EPR and ENDOR study of thermally produced triphenylmethyl radicals

The ENDOR spectrum of the triphenylmethyl radical formed by heat treatment of triphenylmethane has been measured in solution at 131°C. The derived hyperfine coupling constants of 2.770, 2.556 and 1.138 G were used to simulate the extremely well resolved EPR spectrum. These coupling constants are in good agreement with spin densities determined for the planar radical by the McLachlan method.     

Photochemically generated carotenoid radicals on Nafion film and silica gel: an EPR and ENDOR study

Carotenoid cation radicals have been observed at 120 K, by EPR and proton ENDOR measurements, to be formed upon 77 K photolysis of thin films of Nafion or silica gel coated with the carotenoids, β-carotene and canthaxanthin. The powder ENDOR spectra consist of resolvable lines due to couplings larger than 2.8 MHz but smaller than 17 MHz assigned to the methyl protons of the carotenoid cation radical and to an -proton of the planar polyene chain.     

Photo- and chemically-produced phylloquinone biradicals: EPR and ENDOR study

Phylloquinone biradical triplet species were generated by 300 nm irradiation of frozen (77 K) solutions or by treatment with AlCl₃. The shape of the (Δm_s=1) electron paramagnetic resonance (EPR) signal of the triplet is axially symmetric (E=0) with D=19±0.5 mT for photo-induced and D=11.2±0.5 mT for chemically induced radicals. A half-field signal (Δm_s=2) in the region of g≈4 was detected in both cases, confirming its assignment as a triplet. An additional line arising at the center of the (Δm_s=1) signal with g=2.0048±0.0002 was assigned to the phylloquinone radical anion (PhQ⁻). Electron nuclear double resonance (ENDOR) measurements of the triplet revealed the sign of the D parameter. For photo-generated radicals it appeared to be negative, which is the characteristic of radical dimers with well-separated partners (biradicals). Spin–spin distances of 5.3 and 6.3 Å, respectively, were estimated from the D parameter of photo-generated and chemically prepared phylloquinone biradicals.     

An EPR, ENDOR and EIE study of gamma-irradiated poly (lactide-co-glycolide) polymers

Gamma radiation of poly (lactide-co-glycolide) raw polymers and processed microspheres under vacuum and at 77 K results in the formation of a series of free radicals. The resulting powder electron paramagnetic resonance (EPR) spectrum contains a distribution of several different radicals, depending on the annealing temperature, and is therefore difficult to interpret. By utilising the selectivity of the electron nuclear DOuble resonance (ENDOR) and associated ENDOR induced EPR (EIE) techniques, a more direct approach for the deconvolution of the EPR spectrum can be achieved. Using this approach, the radiolytically induced CH3 *CHC(O)R- chain scission radical was identified at 120 K by simulation of the EIE spectrum. At elevated temperatures (250 K), this radical decays considerably and the more stable radicals -O*CHC(O)-, CH3 *C(OR)C(O)- and CH3 *C(OH)C(O)- predominate. This work demonstrates the utility of the EIE approach to supplement and aid the interpretation of powder EPR spectra of radicals in a polymer matrix.     

EPR study of free radicals in bread

The features of the recorded EPR spectra of paramagnetic species formed in bread and rusk are reported. The appearance of free radicals in them is only connected with their thermal treatment since the starting materials (flour and grains) exhibit very weak EPR signal. The obtained EPR spectra are complex and indicate that: (i) the relative number of paramagnetic species depends on the temperature and treating time of the raw product; (ii) the g-values are strongly temperature dependent with a tendency to coincide at t > or = 220 degrees C. Because of the relatively low (150-220 degrees C) temperature of thermal treatment, the studied free radicals can be assumed to appear in the course of the browning (Maillard) reaction and not to the carbonization of the material.     

EPR study of the transformations of fluoroalkenyl radicals

Conclusions Fluorinated alkenyl radicals containing a double bond at position 4 or 5 relative to the radical site are converted into relatively stable radicals, apparently due to their cyclization.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 706–708, March, 1989.We express our gratitude to A. L. Blyumenfel'd for compiling a program for calculation of the EPR spectra.     

EPR and ENDOR study of the frozen ammoniated electron at low alkali-metal concentrations

Ammoniated electrons in dilute frozen solutions are examined using EPR spectroscopy under conditions where the formation of metallic nanoparticles is avoided. Two signals from two different species have been observed. One signal is metastable and decays irreversibly upon annealing. The metastable species saturates at a spin concentration of 10 nM. The annealing temperature for this species amounts to 60 K for frozen solutions of sodium in neat ammonia and is raised upon addition of metal iodide. The observed g value is smaller than the free electron g value and is compatible with a cluster-anion radical rather than with a cavity electron. The wave function of the unpaired electron contains about 6%-10% of 2p character at nitrogen. The observed g shift is fully compatible with previously reported theoretical calculations (Shkrob, I. A. J. Phys. Chem. A 2006, 110, 3967-3976). The second signal cannot be annealed in the frozen state. The line shape is homogeneous, and its width depends on the identity of the metal and at large metal concentrations on the metal concentration itself. Upon increasing alkali metal concentration above 0.15 MPM, the line shape changes from Lorentzian to Dysonian, indicating the presence of metal nanoparticles. A new ENDOR pulse sequence is introduced to investigate the presence of weakly coupled nuclear spins for homogeneous EPR lines. The observations are critically compared with available literature data.     

Matrix ENDOR of polyenyl radicals in polymers

EPR and matrix ENDOR spectra have been examined for polyenyl radicals in γ-irradiated PVF, PVF₂, PVC, and PMMA polymers. Proton matrix ENDOR is observed for all four polymers, and fluorine matrix ENDOR for PVF and PVF₂. By line shape analysis of the ENDOR spectra obtained under comparable conditions, delocalization diameters for the unpaired electron of the polyenyl radical in each polymer are obtained. These diameters indicate extensive delocalization over 5–7 carbon double bonds for the polyenyl radicals investigated. It is suggested that these conjugated and crosslinked radiation products account for the observed nondevelopment of electron beam resist PMMA material at high radiation charge densities.     

EPR, ENDOR and HYSCORE study of X-ray induced centres in K2YF5 thermoluminescent phosphors

X-Ray irradiation at room temperature produces several paramagnetic centres in rare-earth activated K2YF5 crystals, whose thermal annealing behaviour can be linked with the occurrence of thermoluminescence (TL) glow peaks. In this paper, continuous wave (CW) and pulsed paramagnetic resonance techniques are used to study the structure of a very stable radiation-induced centre, which may be involved in the TL peak at approximately 390 degrees C reported for Ce- and Tb-activated crystals. From the spectra the centre's g tensor and hyperfine (nuclear quadrupole) tensors for several 19F and 39K neighbouring nuclei are extracted, but no self-hyperfine interaction could be detected. Based on the analysis of the interaction tensors, a model is constructed consisting of an oxygen-related radical (e.g. O(-) or O2(-)) on a substitutional F(-) position in the mirror plane of the YF7 polyhedra. Such a centre most probably corresponds to a trapped-hole state.     

EPR,ENDOR and ESE investigations on chromium modified PbTiO3 ceramics

EPR and ENDOR studies show that chromium is incorporated in mono crystalline regions of the grains in PbTiO₃ ceramics as Cr⁵⁺ ions at Ti⁴⁺ sites. The local symmetry of the Cr⁵⁺ impurity is lower than of the host Ti⁴⁺ site in the crystallite. ESE measurements of the spin-lattice relaxation of the Cr⁵⁺ centre point to a structural transition at 10 K.     

EPR measurements of the phosphorescent state of 2,1,3-benzothiadiazole

The spin dipolar interaction and the hyperfine structure of the phosphorescent state of 2,1,3-benzothiadiazole have been measured in a naphthalene host crystal at 77 K. Preliminary transient study indicates the short in-plane spin substate is the active component in the triplet population.     

EPR study of monomer-dimer equilibrium of nitroxyl radicals,containing an oxime group

Conclusions The EPR spectra of solutions of nitroxyl radicals, containing a ketoxime group, change from triplet to quintet in nonpolar organic solvents, which is caused by the formation of dimers involving oxime groups, in which the exchange interaction of the unpaired electrons of the paramagnetic centers is realized.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1153–1155, May, 1981.     

Acid-Base and Anion Binding Properties of Tetrafluorinated 1,3-Benzodiazole, 1,2,3-Benzotriazole and 2,1,3-Benzoselenadiazole

The influence of fluorination on the acid-base properties and the capacity of structurally related 6–5 bicyclic compounds – 1,3-benzodiazole 1 , 1,2,3-benzotriazole 2 and 2,1,3-benzoselenadiazole 3 to σ-hole interactions, i. e. hydrogen ( 1 and 2 ) and chalcogen ( 3 ) bondings, is studied experimentally and computationally. The tetrafluorination increases the Brønsted acidity of the diazole and triazole scaffolds and the Lewis acidity of selenadiazole scaffold decreases the basicity. Increased Brønsted acidity facilitates anion binding via the formation of hydrogen bonds; particularly, tetrafluorinated derivative of 1 (compound 4 ) binds Cl⁻. Increased Lewis acidity of tetrafluorinated derivative of 3 (compound 10 ), however, is not enough for binding with Cl⁻ and F⁻ via chalcogen bonds in contrast to previously studied Te analog of 10 . It is suggested that the maximum positive values of molecular electrostatic potential at the σ-holes, V_S,max, can be a reasonable metric for design and synthesis of new anion receptors with selenadiazole-diazole/triazole hybrids as a special target. Related chlorinated compounds are also discussed.     

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.     

EPR study of gamma induced radicals in amino acid powders

To better understand the composite character of amino acids EPR spectra, the radiolysis and reactions which occurred after irradiation of amino acids, a comparative EPR study of a few simple amino acids has been made in order to identify qualitatively and quantitatively the different radiation-induced radicals in amino acid powders. A spin-trapping methodology has been developed and carried out on irradiated glycine, alanine and valine.     

Q-band EPR and ENDOR of low temperature X-irradiated beta-D-fructose single crystals

Beta-D-fructose single crystals were in situ X-irradiated at 80 K and measured using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques at Q-band (34 GHz) microwave frequencies. The measurements revealed the presence of at least four carbon-centered radicals stable at 80 K. By means of ENDOR angular variations in the three principal crystallographic planes, six proton hyperfine coupling tensors could be determined and were assigned to four different radicals by the aid of EIE. Two of the radicals exhibit only beta-proton hyperfine couplings and reveal almost identical EIE spectra. For the other two radicals, the major hyperfine splitting originates from a single alpha-proton hyperfine coupling and their EIE spectra were also quite similar. The similarity of the EIE spectra and hyperfine tensors led to the assumption that there are only two essentially different radical structures. The radical exhibiting only beta-proton hyperfine couplings was assigned to a C3 centered radical arising from H3 abstraction and the other radical suggested to be an open-ring species with a disrupted C2-C3 bond and a double C2-O2 bond. A possible formation mechanism for the latter open-ring radical is presented. By means of cluster density functional theory (DFT) calculations, the structures of the two radicals were determined and a fairly good agreement between the calculated and experimental hyperfine tensors was found.