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We present in-depth profiling by micro FTIR of cross sections for Nafion 115 membranes in membrane-electrode assemblies (MEAs) degraded during 52 or 180 h at open circuit voltage (OCV) conditions, 90 °C and 30% relative humidity. Analysis of optical images showed highly degraded zones in both MEAs. Corresponding 2D FTIR spectral-spatial maps indicated that C-H and C═O groups are generated during degradation. The highest band intensities for both groups appeared at a depth of 82 μm from the cathode in the MEA degraded for 180 h; the same bands were present but less intense at a depth of 22 μm from the cathode. Degradation at these depths is most likely associated with the location of the Pt band formed from Pt dissolution and migration into the membrane. The two degradation bands, C═O and C-H, appeared at the same depths from the cathode, 82 and 22 μm, suggesting that they are generated by a common mechanism or intermediate. This result is rationalized by a very important first reaction: Abstraction of a fluorine atom from the polymer main chain and side chain by hydrogen atoms, H

Antibiotic-resistant superbug bacteria represent a global health problem with no imminent solutions. Here we demonstrate that the combination (termed AB569) of acidified nitrite (A-NO 2 − ) and Na 2 -EDTA (disodium ethylenediaminetetraacetic acid) inhibited all Gram-negative and Gram-positive bacteria tested. AB569 was also efficacious at killing the model organism Pseudomonas aeruginosa in biofilms and in a murine chronic lung infection model. AB569 was not toxic to human cell lines at bactericidal concentrations using a basic viability assay. RNA-Seq analyses upon treatment of P. aeruginosa with AB569 revealed a catastrophic loss of the ability to support core pathways encompassing DNA, RNA, protein, ATP biosynthesis, and iron metabolism. Electrochemical analyses elucidated that AB569 produced more stable SNO proteins, potentially explaining one mechanism of bacterial killing. Our data implicate that AB569 is a safe and effective means to kill pathogenic bacteria, suggesting that simple strategies could be applied with highly advantageous therapeutic/toxicity index ratios to pathogens associated with a myriad of periepithelial infections and related disease scenarios.

The most commonly used proton exchange membranes (PEMs) for hydrogen and direct methanol fuel cells are the perfluorosulfonic acid ionomers (PFSA) such as Nafion, which are based on a Teflon-like backbone with side chains containing sulfonic groups. These materials exhibit good mechanical, chemical, and thermal stability in both oxidative and reductive media; however, the fuel cell is a reactor with strong oxidizing power, capable of reducing the durability of the PEMs. The centrality of hydroxyl radicals, HO·, in the degradation mechanism of PFSA is well documented in numerous papers, including our work. Therefore, membrane durability remains a critical issue for further development of fuel cell applications. We present the use of ceria nanoparticles (NPs) for mitigating the chemical degradation process and describe their effectiveness for scavenging hydroxyl radicals as a function of their size and concentrations. The hydroxyl radicals were generated by UV-irradiation of hydrogen peroxide, and the radic...

We present the effect of the degree of sulfonation (DS) and water content on the phase separation in sulfonated poly(ether ether ketone) (SPEEK) ionomers, using the method of spin probe electron spin resonance (ESR). The ionomer contained the nitroxide radical 4-oxo-TEMPO (TEMPONE) as the spin probe. ESR spectra in the temperature range 110–350 K were measured in order to obtain information on the dynamics of the spin probe, and the local polarity of the probe site in SPEEK membranes with DS = 38%, 58%, and 70%. ESR spectra at 110 K and at 350 K were simulated for each DS value, in samples soaked at ambient temperature and at 60 °C. For all samples the hyperfine splittings for the 14N nucleus, aiso, was in the range 15.4–15.8 G, indicating a hydrophilic environment of the probe, based on the comparison with the aiso value measured for TEMPONE in bulk water at 350 K. The local dynamics of the probe is reflected in the type and number of spectral components as a function of DS and type of soak. At 350 K spe...

We report attempts to synthesize and to study the stability of anion exchange membranes (AEMs) from the perfluorinated 3M membrane precursor having terminal sulfonyl fluoride, –SO2F, groups in the side chains of the backbone. The precursor was obtained in powder form from which flexible films were prepared using hot pressing; the films were reacted with trimethylamine (TMA), 1,4-dimethylpiperazine (DMP), and quaternized 1,4-diazabicyclo[2.2.2]octane (DABCO) amines, following published protocols. The chemical stability of the generated membranes was determined by FTIR, and by 1H and 19F NMR, first after the reaction between the precursor and the amines, and then after addition of KOH. The FTIR spectra indicated the presence of the C–H bands of the amines after reaction, and their disappearance after addition of KOH, a result consistent with 1H NMR spectra of the membranes swollen by deuterated dimethylsulfoxide (DMSO-d6). The 19F NMR spectra indicated the presence of SO3− anions, in the membranes after the initial amination reaction and also after addition of KOH. This deduction was consistent with the detection of the band at 1052 cm−1 in the FTIR spectra of membranes generated after amination, as well as after the addition of KOH. The main conclusion is that no AEMs were generated by the reaction of the 3M precursor with TMA, DMP, and DABCO: The amines acted as catalysts for the hydrolysis of the sulfonyl fluorides and led to the generation of the SO3− anions. The conclusions of this study are expected to be valid for synthetic efforts to obtain AEMs starting with other sulfonyl fluoride polymers, such as the Nafion™ precursor.

We present experiments in a fuel cell (FC) inserted in the resonator of the electron spin resonance (ESR) spectrometer, which allowed separate monitoring of radical formation at anode and cathode sides. The in situ FC was operated at 300 K under closed- and open-circuit voltage conditions, CCV and OCV, respectively, with a membrane–electrode assembly (MEA) based on sulfonated poly(ether ether ketone) (SPEEK). The formation of radicals was monitored by spin trapping ESR, with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin trap. The magnetic parameters and the relative intensities of DMPO adducts were determined by simulation of the ESR spectra. The major adducts detected at both electrodes were DMPO/OOH and DMPO/H. The generation of the DMPO/OOH adduct at the anode can be explained by reaction of crossover oxygen with hydrogen atoms formed at the Pt catalyst; at the cathode this adduct can be generated by H2 crossover to the cathode, reaction at the Pt catalyst, and reaction with O2. The generation of...

Spin trapping ESR experiments were performed in a fuel cell inserted in the resonator of the ESR spectrometer, and allow visualization of crossover processes and separate detection of chemical reactions at anode and cathode. The membrane-electrode assembly (MEA) consisted of the perfluorinated 3M membranes and Pt as catalyst; the detection of unstable intermediates was possible with 5,5-Dimethyl-1-pyrroline N -oxide (DMPO) as the spin trap. The major goal was to determine the effect of membrane thickness on the type and intensity of the spin adducts; MEAs with membrane thicknesses of 25 μm, 50 μm and 100 μm were studied. DMPO/H and DMPO/OOH adducts were detected in most experiments and their presence is explained by the generation of hydrogen atoms, H·, at the catalyst, and reaction with crossover O 2 . A strong effect of membrane thickness was detected: For a 25 μm thickness the relative intensities of DMPO/H and DMPO/OOH are similar at both anode and cathode. For 100 μm thickness the relative intensity of DMPO/H at the cathode decreased significantly, due to decreased H 2 crossover. At the anode DMPO/H was the only adduct detected ; the absence of DMPO/OOH is assigned to negligible O 2 crossover. The results provide evidence for the involvement of hydrogen atoms in the membrane degradation. Enhancement of crossover rates in thinner membranes is expected to reduce membrane durability.

Fuel cells (FCs) convert the chemical energy from the reaction of H2 with O2 to electrical energy, and have become an alternative clean energy source for automotive, portable and stationary applications. FC operation is possible when the membrane located between the electrodes maintains its integrity in the oxidising FC environment. Spin trapping electron spin resonance (ESR) has been used for determining the degradation mechanism of the perfluorinated membranes used in FCs. The study of low molecular weight model compounds confirmed two possible degradation mechanisms in membranes: initiated at the backbone and at the side chain. In situ experiments in a FC inserted in the resonator of an ESR spectrometer offered the ability to monitor radical processes in a FC. The presence of the radicals was determined by addition of 5,5-dimethy-l-pyrroline N-oxide (DMPO) as a spin trap. Taken together, the in situ results pointed to crossover processes, reactions at the catalyst surface, and the involvement of H• ato...

We present an ATR-FTIR study of three major perfluorinated ionomers that are used as proton exchange membranes in fuel cells (FCs) and that differ in their side chains: Nafion, Aquivion and 3M membrane. The choice of the following low-molecular-weight model compounds (MCs) that mimic the membrane side chains was essential for FTIR band assignment: perfluoro(3-methyl-2,4-dioxahexane)sulfonic acid for Nafion, perfluoro(2-ethoxyethane)sulfonic acid for Aquivion, and perfluoro(2-ethoxybutane)sulfonic acid for the 3M membrane. The major goal was to identify spectral bands that can be used for recognizing bonds involved in membrane fragmentation. A major focus was on the signals from the C–O–C bonds, which were assigned in some papers for Nafion and Aquivion membranes. Our ATR-FTIR results for 3M membranes did not conform to these assignments, and DFT calculations of the vibrational frequencies for the MCs were used to resolve this conundrum. The ATR-FTIR spectra of membranes and MCs and the DFT calculations led to an understanding of the fingerprint region of all membranes, and to a re-examination and re-assignment of results for Nafion and Aquivion membranes. The low intensity of the spectral bands for the ether link connected to the backbone (for all membranes) and also in the side chain (for Nafion) suggests that these bands cannot be used for the determination of the extent of degradation.

In this paper, we present the detailed study on the correlation of the nuclear magnetic resonance (NMR) parameters with the results of density functional theory calculations performed for paramagnetic high-spin cobalt(II) complexes with trispyrazolylborate ligands. This work focuses on estimation of dipolar and contact shifts in mono-ligand high-spin cobalt(II) pyrazolylborate systems along with discussion on 1H NMR properties of the mentioned tetra-, penta- and hexacoordinate complexes. The calculation results show frontier molecular orbitals that may be responsible for contact shift. The calculated contact shift values are compared with the dipolar shift and hyperfine ones.

Density functional theory (DFT) calculations were applied for the modeling of the adsorption sites of nitric oxide in Li+ exchanged zeolite A previously studied experimentally. Two model clusters, ...

The stability of Nafion, stabilized Nafion (StNafion), and 3M and Aquivion membranes to attack by the hydroxyl radical, HO•, was compared in their aqueous dispersions at 300 K. HO• radicals were generated by UV irradiation of hydrogen peroxide (H2O2), and radicals were detected by spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Two types of adducts were detected in all dispersions: DMPO/OH and DMPO/CCR, an adduct of carbon-centered radicals (CCRs) derived from the polymers. The competitive kinetics (CK) approach that has been adapted for ranking the polymer stability leads to the determination of their reaction rate constant with hydroxyl radicals. The experiments consisted of measuring the concentration of the DMPO/OH adduct in the presence of the polymers, which are considered to be “competitors” that react with HO• radicals and compete with the spin trapping reaction HO• + DMPO → DMPO/OH. The results indicated an improved stability in StNafion compared with Nafion. The largest effect was, h...

Density functional theory (DFT) has been applied to model the structure of (Ag•CH2OH/A)+ complexes previously experimentally characterized by electron paramagnetic resonance (EPR) in zeolite matrices. The magnetic parameters of (Ag•CH2OH/A)+ were found to depend on the local structure of the zeolite represented by clusters referred to as 3T and 6T, and also on the applied computational method. A spin distribution analysis confirms the one-electron silver-carbon bonding, showing delocalization of an unpaired electron density distributed between the sliver and carbon atoms along the one-electron silver-carbon bond. The results are of relevance for a deeper understanding of the electronic and catalytic properties of zeolites containing silver atoms and clusters. Original Publication:Marek Danilczuk, Dariusz Pogocki and Anders Lund, Interaction of .(CH2OH) with silver cation in Ag-A/CH3OH zeolite: A DFT study, 2009, CHEMICAL PHYSICS LETTERS, (469), 1-3, 153-156.http://dx.doi.org/10.1016/j.cplett.2008.12.060Copyright: Elsevier Science B.V., Amsterdam.http://www.elsevier.com/

We present a detailed study on the correlation of the nuclear magnetic resonance (NMR) parameters with the results of density functional theory (DFT) calculations performed for paramagnetic high-spin cobalt(II) complexes with trispyrazolylborate ligands. This work is a first attempt to calculate dipolar and contact shifts in high-spin cobalt(II) pyrazolylborate systems. The calculation results show frontier orbitals that may be responsible for the contact shift. The contact shift values are compared with the dipolar shift ones. The latter shift values were both obtained from experimental data and calculated using DFT methods. We attempt to explain the dipolar effect, which may have a great share in NMR chemical shift of paramagnetic compounds.

Low-molecular weight model compounds (MCs) for Nafion membranes used in fuel cells were exposed at 300 K to ·OH radicals produced by UV irradiation of aqueous H2O2 solutions. The MCs contained fluorinated and partially fluorinated groups terminated by sulphonic or carboxylic acid groups. The fragmentation process in the MCs was studied by spin trapping electron spin resonance (ESR) methods, using 5,5-dimethylpyrroline-N-oxide (DMPO), N-tert-butyl-α-phenylnitrone (PBN) and 2-methyl-2-nitrosopropane (MNP) as the spin traps. The objective of these experiments was to assess the effect of the type of ionic groups (sulphonic or carboxylic) and of fluorine substitution on the spin adducts detected. DMPO experiments led to the detection of spin adducts of ·OH and of carbon-centred radicals (CCRs), and allowed the determination of the ·OH attack site on the ionic and/or on the protiated or fluorinated groups. CCR adducts were also detected when using PBN as a spin trap; a key point in the interpretation of the PBN results was, however, the realisation that MNP is formed during PBN exposure to UV irradiation and oxygen or other oxidants such as H2O2. Experiments with MNP as the spin trap were the most informative in terms of structural details for adducts obtained from each MC. The results allowed the identification of CCRs present as adducts, based on large hyperfine splittings (hfs) from, and the number of, interacting 19F nuclei; in addition, oxygen-centred radicals (OCRs) as MNP adducts were also identified, with much lower hfs from 19F nuclei. Taken together, the results deduced by spin trapping suggest that both sulphonic acid and acetic acid groups can be attacked by ·OH radicals and confirm two possible degradation mechanisms in Nafion membranes: initiated at the backbone and at the side chain.

During the last few decades microporous and mesoporous materials have been considered for medical use due to biological properties and stability in biological environment. Zeolites have been investigated as drug carriers, and as adjuvants in anticancer therapy, dietetic supplements or antimicrobial agents. This review gives a brief overview of the major aspects of molecular sieves applications in medicine.

Polycrystalline ammonium dithionate has been examined for its radiation response in the low dose range (< 5 Gy) using EPR technique. The •SO3- radical ion was detected as a single EPR line with a peak-to-peak derivative width of ca. 0.44 mT in irradiated samples and its intensity was found to vary linearly with dose. At equal and moderate settings of microwave power and modulation amplitude ammonium dithionate was at least 7 times more sensitive than L-alanine which is the most common EPR dosimeter standard. Pulse experiments were performed on the powder samples to obtain the longitudinal relaxation time. These and microwave saturation experiments served to indicate the optimal microwave power to be applied during measurements as an EPR dosimeter for best sensitivity of this material. It is thus claimed that ammonium dithionate has excellent potential to become an EPR dosimeter with a low limit of the measurable dose for cases where tissue equivalence is not required or can be corrected for. Original publication: M. Danilczuka, H. Gustafsson, M.D. Sastry, E. Lund and A. Lund, Ammonium Dithionate – a New Material for Highly Sensitive EPR Dosimetry, 2008, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy, (69), 1, 18-21. http://dx.doi.org/ 10.1016/j.saa.2007.03.001. Copyright: Copyright: Elsevier B.V., http://www.elsevier.com/

Direct electron spin resonance (ESR) and spin trapping methods are used in our laboratory in order to identify radical fragments in fluorinated membranes (Nafion) used in fuel cells and in model compounds when exposed to reactive oxygen species. Oxygen radicals were generated by the Fenton reaction, by the photo-Fenton method, and by UV-irradiation of aqueous H 2 O 2 solutions. Direct ESR detection led to the identification of fluorinated radical fragments in UV-irradiated Nafion neutralized by Cu(II), Fe(II), and Fe(III) cations and exposed to H 2 O 2 . In Nafion exposed to the Fenton reagent in the presence of DMPO (5,5-dimethyl-1-pyrroline) as the spin trap, radical adducts of hydroxyl and carbon-centered radicals (CCRs) were detected. A fluorinated model compound, perfluro-(2-ethoxyethane) sulfonic acid (CF 3 CF 2 OCF 2 CF 2 SO 3 H, PFEESA), that mimics the side chain of Nafion was chosen for study, with the goal to shed light on the more complicated degradation processes in the polymeric membranes. Analysis of the spin adducts obtained from UV-irradiated PFEESA in the presence of H 2 O 2 and MNP (2-methyl-2-nitrosopropane) as the spin trap suggested that the side chain of Nafion is a possible site of attack by oxygen radicals.

This paper presents studies on paramagnetic intermediates, free atoms and radicals produced in γ-irradiated molecular sieves and their reactions with adsorbate molecules or exchangeable cations. Four different systems have been investigated using EPR spectroscopy, Na-A/CH4, AgNa-A/CH3OH, Ag-SAPO-11/C2H4 and AgCs-rho/NH3. It was found that methyl radicals are formed in two different sites in Na-A/CH4 and in one of them they are stable at room temperature. The formation of Ag·CH2OH+ radical cation with one-electron bond between silver and carbon has been established in AgNa-A/CH3OH by EPR experiments with [13C]CH3OH and DFT calculations. In Ag-SAPO-11/C2H4 the stabilisation of biligand silver/ethylene complex, Ag0(C2H4)2 was postulated based on EPR and DFT results. Tetrameric silver clusters (Ag 4 3+ ) produced radiolytically in AgCs-rho/NH3 strongly interact with two ammonia molecules as was deduced from the changes in superhyperfine structure of high-field EPR line of Ag 4 3+ pentet for zeolite exposed to [14N]NH3 and [15N]NH3. The presented examples clearly show that the combination of radiation methods with EPR technique is very useful to study the structure and reactivity of paramagnetic intermediates.

EPR dosimetry employing L-alpha-alanine has been in vogue during the past few years, due to its tissue equivalence and linear dose response. However, L-alpha-alanine dosimetry has been improved during the past years, the sensitivity of this material is still too low for clinical applications. Polycrystalline lithium formate doped with NiCl2 was therefore examined for radiation response in the dose range of clinical interest (5 Gy) using CW EPR and pulse EPR techniques. At equal and moderate settings of microwave power and modulation amplitude lithium formate doped with 1.6 wt% of NiCl2 was almost four times more sensitive compared to L-alpha-alanine, which is the most common EPR dosimeter standard. It was shown that the nickel-doped lithium formate has an excellent radiation response with a low limit of the measurable dose, and a linear dose response in the range 1-5 Gy. The relaxation and power saturation studies showed that high microwave power can be applied during measurements to improve the sensitivity of this material as an EPR dosimeter. These results show that lithium formate doped with Ni(II) exhibits promising properties required for further development of an EPR dosimeter in the dose range typical for clinical dosimetry.

Irradiated samples of deproteinized powdered human bone (femur) have been examined by electron paramagnetic resonance (EPR) spectroscopy in X, Q and W bands. In the bone powder sample only one type of CO2− radical ion is stabilized in the hydroxyapatite structure in contrast to powdered human tooth enamel, a material also containing hydroxyapatite, widely used for EPR dosimetry and in which a few radicals are stable at room temperature. It is suggested that the use of deproteinized bone for EPR dosimetry could improve the accuracy of dose determination.

The ESR spectrum of the chain-end radical RCF2CF2* detected in Nafion perfluorinated membranes exposed to the photo-Fenton reagent was accurately simulated by an automatic fitting procedure, using as input the hyperfine coupling tensors of the two F alpha and two F beta nuclei as well as the corresponding directions of the principal values from density functional theory (DFT) calculations. An accurate fit was obtained only for different orientations of the hyperfine coupling tensors for the two F alpha nuclei, indicating a nonplanar structure about the C alpha radical center. The fitted isotropic hyperfine splittings for the two F beta nuclei in the Nafion radical, 24.9 and 27.5 G, are significantly larger than those for the chain-end radical in Teflon (15 G), implying different radical conformations in the two systems. The excellent fit indicated that the geometry and electronic structure of free radicals can be obtained not only from single-crystal ESR spectroscopy, but also, in certain cases, from powder spectra, by combination with data from DFT calculations. The optimized structures obtained by DFT calculations for the CF3CF2CF2CF2* or CF3OCF2CF2* radicals as models provided additional support for the pyramidal structure determined from the spectral fit. Comparison and analysis of calculated and fitted values for the hyperfine splittings of the two F beta nuclei suggested that the radical detected by ESR in Nafion is ROCF2CF2*, which originates from attack of oxygen radicals on the Nafion side chain. The combination of spectrum fitting and DFT is considered important in terms of understanding the hyperfine splittings from 19F nuclei and the different conformations of fluorinated chain-end-type radicals RCF2CF2* in different systems, and also for elucidating the mechanism of Nafion fragmentation when exposed to oxygen radicals in fuel cell conditions.

This review gives a brief overview over the major aspects of application of the nicotine alkaloid and its close derivatives in the therapy of some neurodegenerative disorders and diseases (e.g. Alzheimer's disease, Parkinson's disease, Tourette's syndrome, schizophrenia etc.). The issues concerning methods of nicotine analysis and isolation, and some molecular aspects of nicotine pharmacology are included. The natural and synthetic analogues of nicotine that are considered for medical practice are also mentioned. The molecular properties of two naturally occurring nicotine enantiomers are compared--the less-common but less-toxic (R)-nicotine is suggested as a natural compound that may find its place in pharmaceutical practice.

Three-pulse electron spin echo envelope modulation (ESEEM), hyperfine sublevel correlation spectroscopy (HYSCORE) investigations and two-pulse electron spin echo (ESE) measurements of phase memory time TM, were carried out, in the 20-200 K temperature range, on an AsO44- paramagnetic probe stabilized in RbH2PO4 (RDP), NH4H2PO4 (ADP), and dipolar glass Rb0.5(NH4)0.5H2PO4 (RADP). The results obtained on ADP revealed hyperfine interaction of the probe ion with the 14N of the ammonium ion, the coupling constant satisfying the condition of 'cancellation' at a field of 480 mT. The ammonium ion was found to be in two different sites in ADP, which became indistinguishable on the formation of dipolar glass RADP. These results were confirmed by HYSCORE spectral measurements. The fast Fourier transform (FFT) spectra of three-pulse ESEEM decays have clearly revealed the interaction with protons in the bond both in ADP and RDP, and in RADP with an averaged coupling constant. The phase memory times in RADP exhibited strong temperature dependence and were found to be dependent on the nuclear spin quantum number mI of 75As. The temperature dependence of TM exhibited a well-defined maximum around 90 K, coinciding with the temperature of onset of 'freezing' in Rb0.5(NH4)0.5H2PO 4. This is symptomatic of dynamic fluctuations in the dipolar glass phase, with onset around 150 K, going through a maximum around 90 K and slowing down on further cooling. These results suggest that in RADP, a dynamical mechanism with progressive slowing down below 90 K is operative in the glass formation. This implies that the RADP system, with x = 0.5, exists in an ergodic relaxor (R)-state in the 20-200 K temperature range wherein every fluctuating monodomain can be viewed as statistically representative of the whole sample. © IOP Publishing Ltd.

Electron paramagnetic resonance (EPR) studies of X-irradiated lithium dithionate, Li 2 S 2 O 6 ·2H 2 O, doped with Ni and Rh have shown that these impurities enhance the yield of radicals formed by X-irradiation at room temperature. The signal in the doped samples, measured peak-to-peak of the single EPR derivative line attributed to the SO 3 − anion was about 3–4 times that of the pure lithium dithionate and more than 10 times stronger than the alanine signal. These impurities also shortened the spin-lattice relaxation time, T 1 , which gives the possibility to measure the doped samples at a higher microwave power. This implies that sensitivity could be further enhanced in the already sensitive EPR dosimeter material lithium dithionate.

Polycrystalline, formates and dithionates are promising materials for EPR dosimetry, as large yields of radiation induced stable radicals are formed with a linear dose response. Rapid spin relaxation rates were detected in many of the substances, indicating that a high microwave power can be applied during EPR acquisition in order to improve sensitivity. Different techniques used to further improve the sensitivity, such as the replacement of Li-7 with Li-6 or exchange of protons with deuterons in the corresponding crystalline matrices and metal ion doping are discussed. It is concluded that formates and dithionates may be up to 10 times as sensitive as L-alpha-alanine. (C) 2004 Elsevier Ltd. All rights reserved.

Formates and dithionates of Li-6, enriched and Li-7 in natural composition of Li offer a possibility to measure the absorbed dose from photons and thermal neutrons in a mixed radiation field for in ...

Mesoporous aluminosilica molecular sieves AlMCM-41 are loaded with different amounts of Ag+, Pd2+ and Pt2+ ions by varying the Si/Al ratio, which changes the ion exchange capacity. These metal ions are reduced to metal particles by hydrogen and characterized by conduction electron paramagnetic resonance (CEPR). Metal particle sizes were calculated from the Kawabata theory that relates CEPR linewidths to metal particle size. The CEPR linewidths are independent of metal ion loading, which suggests that the metal particles are located inside the AlMCM-41 channels and size constrained by the channel diameter. The calculated metal particle diameters are about 1 nm, which is smaller than the 3.5 nm channel diameter of AlMCM-41. The validity of Kawabata's theory to such systems may need reexamination.

Cationic silver clusters formed in γ-irradiated AgCs-rho zeolite have been studied by EPR spectroscopy. Tetrameric silver is formed efficiently in the dehydrated form of zeolite as well as in samples exposed to ammonia. However, the hyperfine splitting of the Ag43+ pentet in AgCs-rho/NH3 zeolite is distinctly smaller due to the shift of spin density from silver to nitrogen nuclei, suggesting the formation of an Ag43+–ammonia complex. The observed 14N and 15N superhyperfine splitting from interacting 14NH3 and 15NH3 molecules unequivocally proved the presence of a multicore Ag43+(NH3)n complex. Based on the simulation of 14N and 15N superhyperfine interactions a structure of the complex was proposed in which two ammonia molecules are located close to the octagonal windows of two neighboring α-cages and four further molecules are placed in next-neighboring α-cages interacting with the Ag tetramer through square windows. When dehydrated AgCs-rho zeolite containing radiation-induced Ag43+ clusters is exposed to ammonia the Ag hyperfine splitting of silver tetramer decreases from 13.9 to 10.7 mT proving that Ag43+ clusters are located in the sites accessible to adsorbates.

Electron spin resonance (ESR) spectroscopy has been used to study the conduction electron spin resonance (CESR) of small silver particles stabilized in dehydrated Ag-rho zeolite. Silver particles were produced by hydrogen reduction at elevated temperatures and diameter of the stabilized particles was calculated based on the linewidth of CESR signal using Kawabata theory.

The stability of proton conducting polymers under aggressive conditions in a fuel cell (FC) is one of the major challenges in the development of FCs based on proton exchange membranes (PEMs). This study is focused on the determination of the stability of sulfonated poly(benzoyl paraphenylene) (SParmax) membranes exposed to elevated temperature (100–350 °C) and highly reactive radicals. To determine the durability of SParmax, attenuated total reflectance infrared spectroscopy (ATR-FTIR), electron spin resonance (ESR), and thermogravimetric analysis coupled with quadrupole mass spectrometry (TGA/QMS) methods were applied. In situ experiments using a FC inserted into the resonator of the ESR spectrometer combined with spin trapping offered the possibility to detect separately processes at the anode and cathode sides and to monitor the formation of radicals responsible for membrane degradation. The radical species (H • , D • , HO • , DO • ) were detected immediately after the flow of feeding gases started. However, membrane-derived degradation products, such as carbon-centered radicals (CCRs) or formation of carbonyl bonds, were detected only in ex situ experiments when the samples were degraded at 250 °C during 1 h.

We present experiments in an in situ fuel cell (FC) inserted in the resonator of the ESR spectrometer that offered the ability to observe separately processes at anode and cathode sides and to monitor the formation of HO and HOO radicals, H and D atoms, and radical fragments derived from the Nafion membrane. The presence of the radicals was determined by spin-trapping electron spin resonance (ESR) with 5,5-dimethylpyrroline N-oxide (DMPO) as a spin trap. The in situ FC was operated at 300 K with a membrane-electrode assembly (MEA) based on Nafion 117 and Pt as catalyst, at closed and open circuit voltage conditions, CCV and OCV, respectively. Experiments with H(2) or D(2) at the anode and O(2) at the cathode were performed. The DMPO/OH adduct was detected only at the cathode for CCV operation, suggesting generation of hydroxyl radicals from H(2)O(2) formed electrochemically via the two-electron reduction of oxygen. The DMPO/OOH adduct, detected in this study for the first time in a FC, appeared at the cathode and anode for OCV operation, and at the cathode after CCV FC operation ofor=2 h. These results were interpreted in terms of electrochemical generation of HOO at the cathode (HO + H(2)O(2) --H(2)O + HOO) and its chemical generation at the anode from hydrogen atoms and crossover oxygen (H + O(2) --HOO). DMPO/H and DMPO/D adducts were detected at the anode and cathode sides, for CCV and OCV operation; H and D are aggressive radicals capable of abstracting fluorine from the tertiary carbon in the polymer membrane chain and of leading to chain fragmentation. Carbon-centered radical (CCR) adducts were detected at the cathode after CCV FC operation; weak CCR signals were also detected at the anode. CCRs can originate only from the Nafion membranes, and their presence indicates membrane fragmentation. Taken together, this study has demonstrated that FC operation involves processes such as gas crossover, reactions at the catalyst surface, and possible attack of the membrane by reactive H or D that do not occur in ex situ experiments in the laboratory, thus implying different mechanistic pathways in the two types of experiments.

Organometallic radicals are important intermediates in biological and catalytic reactions. The structure and formation mechanism of radicals trapped in γ-irradiated molecular sieves exposed to methanol and ethylene have been studied by EPR spectroscopy. It was found that Ag·CH 2 OH + radical with one-electron bond between Ag and C is formed by the attack of ·CH 2 OH hydroxymethyl radical on Ag + cation. In Ag-SAPO-11/C 2 H 4 zeolite the EPR at 77 K shows the spectra of Ag 0 atoms and ·C 2 H 5 radicals. After annealing at 230 K those species disappeared and then an anisotropic EPR sextet was recorded. Based on DFT calculation the structure of complex was proposed in which two C 2 H 4 ligands adopted eclipsed confirmation on either side of the Ag atom. As a result the overwhelming spin density is localised on ethylene orbitals.

Electron paramagnetic resonance (EPR) spectroscopy was applied to study paramagnetic species stabilized in Na-A zeolite exposed to gaseous methane and gamma-irradiated at 77 K. Two types of EPR spectra were recorded during thermal annealing of zeolite up to room temperature. Owing to the results for the zeolite exposed to (13)CH(4) the multiplet observed at 110 K was assigned to a (.-)CH(3)...Na(+) complex. After decay of the multiplet, the isotropic quartet of methyl radical was recorded in the temperature range of 170-280 K. On the basis of the EPR parameters it is postulated that (.-)CH(3) radicals in this temperature region are able to freely rotate inside the zeolite cage. The structures of the (.-)CH(3)...Na(+) adsorption complex and respective hyperfine coupling constants were calculated by applying DFT quantum chemical methods. Two different models were applied to represent the zeolite framework: the 6T structure of one six-membered ring and the 3T cluster. The hyperfine coupling constants calculated for the (.-)CH(3)...Na(+) adsorption complex for both applied models show very good agreement with those obtained experimentally.

The silver agglomeration process in γ-irradiated AgNaCs-rho zeolites and AgNa-sodalites has been investigated by electron paramagnetic resonance spectroscopy. In dehydrated rho zeolites the subsequent formation of Ag 0 , Ag 2 + , Ag 3 2+ and Ag 4 3+ was observed. Ag 4 3+ cluster, stable at room temperature, after NH 3 adsorption is coordinated by six ammonia ligands, two in a first coordination sphere and four in a second one forming the multicore complex Ag 4 3+ (NH 3 ′) 2 (NH 3 ″) 4 . In dehydrated sodalite the Ag 6 5+ cluster is produced in the course of low temperature radiolysis as the result of electron trapping by the assemble of six Ag + cations located in the same sodalite cage.

The formation of silver–ethylene complexes in dehydrated Ag-SAPO-11 molecular sieve have been observed by electron paramagnetic resonance spectroscopy (EPR) after γ-irradiation at 77 K. Such reactive intermediate can play an important role in catalytic conversion of ethylene on silver loaded molecular sieves. The Ag(C2H4)2 stabilized inside relatively small channels of SAPO-11 molecular sieve it is observed even at 290 K. The ‘gas-phase’ geometries of Ag(C2H4) and Ag(C2H4)2 complexes, and respective hyperfine coupling constants were calculated applying DFT quantum chemical methods. The hyperfine coupling constants calculated for Ag(C2H4)2 complex of D2h symmetry are in very good agreement with those obtained experimentally.