Structures and EPR spectra of binary sulfur-nitrogen radicals from DFT calculations

The scattered electron paramagnetic resonance (EPR) spectroscopic data for binary sulfur-nitrogen (S,N) radicals have been compiled and critically assessed.Many of these are inorganic rings or cages.For each species, possible equilibrium structures in the gas phase and the EPR hyperfine coupling (hfc) constants have been calculated with DFT using the B3LYP functional and basis sets of triple-ζ (or better) quality.Good agreement is obtained between calculated and measured values for the well characterized [S₃N₂]⁺, a planar π-radical for which the s-component of the orbitals is likely to be reasonably independent of minor geometrical changes between gas-phase and condensed-phase states.The cage compounds [S₄N₄]⁻ and [S₄N₅]⁻², for which reliable experimental EPR spectra have been reported, show larger variation between calculated and measured hfc, as a consequence of the dependence of the s orbital content of the molecular orbitals on small structural changes.The very large disagreements between the DFT calculated and experimentally claimed hfc constants for [NS], [SNS] and [S₄N₄]⁻³ in condensed phases lead us to question their assignment.Among binary S,N radicals, ³³S hfc data has only been reported for [S₃N₂]⁺ (through isotopic enrichment).These values were essential for the correct identification of the EPR spectra of this important radical, which previously was misassigned to other species.Our results suggest that ³³S data will be equally important for the correct identification of the EPR spectra of other binary S,N species, many of which are cyclic systems, e.g.[S₃N₃], [S₄N₃] and[S₄N₅].     

Investigation of perfluoroperoxy radicals self-coupling reactions by kinetic EPR spectroscopy

The rate constants and activation parameters for the self-coupling of perfluoroperoxy radicals of structure A and B: C₇F₁₅OO (A) and R_FOCF₂OO (B) have been determined in perfluorohexane solution in the temperature range 228-258 K. The magnitude of the rate constants obtained ranks between 6.6×10⁸ and 2.5×10⁹ l mol⁻¹ s⁻¹ and are therefore, among the largest rate values so far reported in the literature for primary peroxy radicals couplings. The activation energy is positive and lower for the peroxy radicals (A) with respect to the peroxy radicals (B) (10.5 and 23.0 kJ mol⁻¹, respectively).Analysis by kinetic modeling has shown that the peroxy radicals decay curves are compatible with the participation of peroxy radicals↔tetroxide equilibria to the reaction mechanism.Upper limit values of k_bs<10 and <20 s⁻¹ were inferred for the β-scission reactions of the perfluoroalkylperoxy radicals at 228 and 258 K, respectively.     

Use of a range scaling method to determine alanine/water stopping power ratios

A phantom composed of alanine dosimeter material has been constructed and depth–dose measurements made in a 10 MeV electron beam. The results have demonstrated the feasibility of using relative depth–dose measurements to determine stopping power ratios in materials of dosimetric interest. Experimental stopping power ratios for alanine dosimeter material and water agreed with the data of ICRU Report 37 within the uncertainty of the experiment (±1.2% at a 95% confidence level).     

Uncertainties in absorbed dose as measured using PMMA dosimeters

Some Harwell quality-control data on red 4034 PMMA dosimeters are presented, showing that gaussian distribution and probability apply to the specific absorption measurements used to derive dose.

Uncertainties of calibration and use are listed, quantified, and combined in simple quadrature, for an example batch of dosimeters. The overall uncertainty of calibration derived from this listing is shown to agree with the average uncertainty found in a closed-loop inter-laboratory comparison exercise.     

EPR study of persistent free radicals in cross-linked EPDM rubbers

Cross-linking of ethylene propylene diene monomer (EPDM) rubbers containing different amounts of dicyclopentadiene (DCPD), 5-ethylidene-2-norbornene (ENB) and 5-vinyl-2-norbornene (VNB) dienes was examined by EPR spectroscopy. The cross-linking was initiated by thermal decomposition of dicumyl peroxide at 440 K. The concentration of free radicals increased towards the end of the cross-linking process before reaching a maximum and decaying to zero. This is explained by the spatial confinement of the radicals in the cross-linked rubber, which leads to increased life time and, hence, higher radical concentration at a time when most peroxide has decomposed. The EPR spectra showed the presence of two components: a well-resolved spectrum overlapping a broad line. Both components are assigned to allyl radicals possessing different mobility. The more mobile component is assigned to allyl radicals along the EPDM chains, whereas the immobilised allyl radicals are formed in the cross-links. The stability of the allyl radicals decreases in the order DCPD > ENB > VNB. EPDMs containing two dienes show more persistent radicals than their single-diene counterparts. The most persistent radicals are observed for highly cross-linked (e.g., 28% ENB) or mixed diene EPDMs (e.g., 2.2% DCPD-4.4% ENB); the EPR spectra of free radicals in these systems can be observed for several hours.     

The radiation chemistry of acyclic hydrofluoro and perhalogenated ether and hydrocarbon compounds

The radiolytic stability of some hydrofluoroethers and hydrofluorocarbons was investigated and compared with those of perfluoropolyethers (PFPEs) and the CCl₂FCClF₂ (CFC 113). The experimental results indicate that stability depends mainly on the relative abundance of hydrogen atoms in the molecule; however, a significant role is played also by the chemical structure (i.e. the relative positions of the hydrogen atoms in the molecule). As a result, molecules containing hydrogen atoms as OCF₂H chain ends show a higher stability compared with the other hydrofluoro compounds. Based on the analysis of the end products and on the nature of radicals detected by EPR, radiolysis mechanisms are proposed and discussed. Due to their high dipole moments the hydrofluoro compounds and CCl₂FCClF₂ degrade mainly through an ionic mechanism.     

Development of nickel-doped lithium formate as potential EPR dosimeter for low dose determination

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.     

Powder state alanine ESR dosimeter for measuring the absorbed dose of electron beam

This paper describes the application of powder state alanine/ESR dosimeter for measurement of the absorbed dose of electron beam, for transfer the dose standards and for quality controls of the products processed by electron beam irradiation.The dosimeter is a sealed plastic container containing pre-treated alanine powder. Spectra of the internal standard and the alanine sample are measured simultaneously by using dual - cavity of a ESR spectrometer. The internal standard is a CuCl₂. 2H₂O monocrystal which gives stable ESR signals. A diamond sample is regard as working standard. With these two standards, the measurement accuracy can be improved apparently. The standard dose value is determined with a electron beam calorimeter made in our laboratory for dose calibration purpose.The advantages and the dosimetry characteristics of the application of powder state alanine/ESR dosimeters are discussed. This method is proved accurate and easy to use. In the region of 10-10⁴Gy, the dose response show a linear relationship and the precision is better than ± 2%.     

EPR spectroscopy on irradiated nickel tetracyanide in NaCl host lattice: mechanism for the simultaneous formation of reduced and oxidized species

The kinetics of oxidized and reduced Ni²⁺ complexes produced by X-ray irradiation on single crystals of NaCl doped with [Ni(CN)₄]²⁻ is studied by Electron Paramagnetic Resonance at room temperature. The interdependent generation of these two complexes is attributed to migration of the charge compensating vacancy from the reduced to the oxidized complex in a reversible reaction. At higher X-ray doses, there is a predominant formation of the reduced complex.     

EPR of free radicals formed from 3-hydroxyesculetin and related derivatives

EPR spectroscopy of 3-hydroxyesculetin (1, solid and in solution) and of the radicals formed during the aerobic oxidation of alkaline solutions of 1 and related compounds was investigated. 1 in the solid state was studied by pulsed EPR experiments and showed a radical character. The aerobic oxidation of alkaline solutions of 1 was also followed by EPR spectroscopy. A ring contraction occurred leading to a 5,6-dihydroxybenzofuran-2-carboxylate radical. The autoxidation of an alkaline solution of (Z)-3-(3,4-dihydroxyphenyl)-2-hydroxypropenoic acid allowed the observation of a spectrum attributable to 5,6-dihydroxybenzofuran-3-one-2-carboxylate radical. The formation mechanisms of these radicals are discussed.     

EPR study of the radicals formed upon UV irradiation of ceria-based photocatalysts

EPR measurements reveal remarkable differences on the type of radicals produced after UV illumination of TiO₂, CeO₂ and 0.8% CeO₂/TiO₂ photocatalysts. Photoactivation of the TiO₂ sample in vacuum results in the formation of Ti⁴⁺–O⁻ species and a small amount of Ti³⁺ centers. In the presence of adsorbed oxygen, irradiation of this material also generates Ti⁴⁺–O₃⁻ radicals. In the case of the CeO₂/TiO₂ catalyst, the ceria component is present in a highly dispersed state, as indicated by XRD and UV–Vis diffuse reflectance spectra (DRS) results. Accordingly, the only type of Ce⁴⁺–O₂⁻ adducts generated on the CeO₂/TiO₂ sample are indicative of the presence of two-dimensional patches of ceria on the anatase surface. On the other hand, photoactivation of the CeO₂/TiO₂ sample in the presence of oxygen also leads to the formation of some Ti⁴⁺–O⁻ and Ti³⁺ centers. In the case of the CeO₂ sample, superoxide radicals are observed upon irradiation in vacuum and subsequent oxygen adsorption. Further irradiation of this material in the presence of oxygen increases the amount of Ce⁴⁺–O₂⁻ radicals and simultaneously generates new species, which are tentatively assigned to Ce⁴⁺–O₂H radicals. Photocatalytic activity was tested for toluene oxidation, and the results obtained show that the photodegradation rate is slightly lower for CeO₂/TiO₂ than for the TiO₂ sample. However, the selectivity towards benzaldehyde (6–13%) is comparable for both materials. In the case of CeO₂, the photo-oxidation rate is an order of magnitude lower than for TiO₂, although mineralization of toluene is almost complete. Photoactivity results are discussed in connection with the characteristics of the radicals observed.     

An EPR study of thermally and photochemically generated oxygen radicals on hydrated and dehydrated titania surfaces

The formation of a series of oxygen-centred radicals on different TiO₂ samples (P25 and two different rutile materials) under various conditions was investigated using X-band c.w. Electron Paramagnetic Resonance (EPR) spectroscopy. The radicals were formed either on thermally-reduced TiO₂, or by UV irradiation of the oxide under an oxygen atmosphere. The nature and stability of the radicals was also explored as a function of surface hydration. On thermally reduced TiO₂, containing surface and bulk Ti³⁺ centres, oxygen adsorption at 300 K results in the preferential formation and stabilisation of O₂ ^- anions on the P25 surface, but O^- and O₃ ^- anions are generated on the rutile surfaces. Superoxide anions (O^-) and trapped holes (O₂ ^-) were also identified after photo-irradiation of the thoroughly dehydrated TiO₂ samples under oxygen. The O^- anions were only visible at low temperatures under continuous irradiation, while the O₂ ^- anions were stable for days at 300 K. By comparison, on fully hydrated surfaces, no stable oxygen centred radicals could be detected on P25, while O₂ ^- anions were easily observed on the rutile surfaces. On partially hydrated P25, the O^-, O₂ ^- and HO₂ anions were detected after UV irradiation at 77 K; all radicals decayed upon warming to 298 K. On partially hydrated rutile, the O^- and O₂ ^- anions were detected and, unlike the case for P25, were found to be stable for days under the same conditions. The results illustrate the varied formation and stability of the oxygen centred radicals on TiO₂ surfaces depending on the pretreatment conditions.     

Radiation effect on thermoluminescence and electron paramagnetic resonance (EPR) of pink beryl

Morganite, a pink beryl with the chemical formula Be₃Al₂Si₆O₁₈, was investigated in terms of its thermally stimulated luminescence (TL) and electron paramagnetic resonance (EPR) properties due to point defects. Atomic hydrogen H°-centers were induced by γ-radiation and shown to be responsible for a 165°C TL peak. The Fe³⁺-center concentration was found to increase with γ-dose, observed by both TL and EPR measurements, decreasing with thermal treatments beyond 250°C. A mechanism for emission around 315°C is proposed.     

DFT study of the EPR spectral pattern of propagating methacrylic radicals

The EPR spectral pattern observed in the bulk polymerization of methacrylic monomers was theoretically investigated by DFT methodology. The conformational analysis of the propagating radical by the rotation around the C–C_β bond, was performed using the B3LYP/6-31G^* computational scheme. To obtain accurate values of the isotropic hyperfine coupling constants (hfccs) a higher level protocol, B3LYP/TZVP//B3LYP/6-31G^*, was applied. The experimental 13-line spectrum registered at the first stage of the polymerization was assumed to correspond to a free rotating radical in a fluid medium and it was simulated just considering the most stable conformation. The 9-line spectrum registered at high conversions was interpreted in terms of highly hindered rotational conformers frozen in the very viscous medium. This spectrum was well reproduced by a model which considers the sum of the individual spectra of the conformations spread around the most probable. Each of these contributing spectra was obtained on the basis of the computed hfccs for the considered conformations and weighted by his relative Boltzmann population according to the DFT analysis. Besides, the calculated hfccs showed an excellent agreement with those predicted by the Heller–McConnell relationship, which confirms the coherence of the DFT methodology for this kind of calculations.     

New strategy to identify radicals in a time evolving EPR data set by multivariate curve resolution-alternating least squares

Electron paramagnetic resonance (EPR) spectroscopy is a powerful technique that is able to characterize radicals formed in kinetic reactions. However, spectral characterization of individual chemical species is often limited or even unmanageable due to the severe kinetic and spectral overlap among species in kinetic processes. Therefore, we applied, for the first time, multivariate curve resolution-alternating least squares (MCR-ALS) method to EPR time evolving data sets to model and characterize the different constituents in a kinetic reaction. Here we demonstrate the advantage of multivariate analysis in the investigation of radicals formed along the kinetic process of hydroxycoumarin in alkaline medium. Multiset analysis of several EPR-monitored kinetic experiments performed in different conditions revealed the individual paramagnetic centres as well as their kinetic profiles. The results obtained by MCR-ALS method demonstrate its prominent potential in analysis of EPR time evolved spectra.     

Lifetime of alkyl macroradicals in irradiated ultra-high molecular weight polyethylene

The interaction of UHMWPE with an electron beam in vacuum and in the presence of oxygen has been investigated. UHMWPE irradiated to various doses was examined with Electron Paramagnetic Resonance (EPR) and Fourier Transform infra-red (FTIR) spectroscopies. EPR was used to explore the nature of radicals produced in UHMWPE upon irradiation and to follow their decay as a function of time. Hydroperoxides formation and distribution throughout the samples were studied with FTIR spectroscopy.A correlation between the rate of decay of macroradicals and that of hydroperoxide formation has been proposed. Accordingly, the lifetime of secondary alkyl macroradicals in the amorphous phase of the polymer was found to range approximately from 3 to 10 h.     

Uncertainty estimation in Co gamma-ray dosimetry at JAERI involving a two-way dose intercomparison study with NPL in the dose range 1–50 kGy

The uncertainties in ⁶⁰Co gamma-ray dosimetry at JAERI have been estimated by reviewing irradiation and calibration procedures. The uncertainty in dose delivered by the JAERI calibration system has been estimated to be 2.2% (2σ) and the uncertainty in dose measurement using JAERI alanine/ESR transfer dosimeters has been estimated to be 3.4% (2σ). A two-way dose intercomparison study between NPL and JAERI employing alanine-PS, alanine-paraffin, and dichromate dosimeters as transfer dosimeters was performed to confirm the estimated uncertainty over the dose range 1–50 kGy. Dose intercomparison results using both alanine and dichromate dosimeters show agreement within about 2%, which gives confidence in the estimated uncertainty in ⁶⁰Co gamma-ray dosimetry at JAERI.     

An EPR and molecular mechanics study of radical adducts of phosphonyl radicals with 2-hydroperfluoro-4-methyl-and 4,4-dimethyl-2-pentene

EPR spectra of radical adducts of phosphonyl radicals with 2-hydroperfluoro-4-methyl- and 4,4-dimethyl-2-pentene have been studied. The molecular mechanics method has been used to determine the preferred conformation of the (CF₃)₂CF-CF-HCF₃P(O)(OMe)₂ radical. The eclipsed conformation of the C–P bond and 2p _z -orbital of an unpaired electron is stabilized due to steric factor and hyperconjugation.A. N. Nesmeyanov Institute of Organoelemental Compounds, Russian Academy of Sciences, 117813 Moscow. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 11, pp. 2550–2554, November, 1992.     

Studies on the uncertainties in quantitative EPR estimations due to the construction of the cavities used

The magnitude of the continuous wave electron paramagnetic resonance (CW-EPR) response of a given sample is governed by several cavity parameters. Two of them are the distribution of the modulation magnetic flux density and the microwave magnetic component over the sample length. Their mutual configuration could have a significant influence on quantitative EPR estimations. In the present paper, we report the results obtained with 14 different cavities of TE₁₀₂, TE₁₀₄, TM₁₁₀ and TM₀₁₁ modes commercially available and used in many EPR laboratories with respect to the magnitude of their EPR response as a function of the sample length. The results show that due to their technical construction the widely used cavities in CW-EPR spectroscopy could be classified into three types in which: (i) the distribution of the microwave magnetic component in the cavity coincides with the modulation magnetic flux; (ii) the modulation magnetic flux is wider than the microwave magnetic component in the cavity. In this type, the magnitude of the EPR response along the sample length is governed only by the distribution of the microwave magnetic component in the cavity, (iii) the modulation magnetic flux is narrower than to the microwave magnetic component in the cavity. In the cavities of this type, the magnitude of the EPR response is governed by the distribution of the modulation magnetic flux. Whereas, in both former types, the EPR response corresponds to the full length of the sample along the z-axis of the cavity, an EPR silent zone appears in the latter type. In some of them this zone takes ca. 40% of their full length. Therefore, the assumption that the EPR active zone corresponds to the full cavity length (along the sample axis) is not valid for the latter type cavities. Their use will give 2-3% error within a series of samples, but the results will be burdened by a systematic error and could not be compared with the data obtained with another cavity even on the same spectrometer. In view of this, we should be familiar with the features of every cavity before using it for quantitative EPR estimations. Two ways are recommended to overcome the effect of EPR silent zones of an unknown cavity in order to get comparable results in quantitative EPR estimations: (i) to use samples of less than 5-6 mm length, symmetrically situated with respect to the cavity center in consecutive procedures of estimations independently of the cavity type, (ii) in the cases of simultaneous recording of unknown sample and a standard they both should be homogeneously mixed or coaxially situated along their full length.     

The effect of chloroalkane/alcohol solvent composition on the photochemical properties of copper(II) dithiocarbamate mixed-ligand complexes. EPR study

The EPR technics has been used to study the effect of solvent composition on the photochemical conversion of Cu(II) dithiocarbamate mixed-ligand complexes Cu(Et₂dtc)X (X=Cl⁻, Br⁻) and Cu(Et₂dtc)⁺…Y⁻ (Y⁻=ClO₄⁻, NO₃⁻) in chloroalkane/alcohol solutions, where chloroalkane=CCl₄, CHCl₃ or CH₂Cl₂ and alcohol=MeOH, EtOH, i-PrOH or i-BuOH. The obtained results allow to get some insight into the behaviour of the mixed-ligand complexes towards the halogen donation power of chloroalkanes and the co-ordination abilities of alcohols. The paper deals with the nature of the complexes obtained as intermediate products of photolysis.