Electron Paramagnetic Resonance Evidence Research Articles

EPR Evidence for Mechanistic Diversity of Cu(II)/Peroxygen Oxidation Systems by Tracing the Origin of DMPO Spin Adducts.

Electron paramagnetic resonance (EPR) has been extensively used for the identification of free radicals that are generated from advanced oxidation processes (AOPs) so as to establish the reaction mechanism. However, some misinterpretations or controversies on the identity of detected EPR signals remain in the literature. This study, with Cu(II)-based AOPs as examples, comprehensively investigated the origin of 5,5-dimethyl-l-pyrroline N-oxide (DMPO) adducts in Cu(II) alone, Cu(II)/H2O2, Cu(II)/peroxymonosulfate (PMS), and Cu(II)/peroxydisulfate (PDS) systems. In most Cu(II) systems, DMPO-OH signals can be detected even without any peroxygens, indicating the presence of other origins of this adduct in addition to the genuine spin trapping of •OH by DMPO. According to the formed secondary radical adducts (DMPO-OCH3 from a nonradical process or DMPO-CH2OH from a radical oxidation) derived from methanol quenching, we propose that CuO+, instead of free radicals, is involved in the Cu(II)/PMS system, while •OH is indeed generated in the Cu(II)/H2O2 and Cu(II)/PDS systems under neutral conditions. Notably, 17O-incorporation experiments demonstrate that -OH in the detected DMPO-OH adduct originates 100% from water in the Cu(II) alone system but the amount of -OH is over 99.8% from the oxidant while peroxygens are added. In addition, DMPO-O2- appears only in the Cu(II)/PDS system under highly alkaline conditions and H2O is not involved in superoxide formation.

Identification of black sturgeon caviar pigment as eumelanin

Reported herein is the purification of the pigment of black sturgeon caviar and its unambiguous identification as a typical eumelanin by means of chemical degradation coupled with electron paramagnetic resonance (EPR) evidence. HPLC and LC-MS analysis of oxidative degradation mixtures revealed the formation of pyrrole-2,3,5-tricarboxylic acid (PTCA), a specific marker of eumelanin pigments, in yields compatible with a 6.5% w/w pigment content. EPR spectral features and parameters were in close agreement with those reported for a typical natural eumelanin such as Sepia melanin from squid ink. The identification for the first time of eumelanin in a fish roe is expected to provide a novel molecular basis for the valorization of black caviar and production wastes thereof in food chemistry and diet.

Characterization of a Mixed-Valence Ru(II)/Ru(III) Ion-Pair Complex. Unexpected High-Frequency Electron Paramagnetic Resonance Evidence for Ru(III)–Ru(III) Dimer Coupling

In this contribution, we report the synthesis and full characterization of the first mixed-valence Ru(II)/Ru(III) ion-pair complex, [RuII(bipy)2(pic)]+[cis-RuIIICl2(pic)2]-, in the solid state and in aqueous solution, where bipy = 2,2'-bipyridine and pic- = picolinate. In addition, unexpected high-frequency electron paramagnetic resonance evidence for interactions between two neighboring Ru(III) ions, resulting in a triplet state, S = 1, was found.

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

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

EPR detection of sulfanyl radical during sulfhemoglobin formation – Influence of catalase

Hemoglobin in its ferryl form oxidizes hydrogen sulfide and is transformed to sulfhemoglobin, where the sulfur is inserted covalently at the heme edge. Shown here is evidence that—as previously proposed by others—this process involves oxidation of hydrogen sulfide to a sulfanyl radical detectable by spin-trapping in electron paramagnetic resonance (EPR) spectroscopy. The yields and rates of formation of sulfhemoglobin as well as of the sulfanyl radical are affected by the same factors that affect the reactivity of hemoglobin ferryl, in bovine hemoglobin and in phytoglobins as well. A freely-diffusing sulfanyl radical is thus proposed to be involved in sulfhemoglobin formation. Catalase is shown to accelerate this process due to a previously described hydrogen sulfide oxidase activity, within which EPR evidence for sulfanyl generation is shown here for the first time. The reaction of preformed ferryl with hydrogen sulfide—in absence of hydrogen peroxide—is studied by stopped-flow at several pH values and explained in light of reactivity and redox potential control.

The thermal charge-transfer reduction of uranyl UO22+(VI) to UO2+(V) by various functionalized organic compounds, and evidence for possible spin-spin interactions between UO2+(V) and hydroxymethyl ([rad]CH2OH) radical and between UO2+(V) and diphenyl sulfide radical cation (Ph2S[rad]+)

The linear uranyl UO22+(VI) cation (D∞h symmetry) exhibited strong and broad absorptions at 350–400 nm in anhydrous methanol and methanol-water mixtures in the UV-Vis spectra. The intensity of the absorptions (represented by absorbance at 375 nm) is directly proportional to molar concentrations of methanol and UO22+(VI), respectively. The linear relationships indicate formation of an electron-donor-acceptor (EDA) complex [UO22+, CH3OH]. The absorptions at 350–400 nm originate from the charge-transfer (single-electron transfer) from CH3OH (electron donor) to UO22+ (electron acceptor) within the [UO22+, CH3OH] complex. Electron paramagnetic resonance (EPR) studies of various mixtures of UO22+-CH3OH and UO22+-CH3OH-H2O have shown that the charge-transfer also took place slowly in the dark, resulting in thermal reduction of UO22+(VI) to UO2+(V) (singlet, g = 2.08) by CH3OH, and CH3OH was oxidized to the hydroxymethyl CH2OH radical (generating an axial signal). The charge-transfer oxidation-reduction reaction is believed to take place via the EDA [UO22+, CH3OH] complex. EPR studies suggested spin-spin coupling between UO2+(V) and CH2OH in anhydrous methanol, supporting the formation of a [UO2+, CH2OH] ion-radical pair. The EPR studies have also shown that UO22+(VI) was reduced to UO2+(V) thermally by other alcohols (ethanol, 2-propanol, and cyclohexanol), and by diphenyl sulfide (Ph2S), l-ascorbic acid (AA), and 2-methyl-5-(propan-2-yl)phenol (carvacrol, ArOH), respectively. Ph2S, AA, and ArOH were oxidized to the diphenyl sulfide Ph2S+ radical cation (singlet, g = 2.00), ascorbic acid AA radical (singlet, g = 2.00), and carvacrol ArO radical (singlet, g = 1.98), respectively. Both EPR and UV-Vis studies indicate that the reactions followed the ground-state charge-transfer mechanisms similar to that of the UO22+/methanol reaction. EPR evidence supported formation of the [UO2+, Ph2S+] ion-radical pair in the charge-transfer reaction of UO22+ and Ph2S and spin-spin interactions within the ion-radical pair. The sulfuric-acid-catalyzed isomerization of CH2OH to CH3O was found by EPR studies.

EPR Evidence of Liquid Water in Ice: An Intrinsic Property of Water or a Self-Confinement Effect?

Liquid water (LW) existence in pure ice below 273 K has been a controversial aspect primarily because of the lack of experimental evidence. Recently, electron paramagnetic resonance (EPR) has been used to study deeply supercooled water in a rapidly frozen polycrystalline ice. The same technique can also be used to probe the presence of LW in polycrystalline ice that has formed through a more conventional, slow cooling one. In this context, the present study aims to emphasize that in case of an external probe involving techniques such as EPR, the results are influenced by the binary phase (BP) diagram of the probe-water system, which also predicts the existence of LW domains in ice, up to the eutectic point. Here we report the results of our such EPR spin-probe studies on water, which demonstrate that smaller the concentration of the probe stronger is the EPR evidence of liquid domains in polycrystalline ice. We used computer simulations based on stochastic Liouville theory to analyze the lineshapes of the EPR spectra. We show that the presence of the spin probe modifies the BP diagram of water, at very low concentrations of the spin probe. The spin probe thus acts, not like a passive reporter of the behavior of the solvent and its environment, but as an active impurity to influence the solvent. We show that there exists a lower critical concentration, below which BP diagram needs to be modified, by incorporating the effect of confinement of the spin probe. With this approach, we demonstrate that the observed EPR evidence of LW domains in ice can be accounted for by the modified BP diagram of the probe-water system. The present work highlights the importance of taking cognizance of the possibility of spin probes affecting the host systems, when interpreting the EPR (or any other probe based spectroscopic) results of phase transitions of host, as its ignorance may lead to serious misinterpretations.

High Resolution Crystal Structures of the Acetohydroxyacid Synthase‐Pyruvate Complex Provide New Insights into Its Catalytic Mechanism

AbstractAcetohydroxyacid synthase (AHAS) is the first enzyme in the biosynthesis pathway of the branched‐chain amino acids, catalyzing the condensation of pyruvate with another molecule of pyruvate or with 2‐ketobutyrate, to produce 2‐acetolactate or 2‐acetohydroxybutyrate, respectively. The catalytic subunit of the dimeric enzyme has thiamin diphosphate (ThDP), a divalent metal ion, flavin adenine dinucleotide (FAD), and two molecules of oxygen (O2(I) and O2(II)) as cofactors. Here, crystal structures of Saccharomyces cerevisiae AHAS in complex with pyruvate provide novel insights into the mechanistic features of this enzyme including: i) The precise position taken by pyruvate molecules as they enter the active site (i. e. prior to catalysis occurring) with conformations suitable for the transfer of electrons to/from O2(I) and FAD; ii) The formation of ternary donor‐acceptor‐O2(I) complexes and iii) The location of O2(II) relative to the substrate showing that it plays a critical role in the organization of substrate for catalysis. These structural data, accompanied by electron paramagnetic resonance evidence that a radical is produced during AHAS catalysis, lead to the proposal that FAD and O2 are involved in an indirect one‐electron redox cycle. In this mechanism, the spatial configurations of O2 and FAD in the active site can allow electrons to be exchanged with the substrates and catalytic intermediates to satisfy and control the overall AHAS catalyzed reaction.

X-ray Absorption and Electron Paramagnetic Resonance Guided Discovery of the Cu-Catalyzed Synthesis of Multiaryl-Substituted Furans from Aryl Styrene and Ketones Using DMSO as the Oxidant.

The first example of DMSO serving not only as a solvent but also as an oxidant to promote the oxidation of Cu(I) to Cu(II) has been demonstrated. X-ray absorption and electron paramagnetic resonance evidence revealed a single-electron redox process where DMSO could oxidize Cu(I) to Cu(II). The novel discovery guided the rational design of copper-catalyzed oxidative cyclization of aryl ketones with styrenes to furans, providing a new method for the synthesis of multiaryl-substituted furans from cheap and readily available starting materials.

Direct EPR Evidence of Cathodic ECL Coreactant Intermediates of Benzoyl Peroxide and Ammonium Persulfate

Electrogenerated chemiluminescence (ECL) coreactants are crucial towards their roles in forming radical intermediates for chemical oxidation or reduction of the ECL emitters, such as rubrene (RUB) or 9,10-diphenylanthracene (DPA), to form excited state species that emit light. During the cathodic potential scanning, ECL coreactants such as benzoyl peroxide (BPO) and ammonium persulfate were believed to generate benzyloxy free radical (C6H5CO2 • or BPO•) and sulfate anion radical (SO4 -•), respectively, resulting in the chemical oxidation of the ECL emitter and eventually the ECL production. The direct electron paramagnetic resonance (EPR) evidence of the above radical intermediates, generated via chemical reduction of the ECL coreactant benzoyl peroxide in MeCN and ammonium persulfate in water using hydrazine as the reducing agent and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin trapping agent, was obtained. Additionally, EPR spectra of C6H5CO2 • (without DMPO trapping) generated from solid state using UV irradiation were consistent with the spectra obtained from solution phase. The EPR spectra of SO4 -•(with DMPO trapping) were also obtained. Simulations of the experimentally acquired EPR spectra were conducted with EPR simulating software, such as PEST winsim. Financial support from the NSF CAREER Award (CHE-0955878) is gratefully acknowledged.

Disappearance of electron-hole asymmetry in nanoparticles of Nd1−xCaxMnO3(x=0.6,0.4): magnetization and electron paramagnetic resonance evidence

We study and compare magnetic and electron paramagnetic resonance behaviors of bulk and nanoparticles of Nd1−xCaxMnO3 in hole doped (x=0.4;NCMOH) and electron doped (x=0.6;NCMOE) samples. NCMOH in bulk form shows a complex temperature dependence of magnetization M(T), with a charge ordering transition at ∼250 K, an antiferromagnetic (AFM) transition at ∼150 K, and a transition to a canted AFM phase/mixed phase at ∼80 K. Bulk NCMOE behaves quite differently with just a charge ordering transition at ∼280 K, thus providing a striking example of the so called electron-hole asymmetry. While our magnetization data on bulk samples are consistent with the earlier reports, the new results on the nanoparticles bring out drastic effects of size reduction. They show that M(T) behaviors of the two nanosamples are essentially similar in addition to the absence of the charge order in them thus providing strong evidence for vanishing of the electron-hole asymmetry in nanomanganites. This conclusion is further corroborated by electron paramagnetic resonance studies which show that the large difference in the “g” values and their temperature dependences found for the two bulk samples disappears as they approach a common behavior in the corresponding nanosamples.

EPR Evidence of Unusual Dopant Valency States in Nanocrystalline Er-doped CeO2

The structure and oxidation state of the Er dopant cation in CeO2 single crystal and nanocrystals with size ranging from 22 to 300 nm are studied using electron paramagnetic resonance (EPR) spectroscopy at X- and at Q-band near liquid-He temperatures. Besides the expected EPR line due to Er3+ in cubic sites in the lattice, unusual EPR lines with g values around 14 and 20 are observed in nanocrystalline CeO2. The appearance of these lines suggests the formation of non-Kramers Er2+, Er4+ ions, which becomes increasing favorable with decreasing nanoparticle size. Formation of rare earth ions with such unusual oxidation states in nanoparticles can be exploited in tuning their catalytic activity and optical properties.

Direct Observation of Reduction of Cu(II) to Cu(I) by Terminal Alkynes

X-ray absorption spectroscopy and in situ electron paramagnetic resonance evidence were provided for the reduction of Cu(II) to Cu(I) species by alkynes in the presence of tetramethylethylenediamine (TMEDA), in which TMEDA plays dual roles as both ligand and base. The structures of the starting Cu(II) species and the obtained Cu(I) species were determined as (TMEDA)CuCl2 and [(TMEDA)CuCl]2 dimer, respectively.

Phosphorous doping of superlattice silicon quantum dots in silicon dioxide

AbstractHere we investigate the effects of phosphorous (P) doping of silicon quantum dots in a silicon dioxide superlattice structure. The doping occurs by diffusion of P during high‐temperature annealing from P‐rich silicon dioxide regions to directly adjacent layers containing the quantum dots embedded in oxide. Secondary ion mass spectroscopy (SIMS) is used to clearly show that after annealing, the P preferentially resides in the regions containing the dots. Once in these regions, electron paramagnetic resonance (EPR) shows that a relatively small fraction of this P is used to passivate dangling bonds on the surfaces of the quantum dots. Additionally, EPR shows no evidence that any of the P is located in the interior of the dot. Using calibrated SIMS values along with the EPR evidence allows us to conclude that the P is concentrated in the area surrounding the dots in a non‐paramagnetic configuration. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Light-induced electron paramagnetic resonance evidence of charge transfer in electrospun fibers containing conjugated polymer/fullerene and conjugated polymer/fullerene/carbon nanotube blends

Electrospun sub-micron fibers containing conjugated polymer (poly(3-hexylthiophene), P3HT) with a fullerene derivative, phenyl-C61-butyric acid methylester (PCBM) or a mixture of PCBM and single-walled carbon nanotubes (SWCNTs) were studied by light-induced electron paramagnetic resonance spectroscopy. The results provide experimental evidence of electron transfer between PCBM and P3HT components in both fiber systems and suggest that the presence of a dispersing block-copolymer, which acts via physical adsorption onto the PCBM and SWCNT moieties, does not prevent electron transfer at the P3HT-PCBM interface. These findings suggest a research perspective towards utilization of fibers of functional nanocomposites in fiber-based organic optoelectronic and photovoltaic devices. The latter can be developed in the textile-type large area photovoltaics or individual fiber-based solar cells that will broaden energy applications from macro-power tools to micro-nanoscale power conversion devices and smart textiles.

EPR evidence of antiferromagnetic ordering in single‐layer graphene

AbstractWe report electron paramagnetic resonance (EPR) evidence of the antiferromagnetic ordering in pristine single‐layer graphene. Temperature dependences of the parameters of EPR spectra obtained for vacuum‐processed samples were studied within the temperature range of 4.2–300 K. Our experiment has confirmed recent theoretical predictions that in single‐layer graphene the carrier‐mediated exchange interaction leads to antiferromagnetic coupling. We note some quantitative discrepancies between the theory and experimental findings and discuss their origins. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

EPR evidence for the restricted mobility of NO 2 in gamma irradiated thorium nitrate pentahydrate Th(NO 3) 4·5H 2O

Electron paramagnetic resonance (EPR) studies were conducted on gamma irradiated polycrystalline sample of thorium nitrate pentahydrate, Th(NO 3) 4·5H 2O, in the temperature range of 100–300 K. The most prominent species with triplet hyperfine structure in the EPR spectrum was identified as NO 2. The EPR spectrum gave evidence for the stabilization of NO 2 in at least three different sites slightly differing in spin Hamiltonian parameters (Site 1: g x = 2.0042, g y = 1.9911, g z = 2.0020, A x = 54.20 G, A y = 48.50 G and A z = 65.25 G; Site 2: g x = 2.0042, g y = 1.9911, g z = 2.0020, A x = 54.20 G, A y = 48.50 G and A z = 67.85 G; Site 3: g x = 2.0045, g y = 1.9911, g z = 2.0015, A x = 54.20 G, A y = 49.05 G and A z = 72.45 G). The EPR spectra for Site 1 revealed molecular dynamics of NO 2 from a slow motion region to fast motion region as the sample temperature was varied from 100 to 300 K. This led to a change in EPR spectrum from orthorhombic to axial, indicating preferred rotation of NO 2 molecule about the O–O bond direction. However, the NO 2 molecule at Site 2 was found to be rigid throughout the entire temperature range. The differences in the mobility of NO 2 molecules occupying the two sites could be attributed to the fact that in one case NO 2 was bonded to thorium or water and in the other case it was weakly bound. The NO 2 bound to thorium through two oxygen atoms or bound to thorium on one side through one oxygen atom and hydrogen bonded to water on the other side remains rigid throughout the entire temperature range, while NO 2 situated at interstitial sites or adsorbed on the surface exhibits mobility with increase in temperature above 100 K.

EPR Evidence for Co(IV) Species Produced During Water Oxidation at Neutral pH

Thin-film water oxidation catalysts (Co-Pi) prepared by electrodeposition from phosphate electrolyte and Co(NO(3))(2) have been characterized by electron paramagnetic resonance (EPR) spectroscopy. Co-Pi catalyst films exhibit EPR signals corresponding to populations of both Co(II) and Co(IV). As the deposition voltage is increased into the region where water oxidation prevails, the population of Co(IV) rises and the population of Co(II) decreases. The changes in the redox speciation of the film can also be induced, in part, by prolonged water oxidation catalysis in the absence of additional catalyst deposition. These results provide spectroscopic evidence for the formation of Co(IV) species during water oxidation catalysis at neutral pH.

Hydroxyl radical release from dental resins: Electron paramagnetic resonance evidence

It is well known that polymeric free radicals remain trapped inside dental resins for a long time after photopolymerization. Moreover, although these high molecular mass compounds have very limited mobility, there is evidence to suggest that they disappear progressively over time. The purpose of this study was to provide new experimental data to help understand this phenomenon. To determine whether low molecular mass free radicals are released by dental composites stored in hydrophilic media, we used electron paramagnetic resonance spectroscopy to perform spin-trapping experiments on experimental and commercial samples stored in ethanol. Under these conditions, ethoxy radicals were produced. Further experiments demonstrated that (1) hydroxyl radicals were released from the methacrylated resin and (2) they reacted with ethanol molecules to produce "secondary" ethoxy free radicals. In addition to the well-known monomer toxicity of methacrylated resins, we may have identified a new source of concern for these biomaterials.

EPR Evidence for Premonitory Charge-Ordering Fluctuations in Nanomanganite Pr0.57Ca0.41Ba0.02MnO3

Electron paramagnetic resonance (EPR) and magnetic properties of nanocrystalline $Pr_{0.57}Ca_{0.41}Ba_{0.02}MnO_3$ are studied and compared with those of the bulk material. The nanoscale samples prepared by the sol–gel method were annealed at different temperatures to obtain particles of different sizes. The crystallite and particle sizes were estimated by X-ray diffraction and transmission electron microscopy and found to be approximately equal to 30, 60 and 100 nm. Magnetization studies on the 60 and 100 nm particles show charge-ordering at 230 K similar to the bulk sample prepared by solid-state reaction, while in the 30 nm particles the charge-ordering signature is absent. At low temperatures all the three nanosamples show ferromagnetic transitions. This result is confirmed by the EPR intensity behavior as well. However, the EPR line width, which is reflective of the spin dynamics, shows a shallow minimum for the 30 nm particles at the temperature of 230 K, corresponding to the charge-ordering transition. We interpret this result as an indication of the presence of charge-ordering fluctuations in the nanoparticles of 30 nm size even though the static charge order is absent, thus heralding the occurrence of charge order in the larger particles.