies that asymmetrical, dipole-like derivatives exhibit stronger light-induced charge transfer properties than their symmetrical counterparts. The observed inhomogeneous broadening of the ESR lines indicated a large disorder of all polymer–fullerene derivative blends. The density functional theory was applied to explain the results of the optical absorption experiments.

Copper catalysts Cu – (Zr + Ce)O2 – Al2O3 as carrier in the process of ethanol conversion: effect of catalyst composition and ethanol concentration

The impregnation technique was used to produce a range of copper catalysts with different concentrations of copper and with a complex-oxide ceramic carrier 50% (Zr + Ce)O2 – 50% Al2O3. The characterization was done based on X-ray phase analysis and electron paramagnetic resonance (EPR). BET method was used to measure the specific surface area. The catalysts were tested in aqueous solutions of ethanol of different concentrations. The EPR method helped establish that copper(II) oxide only is present on the surface of the catalyst after it has been heat treated in inert atmosphere, while copper ions form during catalysis. Copper was found to change its forms during catalysis and interact with the Ce3+ ions of the carrier in the case of the specimen with a high concentration of copper. The results showed that the activity of the catalysts is governed by the concentration of copper in the specimen and their prior processing. To obtain a high-performance dehydrogenation catalyst, the optimum amount of deposited copper phase is 5–10%. The resultant catalysts can be effectively used for the dehydrogenation of aqueous solutions of ethanol in the temperature range of 320 to 360 oC. The highest amount of acetaldehyde was observed on the specimen with 5 wt % copper without prior reduction treatment: 76% at Т = 380 oC with a 98% selectivity. Hydrogen flow reduction of the catalyst affects its catalytic activity bringing it down by 10–15%. The presence of water in ethanol does not impact the catalytic acti vity of the catalysts. The output of acetaldehyde in ethanol conversions with 50 wt % is higher than in ethanol conversions with 96.6 wt %.The EPR study was carried out at the Nanochemistry and Nanomaterials Shared Knowledge Centre at the Lomonosov Moscow State University as part of the University’s Development Plan. The authors thank senior researcher A. V. Fionov, PhD (Chemistry), for his support.This research was funded by the Russian Science Foundation under Research Project No. 22-29-01135.

Highly Active Rutile TiO2 for Photocatalysis under Violet Light Irradiation at 405 nm

Anatase TiO2 is a widely investigated photocatalyst; however, it can only work under ultraviolet (UV) light with wavelengths less than 390 nm (band gap 3.2 eV). Rutile TiO2 can absorb visible light at wavelengths less than 410 nm (band gap 3.0 eV); however, its photocatalytic activity is not high. Herein, we activated rutile TiO2, which was prepared from Evonik TiO2 P 25 through calcination at 800 °C using hydrogen reduction treatment at 700 °C. The photocatalytic activity of the hydrogen-treated TiO2 was as high as P 25 under UV irradiation at 380 nm, which was significantly higher than P 25 under violet light irradiation at 405 nm for the oxidative decomposition of acetic acid in water. Electron spin resonance studies indicate that charge separation is enhanced in reduced TiO2, and their oxygen reduction pathways differ between anatase and rutile. The formation of H2O2 was observed on rutile TiO2; however, it was consumed during photocatalysis to accelerate acetic acid decomposition.

Redox imaging of dextran sodium sulfate-induced colitis mice treated with nitric oxide synthase inhibitors

Ulcerative colitis (UC) is an inflammatory bowel disease of unknown cause. Redox imbalance, as characterized by excessive production of reactive oxygen species (ROS) and nitric oxide (NO), and deficient levels of endogenous antioxidants, is associated with the pathogenesis of UC. Overhauser-enhanced magnetic resonance imaging (OMRI) combined with a spin-probe method produces high-resolution images of redox imbalance in disease states. We aimed to investigate the effect of NO synthase (NOS) inhibitors on colonic ROS generation in mice with colitis induced by dextran sodium sulfate (DSS) using the OMRI/spin-probe method. Symptoms of DSS-induced colitis were ameliorated by an inducible NOS (iNOS) inhibitor, aminoguanidine (AG), but not by a non-selective NOS inhibitor, NG-Nitro-L-arginine methyl ester (L-NAME). Intracellular ROS generation, observed as the enhancement of OMRI contrast decay rates of a 3-methoxy-2,2,5,5-tetramethylpyrrolidine-1-oxyl probe in the colons of DSS-induced colitis mice, was suppressed by AG, in agreement with the results of an in vivo electron spin resonance study. AG had an inhibitory effect on ROS production at the upper colon and rectum during the initiation stage of colitis, and at the upper and lower colon during the advanced stage. Additionally, there were partial differences in the localization of iNOS protein and nitrite/nitrate levels. L-NAME treatment showed a tendency for increased ROS generation at the upper colon during the advanced stage. The OMRI/spin-probe method provides a novel tool for screening ROS inhibitory activity in vivo in DSS-induced colitis mice.

Electron spin resonance study on free radicals in cyclic olefin copolymers irradiated by gamma rays at cryogenic and room temperatures

Cyclic olefin copolymers (COCs) are amorphous thermoplastic polymers that require radiation sterilizations when used in prefilled syringes and drug packaging. In the radiation sterilization process, COCs are ionized, and thus, they produce free radicals, which lead to free radical reactions that affect their performance and biosafety. In this study, species of free radicals generated in COCs by γ-ray irradiation and their decay behaviors under different conditions were investigated using electron spin resonance spectroscopy. Results indicated that many alkyl radicals were produced by the irradiation of COCs in liquid nitrogen, which were transformed to peroxy free radicals upon exposure to air. It was confirmed that free radicals centered on secondary and tertiary carbons were predominant after being irradiated at room temperature by an isotropic simulation, and their concentrations depended on the absorbed dose. After 576 h of storage at room temperature, free radicals were nearly annihilated, or almost decayed, near the glass transition temperature. In addition, this study proposes a free radical reaction mechanism for COCs under different post-processing conditions.

EPR Study on the Oxidative Degradation of Phenyl Sulfonates, Constituents of Aromatic Hydrocarbon-Based Proton-Exchange Fuel Cell Membranes.

Sulfonated aromatic hydrocarbon-based ionomers are potential constituents of next-generation polymer electrolyte fuel cells (PEFCs). Widespread application is currently limited due to their susceptibility to radical-initiated oxidative degradation that, among other intermediates, involves the formation of highly reactive aromatic cation radicals. The intermediates undergo chain cleavage (dealkylation/dearylation) and the loss of protogenic sulfonate groups, all leading to performance loss and eventual membrane failure. Laser flash photolysis experiments indicated that cation radicals can also be formed via direct electron ejection. We aim to establish the major degradation pathway of proton-exchange membranes (PEMs). To this end, we irradiated aqueous solutions of phenyl sulfonate-type model compounds with a Xe arc lamp, thus generating radicals. The radicals were trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and the formed adducts were observed by electron paramagnetic resonance (EPR). The formed DMPO spin adducts were assigned and relative adduct concentrations were quantified by simulation of the experimental EPR spectra. Through the formation of the DMPO/•SO3 - adduct, we established that desulfonation dominates for monoaromatic phenyl sulfonates. We observed that diaryl ether sulfonates readily undergo homolytic C-O scission that produces DMPO/•aryl adducts. Our results support the notion that polyphenylene sulfonates are the most stable against oxidative attack and effectively transfer electrons from DMPO, forming DMPO/•OH. Our findings help to identify durable moieties that can be used as building blocks in the development of next-generation PEMs.

EPR and optical absorption of VO2+ doped tetramethylammonium tetrachloro-zincate

• EPR studies of VO 2+ ions in TMZ single crystals are done at room temperature. • EPR analysis indicates four magnetically equivalent VO 2+ sites. • The vanadyl ion is found to take up substitutional position. The principal g and A are determined. • The optical absorption study is also carried out. • By correlating EPR and optical data the nature of bonding in the crystal is discussed. Electron paramagnetic resonance (EPR) studies of VO 2+ ions in [N(CH 3 ) 4 ] 2 ZnCI 4 (TMZ) single crystals are done at room temperature. Four magnetically equivalent VO 2+ sites are observed from EPR analysis. VO 2+ ion substitutes at Zn 2+ site. The principal g and A tensors are determined from angular variation of the EPR spectra. The optical absorption study is also performed. Using EPR and optical data the nature of bonding in the crystal is discussed.

Catalytic pyrolysis of lotus leaves for producing nitrogen self-doping layered graphitic biochar: Performance and mechanism for peroxydisulfate activation.

In this study, nitrogen self-doping layered graphitic biochar (Na-BC900) was prepared by catalytic pyrolysis of lotus leaves at 900°C, in the presence of NaCl catalyst, for peroxydisulfate (PDS) activation and sulfamethoxazole (SMX) degradation. NaCl as catalyst played a crucial part in the preparation of Na-BC900 and could be reused. The SMX degradation rate in Na-BC900/PDS system was 12 times higher than that in un-modified biochar (BC900)/PDS system. The excellent performance of Na-BC900 for PDS activation was attributed to its large specific surface areas (SSAs), the enhanced graphitization structure and the high graphitic N content. The quenching and electrochemical experiments, electron paramagnetic resonance (EPR) studies inferred that the radicals included SO4•-, •OH, O2•- and the non-radical processes were driven by 1O2 and biochar mediated electron migration. Both radical and non-radical mechanisms contributed to the removal of SMX. Additionally, this catalytic pyrolysis strategy was clarified to be scalable, which can be applied to produce multiple biomass-based biochar catalysts for restoration of polluted water bodies.

Noninvasive detection of the endogenous free radical melanin in human skin melanomas using electron paramagnetic resonance (EPR).

We explored the capability of low-frequency Electron Paramagnetic Resonance (EPR) to noninvasively detect melanin (a stable semiquinone free radical) in the human skin. As previous in vitro studies on biopsies suggested that the EPR signal from melanin was different when measured in skin melanomas or benign nevi, we conducted a prospective first-in-man clinical EPR study in patients with skin lesions suspicious of melanoma. EPR spectra were obtained using a spectrometer operating at 1GHz, with a surface coil placed over the area of interest. Two clinical studies were carried out: 1) healthy volunteers (n=45) presenting different skin phototypes; 2) patients (n=88) with skin lesions suspicious of melanoma (n=100) requiring surgical resection. EPR data obtained before surgery were compared with histopathology results. The method was not sensitive enough to measure differences in melanin content due to changes in skin pigmentation. In patients, 92% of the spectra were analyzable. The EPR signal of melanin was significantly higher (p<0.0001) in melanoma lesions (n=26) than that in benign atypical nevi (n=62). A trend toward a higher signal intensity (though not significant) was observed in high Breslow depth melanomas (a marker of skin invasion) than in low Breslow lesions. To date, no naturally occurring free radicals have been detected by low-frequency EPR systems adapted for clinical studies. Here, we demonstrated for the first time the ability of this technology to detect an endogenous free radical, opening new avenues for evaluating clinical EPR as a potential aid in the diagnosis of pigmented skin lesions.

Electron spin resonance (ESR) study on gamma irradiated some modern paper samples

In this work, we use paramagnetic defects induced by gamma radiation in some modern paper samples to determine their main dosimetric properties in the decontamination dose range (0.5–10 kGy) of ancient papers. Electron spin resonance (ESR) spectra of photo paper, cigarette paper, white paper napkin (napkin), cash register receipt, straw paper and white photocopy paper recorded before and after irradiation with gamma rays were reported. Photo paper, cigarette paper and napkin samples displayed the radiation-induced cellulose-like ESR signals well known from the literature on cellulose-containing foods. Cash register receipt, straw paper, and white photocopy paper are another set of samples where different ESR spectra were recorded. The ESR signals of these three paper samples were shown to originate from carbonate mineral filler grains. Inorganic mineral fillers are commonly used in paper manufacturing. The dose-response curves associated with the radiation-induced sharp ESR signals originating from the CaCO3 filler of the cash register receipt (g = 2.0027), the straw paper (g = 1.9995), and the white photocopy paper (g = 1.9996) were found to follow a single exponential saturation function. Despite the observed decreases with time in the signal intensities of cash register receipt and white photocopy paper, detectable ESR signals were obtained even 100 days after irradiation for them and straw paper. The radiation-induced free radicals responsible for the ESR spectrum of straw paper were more stable than those of cash register receipt and white photocopy paper. The results indicate that free radicals induced in mineral CaCO3 filler grains incorporated within papers have the potential to be used for application to retrospective ESR dosimetry in the range of 0.5–10 kGy.

EPR study of intrinsic defects within NaYGeO4 olivine

An intense electron paramagnetic resonance (EPR) spectrum has been unexpectedly observed at room temperature in polycrystalline samples of NaYGeO4 with olivine crystal structure. Signals were registered in a wide range of fields up to 6000 ​G. Along with strongly broadened unresolved resonance lines, the relatively narrow spectral components were found. It is shown that the spectrum profile depends on synthesis conditions and mechanical treatment of samples, all suggesting a partial orientational order of microcrystallites in the powders. The spectra are interpreted under the assumption of strong superhyperfine interaction between the oxygen-deficient centers like [GeO3]3- and the nuclei of Na+ cations. In addition, a sharp anisotropic resonance at g-values exceeding free-electron value is attributed to oxygenic-hole centers. The density functional theory (DFT) calculations are employed for the estimation of the thermodynamic stability of various intrinsic defects.

Pulsed EPR Study of the Initial Steps of Radical-Scavenging Reactions of C70 with Diphenylphosphine Oxide, Hydroxycyclohexyl, and 2-Hydroxypropyl Radicals

The radical scavenging reactions of C70 were investigated using time-resolved (TR-) and pulsed electron paramagnetic resonance (EPR) methods. In the diphenylphosphine oxide (DPPO) radical and C70 system, structural isomers of the adduct radicals of C70 appeared in the TR-EPR spectrum with clear hyperfine structures due to the phosphorus atom. Four isomers were identified through the analysis of the hyperfine coupling constants. The assignment of the adduct radicals was confirmed by the semiempirical calculation of the relative addition reaction rate constants that produce each isomer. The radical scavenging rate constants, ksca, of C70 in toluene were determined for DPPO, hydroxylcyclohexyl, and 2-hydroxypropyl radicals through electron spin echo observations using the pulsed-EPR method. The ksca values were nearly 109 mol-1 dm3 s-1 and were almost equal to the diffusion-controlled rate constant in toluene. This proves that C70 acts as an excellent radical scavenger. In addition, the radical addition rate constants, kadd, of C70 for varying carbon atoms in C70 were obtained by considering the peak intensity ratio of the adduct radicals in the TR-EPR spectrum. In this study, we demonstrated that the large number of carbon atoms in pentagons (five-membered rings) is responsible for the high reactivity of fullerenes.