Radical decomposition of <Emphasis Type="Italic">N,N</Emphasis>-bis-(3-chloro-1,4-naphthoquinon-2-yl) amine in basic conditions followed by EPR

EPR spectroscopy was used to assess the radicals produced upon basic decomposition of N,N-bis-(3-chloro-1,4-naphthoquinon-2-yl) amine (BClNQA). Three radicals have been trapped and identified: N-bis(3-chloro-1,4-naphthoquinone) hydrazine radical (6), 2-hydroxy-3-chloro-1,4-naphthoquinone anion radical (9) and 2-amino-3-chloro-1,4-naphthoquinone radical (8). The probable reaction mechanism, the structure of intermediates as well as the reaction profile are discussed.     

The use of O_2^{\begin{array}{*{20}c}--\\\cdot \\ \end{array} } radical anions as spin probes for testing nanostructured materials based on zirconium oxide

The conditions of formation of $O_2^{\begin{array}{*{20}c}--\\\cdot \\ \end{array} } $ radical anions on the surface of zirconium oxide systems including nanostructured pillared clays, in interaction with hydrogen peroxide and an NO + O₂ mixture of gases were studied by the EPR method. The special features of the formation of these radicals depending on the phase composition, structure, and degree of hydroxylation of the surface are discussed.     

Multiple functionalization of benzophenones inside polyphenylene dendrimers--toward entrapped ions and radicals

Polyphenylene dendrimers possessing a defined number of keto groups in the dendritic scaffold have been synthesized by using a benzophenone-functionalized tetraphenylcyclopentadienone branching unit. A postsynthetic functionalization of the polyphenylene backbone was achieved by reacting the entrapped keto groups with organolithium reagents yielding monodisperse alcohol products. To investigate the accessibility and reactivity of the embedded groups, many functions of different size and nature, for example, the chromophore pyrene, were introduced. Moreover, suitable precursors for the synthesis of dendrimer entrapped species, trityl cations, trityl radicals, and ketyl radical anions, were obtained. To gain insight into the structure of these newly functionalized dendrimers, UV/vis, EPR, and NMR measurements have been performed. They showed a delocalization of the charge/spin into the polyphenylene dendritic arms leading to a stabilization of the ions/radicals. Remarkably, for the ketyl radicals, EPR measurements indicated the occurrence of intermolecular metal-bridged biradicals. They suggest the existence of a dendritic radical network of the dendrimers themselves.     

Intermolecular interaction in complexes of tetracyanoethylene with amines

Solutions of tetracyanoethylene (TCE) in pyridine and its homologs (piperidine and triethylamine) and also a solution of the complex of 2,2-dipyridyl with TCE in acetone were investigated. The EPR spectra of these systems show hyperfine structure of nine components with splitting constants characteristic of TCE radical anions. This indicates complete electron transfer from the nitrogen atom in the amine to the TCE molecule. In isolated cases, there was a more complex structure in the EPR spectra, and this was interpreted as the result of formation of associates between the TCE radical anions and the amine molecules. The existence of such associates was confirmed by considerable difference in the proton relaxation times T₁ and T₂ in TCE solutions in pyridine, -picoline, and 2,6-lutidine. A formation mechanism and proposed structure are given for associates in these systems.T. Ya. Lavrenyuk and V. M. Ogenko took part in the experimental work.The author thanks Academician A. I. Brodskii of the Academy of Sciences of the Ukrainian SSR for discussing the results.     

EPR spectra and structure of the chlorotrifluoroethylene and bromotrifluoroethylene radical anions

EPR spectra attributable to the halogenotrifluoroethylene radical anions have been generated by electron attachment in solid solutions at low temperatures. The spin density distribution in these radicals strongly suggests that the unpaired electron occupies a σ^* orbital, a conclusion which is supported by CNDO/2 calculations.     

Degradation of γ-irradiated linear perfluoroalkanes at high dosage

The effects of the carbon backbone chain length on the EPR spectra of linear perfluoro-n-alkanes (PFAs) γ-irradiated at 77 K was studied for the short chain n-C₆F₁₄, n-C₈F₁₆, n-C₁₂F₂₆, and n-C₁₆F₃₄ molecules as well as the polymer polytetrafluoroethylene (PTFE). The experimental data show that the processes occurring during radiolysis of perfluoro-n-alkanes and polytetrafluoroethylene are very similar. EPR spectra of irradiated perfluoro-n-alkanes at low radiation dose show superimposed signals from three radicals: -F₂CCFCF₂-, -CF₂CF₂ and F₃C. The signal intensity decreases with perfluoro-n-alkanes chain length. At doses above 2.0 MGy, a constant increase in concentration of the radicals -F₂CCFCF₂- and -CF₂CF₂ is observed with decreasing chain length. The concentration of these radicals formed during radiolysis of PFA is described by the ratio: [-CF₂CF₂]/[-F₂CCFCF₂-] ≈3/(n − 2), where n is the number of carbon atoms in the linear perfluoroalkanes. Density functional theory was used to calculate the structures of the radicals and C-F bond energies in model perfluoro-n-alkanes as well as the EPR spectra of the associated radicals. This data is used to provide further insight into the radiation stability of PTFE. Four topographical structures of polytetrafluoroethylene, one amorphous and three crystalline, were identified by thermomechanical analysis. In the crystal phase, γ-irradiation results in their transformation to the amorphous form. The helical structure of individual perfluroalkanes readily distorts on removal of a fluorine and this will have an impact on the overall structure of the material. Such structural reorganization can lead to loss of the mechanical stability of polytetrafluoroethylene.     

Coordination radical polymerization of methyl methacrylate,initiated by the azobis(isobutyronitrile)–tri-<Emphasis Type="Italic">n</Emphasis>-butylborane binary system

A procedure for coordination radical polymerization of methyl methacrylate in the bulk, initiated by the azobis(isobutyronitrile)–tri-n-butylborane binary system at 60°С, was developed. Butyl radicals participating in the initiation step were detected by EPR. These radicals are generated by complexation of cyanoisopropyl radicals with trialkylborane. The radical at the boron atom transforms from the C-centered to N-centered structure with the subsequent S_R2 reaction and fast generation of n-butyl radicals. As found by NMR and confirmed by kinetic studies, the tri-n-butylborane–initiator–monomer system stimulates generation of propagating poly(methyl methacrylate) radicals.     

Epr study of tertiary cyclic perfluorinated radicals: Methylcyclopentyl and decalyl radicals

EPR spectroscopy was used to study new cyclic perfluorinated radicals, namely, methylcyclopentyl and decalyl radicals obtained by the photolysis of the corresponding bromides in the presence of dicarboranylmercury. The EPR spectrum of the perfluorocyclopentyl radical shows line broadening, which indicates an insufficiently high rate of inversion of the five-membered ring. The constants for coupling with the -fluorine atoms in the decalyl radical indicate considerable comparison of the radical site. The motion of the substituants inthe decalyl radical is hindered. The much lower dimerization rate constant in comparison with the corresponding acyclic radicals may be related to the stereochemical rigidity of the radical site.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2371–2374, October, 1989.     

Single‐Line EPR Spectra from Radicals Encapsulated in Aggregates of Amphiphilic Block Copolymers with Hydrophobic Dendritic Pendants in Water

Summary: A water‐insoluble organic 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) radical is solubilized in aqueous solutions of aggregates of amphiphilic block copolymers with hydrophobic dendritic pendants. The number (100–200) of DPPH molecules solubilized in an aggregate, which is evaluated from UV‐visible absorption spectra, is in agreement with that of the DPPH radicals determined from electron paramagnetic resonance (EPR) spectroscopy. The DPPH radicals are stably solubilized without decomposition in the polymer aggregates. The radicals exhibit a single‐line EPR absorption, which is narrowed by the interspin interaction, and indicates the assembly formation of DPPH radicals in polymer aggregates. These results suggest the effective utilization of the DPPH radical as a spin‐probe indicator in aqueous solutions.

When DPPH is solubilized in aqueous solutions of NaAMPS‐b‐G2(n3), the polymer solutions become purple colored, which is characteristic of the DPPH radical.     

Copper‐Catalyzed Radical/Radical CH/PH Cross‐Coupling: α‐Phosphorylation of Aryl Ketone O‐Acetyloximes

The selective radical/radical cross‐coupling of two different organic radicals is a great challenge due to the inherent activity of radicals. In this paper, a copper‐catalyzed radical/radical C? H/P? H cross‐coupling has been developed. It provides a radical/radical cross‐coupling in a selective manner. This work offers a simple way toward β‐ketophosphonates by oxidative coupling of aryl ketone o‐acetyloximes with phosphine oxides using CuCl as catalyst and PCy₃ as ligand in dioxane under N₂ atmosphere at 130 °C for 5 h, and yields ranging from 47 % to 86 %. The preliminary mechanistic studies by electron paramagnetic resonance (EPR) showed that, 1) the reduction of ketone o‐acetyloximes generates iminium radicals, which could isomerize to α‐sp³‐carbon radical species; 2) phosphorus radicals were generated from the oxidation of phosphine oxides. Various aryl ketone o‐acetyloximes and phosphine oxides were suitable for this transformation.     

Characterization of electron donor sites on Al2O3 surface

Formation of radical anions after adsorption of 1,3,5-trinitrobenzene (TNB) on electron donor sites of fully oxidized Al(2)O(3) samples with different phase compositions is studied by EPR. It is shown that the maximum concentration of the radical anions does not substantially depend on the choice of solvent and reaction temperature, and can be used to measure the total concentration of the donor sites. The donor sites are observed in almost the same concentration about 5 × 10(16) m(-2) on all alumina polymorphs except for α-Al(2)O(3). The formation rate of the TNB radical anions and the activation energy of this process are found to depend on the donor properties of the solvent. The EPR in situ experiments showed that a substantial amount of the adsorbate forming a liquid phase is required for generation of the radical anions. These results prove that the sites measured by the formation of the TNB radical anions are not genuine electron donor sites capable of direct electron transfer to the adsorbed TNB molecules. A model of the observed paramagnetic species based on the obtained experimental data and the results of quantum chemical simulations is suggested. According to this model, a TNB radical anion substitutes a hydroxyl group forming a neutral ion pair with a surface aluminum cation. The suggested mechanism for the formation of such ion pairs involves the migration of simple radicals and does not require long-distance charge separation. It is supposed that the donor site where the process is initiated includes a negatively charged surface hydroxyl group.     

Study of radical species formed by radiolysis of polycrystalline calcium selenite (CaSeO3)

The paper presents spin density values on oxygen and selenium orbitals, derived from EPR spectra of SeO ₂ ^– radical, formed in the -radiolysis of calcium selenite (CaSeO₃). The kinetics of thermal annealing of radicals have been studied. A mechanism for radiolysis is proposed.     

Copper‐Catalyzed Radical/Radical C?H/P?H Cross‐Coupling: α‐Phosphorylation of Aryl Ketone O‐Acetyloximes

The selective radical/radical cross‐coupling of two different organic radicals is a great challenge due to the inherent activity of radicals. In this paper, a copper‐catalyzed radical/radical C H/P H cross‐coupling has been developed. It provides a radical/radical cross‐coupling in a selective manner. This work offers a simple way toward β‐ketophosphonates by oxidative coupling of aryl ketone o‐acetyloximes with phosphine oxides using CuCl as catalyst and PCy₃ as ligand in dioxane under N₂ atmosphere at 130 °C for 5 h, and yields ranging from 47 % to 86 %. The preliminary mechanistic studies by electron paramagnetic resonance (EPR) showed that, 1) the reduction of ketone o‐acetyloximes generates iminium radicals, which could isomerize to α‐sp³‐carbon radical species; 2) phosphorus radicals were generated from the oxidation of phosphine oxides. Various aryl ketone o‐acetyloximes and phosphine oxides were suitable for this transformation.     

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].     

EPR of gamma irradiation induced radicals in NaHCO3, CsHCO3 and Na2CO3

In this study gamma irradiated NaHCO₃, CsHCO₃ and Na₂CO₃ were investigated at room temperature. The radicals induced by gamma irradiation in NaHCO₃ were found to be CO⁻₃, HCO₃ and CO⁻₂; in CsHCO₃ the species were attributed to HCO₃; and in Na₂CO₃ to CO⁻₃ and CO⁻₂ radicals. The hyperfine parameters for the hydrogen in HCO₃, and the ¹³C nucleus in CO⁻₂ radical have been determined. The results were compared with literature data for similar compounds and the EPR properties of the CO⁻₂ radical were discussed.     

Mechanism of the electrochemical reduction of 2-halo-5-nitrothiophenes in dimethylformamide

The reduction of 2-bromo-5-nitrothiophene and 2-iodo-5-nitrothiophene in dimethylformamide (DMF) was studied by means of classical and commutated polarography, EPR spectroscopy, a rotating platinum disk electrode with a ring, and preparative electrolysis. It was established that, depending on the nature of the halogen, their anion radicals may undergo further reduction to 2-nitrothiophene anion radicals or decomposition with splitting out of a halide anion and conversion to nitrothienyl radicals, which were identified on the ring electrode. The latter are capable of undergoing dimerization to 2,2-dinitro-5,5-dithienyl. The spectra of the anion radicals of 2-nitrothiophene and 2,2-dinitro-5,5-dithienyl were recorded by means of EPR. A mechanism for the reduction of halonitrothiophenes is proposed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 195–198, February, 1981.     

α-Branched amines through radical coupling with 2-azaallyl anions,redox active esters and alkenes

α-Branched amines are fundamental building blocks in a variety of natural products and pharmaceuticals. Herein is reported a unique cascade reaction that enables the preparation of α-branched amines bearing aryl or alkyl groups at the β- or γ-positions. The cascade is initiated by reduction of redox active esters to alkyl radicals. The resulting alkyl radicals are trapped by styrene derivatives, leading to benzylic radicals. The persistent 2-azaallyl radicals and benzylic radicals are proposed to undergo a radical–radical coupling leading to functionalized amine products. Evidence is provided that the role of the nickel catalyst is to promote formation of the alkyl radical from the redox active ester and not promote the C–C bond formation. The synthetic method introduced herein tolerates a variety of imines and redox active esters, allowing for efficient construction of amine building blocks.

A mild method for the construction of α-branched amine derivatives is presented. SET processes between the Ni catalyst, redox active esters and 2-azaallyl anions generate azaallyl radicals and alkyl radicals that functionalize the alkenes.     

Guaiazulene-based phenolic radical scavengers: synthesis, properties, and EPR studies of their reaction with oxygen-centred radicals

A variety of phenolic derivatives 4, carrying the guaiazulene moiety, were prepared starting from guaiazulene. Compounds 4 react with oxygen-centred radicals exhibiting chromotropic behaviour. The radical scavenging power of these compounds was evaluated by different methods. Compounds 4 are less efficient than some of the most common radical scavengers but show quite selective behaviour towards different oxygen-centred radicals. A correlation is found between the antioxidant activity of the compounds 4 and the corresponding phenolic O-H bond dissociation energy. Some aspects of the reaction of the compounds 4 with oxygen-centred radicals were elucidated by EPR and DFT studies.     

Production of radicals in polyoxymethylene by ultraviolet photolysis: An EPR study

Polyoxymethylene (POM) was photolyzed at 2537 and at 3130 Å at ?196°C. The EPR spectra of the radical intermediates were recorded. Photolysis in vacuo produces a small number of radicals, apparently due to the presence of traces of chromophores. Photolysis in oxygen, however, is a type of photo-oxidation. The radicals HCO, , CH₃·, and HOO· were detected and identified as intermediate products of photolysis. Hydrogen atoms and hydroxyl radicals were too reactive (i.e., mobile) at ?196°C to be observed. Alkoxy and alkyl radicals and the POM peroxy radical were probably formed as well but could not be characterized with certainty.     

Radical anions from some photochromic nitrocompounds. An electron paramagnetic resonance and electrochemical study

Five 6′-nitrospiro[indole-benzopyrans] have been reduced electrochemically in ACN to the corresponding radical anions. Their reduction potentials were in the range -1.32 to -1.40 V versus SCE, that is in the range expected for substituted nitrobenzenes. The EPR spectra of the species resulting from chemical reduction of the same compounds were also recorded, and suggested that the structure of the observed radicals varies with the reduction media: thus, while the spectra in DMSO could be attributed to the radical anions structurally similar to the starting compounds, those observed in THF under UV irradiation were assigned to the anions of the open merocyanines. It appears that also the species observed by reduction in DMF ought to be assigned the open structure.