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.     

ESR study of the rotation of peroxy radicals in polymers

The nature and the frequencies of rotation of peroxy radicals in polyethylene and polytetrafluorethylene are determined over a wide temperature range theoretically calculated ESR spectra.     

Formation of free radicals in the low-temperature fluorination of polymers

The formation of free radicals in the process of direct fluorination of natural and synthetic polymers at temperatures close to 77 K was studied by ESR. The maximum concentrations of radicals, (10¹⁷–5·10¹⁸ spin g^–1) and their complete oxidation were observed in the temperature interval from 77 to 250 K at a fluorine pressure of 30 Torr. The initiation of chain halogenation, which consists of homolytic breaking of chemical bonds to yield free-radical intermediates. was examined in the framework of the multi-center synchronous transitions model.     

Longer polyenyl cations in relation to soliton theory

The carotene-like polyenes decapreno-beta-carotene (C50), C54-beta-carotene (C54, first synthesis) and dodecapreno-beta-carotene (C60) with 15, 17 and 19 conjugated double bonds, respectively, were synthesized by double Wittig reactions. Introduction of a leaving group in allylic position failed, and cations were obtained by hydride elimination effected by i) triphenylcarbenium tetrafluoroborate-d15, prepared by a new method, or ii) treatment with trifluoroacetic acid-d. Deuterated reagents were employed for product analysis by 1H NMR. Parallel experiments were performed with beta,beta-carotene (C40). NIR spectra at room temperature and at -15 degrees C were employed for characterisation and stability studies of the cationic products. In CH2Cl2lambdamax in the 900-1350 nm region was recorded. NMR data for the cationic product of beta,beta-carotene obtained by the two new preparation methods were consistent with the two monocations previously characterised. The cationic products of the longer polyenes provided downfield-shifted, broadened signals, compatible with C50-monocation, mixed C54-mono- and dication and C60-dication. Combined NIR and NMR data suggest that the extent of charge delocalisation is limited by the maximum soliton width for cations obtained from linear polyenes with more than ca. 20 sp2-hybridized carbon atoms.     

Pulse EPR and ENDOR study of 1,2,3-benzodithiazolyl, 2,1,3-benzothiaselenazolyl and 1,2,3-benzodiselenazolyl radicals

1,2,3-Benzodithiazolyl, 2,1,3-benzothiaselenazolyl and 1,2,3-benzodiselenazolyl radicals were generated by the reduction of the corresponding cations and investigated by pulse EPR and ENDOR in frozen CHCl(3) solutions at 30 and 80 K. These methods, in combination with density functional theory calculations, were used to study the magnetic parameters of the radicals, namely the principal values of the nitrogen and proton hyperfine interactions and g-tensors. The spin density distribution was shown to be nearly the same for all investigated radicals and, therefore, replacement of sulfur by selenium leads to a limited perturbation of the radicals' electronic structure. A high anisotropy of the g-tensors was found for the selenium-containing radicals.     

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.     

Kinetics of the geminal recombination of aromatic radicals in liquid polymers

Conclusions Laser photolysis was used to determine the kinetics of the geminal and bulk recombination of radical pairs formed in the transfer of a hydrogen atom from p-cresol to triplet benzophenone in liquid polymers. In a very viscous polymer matrix containing a nonviscous plasticizer, the bulk recombination of the low-molecular-weight radicals proceeds at a very high rate as in nonviscous liquids.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 437–438, February, 1987.     

Formation of free radicals during machining and fracture of polymers

Electron paramagnetic resonance measurements of the number of free radicals formed during cutting and grinding of polymers are described. It was found in the semicrystalline polymers studied that below the glassy transition temperature that the number of free radicals formed is about 2 × 10¹³/cm² of surface formed. It is proposed that this number results from the crack progressing selectively through the glassy regions about more ordered regions in the polymers.     

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.     

Laser photolysis studies of the photoinduced production of ion radicals in polymers

N₂-laser flash photolysis measurements of poly (N-vinylcarbazole) (PVCz) and N-isopropylcarbazole (NIPC) with p-dicyanobenzene (DCNB)_in dimethylformide (DMF) show an excitation intensity dependent quantum yield for for ion-radical production. The transient absorbance associated with the ion radical species together with light intensity dependent fluorescence decay curves demonstrate that excitation annihilation processes can compete effectively with ion-radical formation at high excitation intensities in this polymer.     

Exchange and dipole interactions and local concentrations of stable radicals in polymers

Theoretical and Experimental Chemistry -     

Redox polymers incorporating pendant 6-oxoverdazyl and nitronyl nitroxide radicals

Polymers comprised of redox-active organic radicals have emerged as promising materials for use in a variety of organic electronics, including fast-charging batteries. Despite these advances, relatively little attention has been focused on the diversification of the families of radicals that are commonly incorporated into polymer frameworks, with most radical polymers being comprised of nitroxide radicals. Here, we report two new examples prepared via ring-opening methathesis polymerization containing 6-oxoverdazyl and nitronyl nitroxide radicals appended to their backbones. The polymerization reaction and optoelectronic properties were explored in detail, revealing high radical content and redox activity that may be advantageous for their use as semiconducting thin films. Initial studies revealed that current–voltage curves obtained from thin films of the title polymers exhibited memory effects making them excellent candidates for use in resistive memory applications. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 309–319     

Matrix elements for operators for physical quantities in one-configuration functions of radicals

Some basis vectors are constructed for singly and doubly excited configurations of molecules and radicals for the method of configuration interaction in the second-quantization representation. The matrix elements of the Hamiltonian are derived for these. A method is given for calculating bond orders, transition moments, spin densities, and electron densities on atoms in the configuration-interaction method. An analog of Brillouin's theorem is obtained for radicals.Read at the II-nd Ukranian Republic Symposium on Quantum Chemistry. (Kiev, Dec. 1968).     

Water‐proton relaxivity of hyperbranched polymers carrying TEMPO radicals

High water‐soluble hyperbranched poly(styrene) (HPS) polymers carrying stable 2, 2, 6, 6‐tetramethylpiperidine‐1‐oxyl (TEMPO) radicals, HPS‐N‐TEMPO, HPS‐Im‐TEMPO, and HPS‐Im‐(TEMPO)₂, were prepared in ca. 60% introducing yield. HPS‐N‐TEMPO and HPS‐Im‐TEMPO were determined to be nearly spherical shapes of the diameter of 2.4 ± 0.6 and 2.2 ± 0.6 nm, respectively, by transmission electron microscope (TEM) images. The values of water‐proton relaxivity, r₁, at 25 MHz, 0.59 T, and 25 °C were 6.0, 5.2, and 14 mM^?1 sec^?1 for HPS‐N‐TEMPO, HPS‐Im‐TEMPO, and HPS‐Im‐(TEMPO)₂, respectively. The spin‐lattice relaxation time (T₁)‐weighted images in phantom were also observed. Copyright © 2009 John Wiley & Sons, Ltd.