I. M. Barkalov

Effect of γ-radiation on the molecular-topological structure of polytetrafluoroethylene

Four topological structures of polytetrafluoroethylene (PTFE): amorphous and three crystalline (high-melting, intermediate, and low-melting forms) blocks, were revealed by the technique of thermomechanical spectroscopy. The γ-irradiation of PTFE leaves both free volume and the molecular-mass characteristics in the pseudo-network structure of the amorphous block practically intact. In the crystalline phase, γ-irradiation decreases the molecular mass of crystallized chains of the low-temperature modification and leads to the disappearance of the intermediate and high-temperature crystalline phases as a result of their transformation into the amorphous phase.

Analysis of the ESR spectra of the ~ CF2CFCF2~ macroradical trapped in a γ-irradiated polytetrafluoroethylene matrix at 77 K

The Β-fluorine atoms in the ~CF2CFCF2~ radical trapped in γ-irradiated polytetrafluoroethylene (PTFE) were found to be nonequivalent and, hence, responsible for a doublet-triplet-triplet ESR spectral pattern. The conformational angle between the axis of the unpaired-electron orbital and the projection of CΒ-Cγ bond is equal to 60‡. The hyperfine coupling constants for α- and Β-fluorine atoms were determined. The hyper-fine splitting constants for two equivalent Β-fluorine atoms were found to be 7.2 mT. The corresponding value for the two other equivalent fluorine atoms is 1.8 mT, whereas the hyperfine splitting due to α-fluorine atom is 23.8 mT.

Formation of stable radicals from organofluorine compounds

Conclusions1.The radiolysis of a series of individual perfluorinated compounds leads to the formation of radicals which are stable not only in the liquid phase in solutions but also in the gas phase.2.The discovered stable radicals do not react with oxygen either during prolonged storage or during γ irradiation at 300°K.3.The radiolysis of the compounds in the liquid and solid phases leads to the formation of various unstable radicals.

Reasons for Selective C–C Bond Rupture upon Photodegradation of the Long-Lived Perfluoro-2,4-dimethyl-3-ethylpentyl-3 Radical

A difference in the mechanisms of photoinduced and thermal decomposition of [(CF3)2CF]2C · C2F5 radical was demonstrated on the basis of comparison of experimental results and quantum-chemical calculation data. Irradiation with light of λ < 320 nm resulted in transition of the radical to an excited state followed by the rupture of the CF2–CF3 bond having the greatest length in the ground state. In the thermal degradation of this radical (heating up to 373 K), the rupture >CF–CF3 bonds is more favorable because of low activation barriers for the process (42–54 kJ/mol). The CF2–CF3 bond rupture during thermolysis is unlikely, since the corresponding activation barrier is as high as ≈84–109 kJ/mol.

The influence of γ- and UV-radiation dose on the buildup and decay of paramagnetic centers in solid hydrogen cyanide

A decrease in the concentration of paramagnetic centers (PMCs) stabilized at 77 K in crystalline hydrogen cyanide was observed when ionizing radiation doses exceeded 3000 kGy. It was assumed that the decrease is due to a change in the physical properties of the matrix as a result of its deformation, loosening, amorphization, and formation of metastable structures. The decay of the PMCs with an increase in temperature of the irradiated samples depends on the dose and type (UV, γ) of the radiation. The higher the dose of preliminary γ-irradiation, the lower the decay temperature. In UV-irradiated HCN, the paramagnetic centers that occur in the thin photolyzed layer are more stable, since they are localized at defect sites of the crystalline matrix, which hinder the migration of free valence.

Low-Temperature Radiation-Initiated Hydrobromination of Allene as Studied by Kinetic Calorimetry

The spontaneous and radiation-initiated cryochemical reactions of allene hydrobromination were studied. The spontaneous hydrobromination of allene proceeded in the liquid phase in the temperature range 170–280 K, yielding primarily 2-bromopropene. Postradiation hydrobromination was observed at much lower temperatures (80–120 K) with the formation of allyl bromide and 1,3-dibromopropane.

An ESR and Optical Spectroscopic Study of the Mechanism of Photostimulated Reactions in Hydrogen Cyanide γ-Irradiated at 77 K

Changes in the electronic absorption spectra and ESR spectra in the course of photobleaching of radiolyzed solid HCN with light of different wavelengths (236–600 nm) were studied by ESR and optical spectroscopy. Two bands at 270 and 290 nm in the optical spectrum were attributed to the presence of H2C=N⋅ and HC⋅=NH radicals, respectively (the molar absorption coefficients are k270≈ 2.7 × 102l mol–1cm–1and k290≈ 1.5 × 102l mol–1cm–1, respectively). Structureless broad bands with maximums at 313 and 465 nm, which were detected after the exposure of a sample to light with λ ≥ 300 nm, can belong to the cyanide ions (CN–) and H2C=N+cations (the molar absorption coefficients of the ions are kion= (0.4–1.0) × 102l mol–1cm–1). In the photobleaching of γ-irradiated HCN (λ = 236–280 nm), H2C=N+radicals were additionally formed by the photoinduced reaction of electron transfer from the CN–anion to the H2C=N+cation. The amount of these radicals generated in the course of photobleaching is several times greater than that of the same radicals formed in the radiolysis via hydrogen atom addition to the multiple bond of HCN molecules.

Gas-phase fluorination of fluorochloromethanes by CF3OF in the presence of oxygen

Abstract Systematic IR-spectroscopic and thermographic studies of product formation and kinetics of gas-phase branched chain fluorination of halomethanes (RH) by trifluoromethylhypofluorite (CF 3 OF) are reviewed. A model of the complicated influence of both the oxygen and the intermediate olefins in the process during the fluorination delays could possibly be extended to other gas-phase fluorination systems with similar chain branching.

Photochemistry of Long-Lived [(CF3)2CF]2C·C2F5 Radicals in a Hexafluoropropylene Trimer Matrix

The kinetics of photodecomposition of long-lived [(CF3)2CF]2C·C2F5 radicals (I) in glassy and liquid hexafluoropropylene trimer was studied at 77 and 300 K, respectively. It was found that the phase state of the hexafluoropropylene trimer matrix did not affect the photodecomposition mechanism. In both cases, I eliminates ·CF3 radical from the perfluoroethyl fragment. The molar absorption coefficient of I was determined: ε250 = 49 m2 mol–1. It was shown that the photodecomposition of I in a liquid matrix at 300 K led to the formation of other long-lived radicals. One of these species is [(CF3)2CF]3C· radical, which results from addition of ·CF3 radical to the double bond of a hexafluoropropylene trimer molecule.

Formation and decay of trifluoromethyl radicals during photodecomposition of the long-lived perfluoro-2,4-dimethyl-3-ethylpent-3-yl radical

UV irradiation of the long-lived radical [(CF3)2CF]2C·C2F5 (1) in a hexafluoropropylene trimer (HFPT) glassy matrix at 77 K and in a HFPT solution at 300 K leads to its decomposition to the ·CF3 radical and perfluoroolefin molecule. About 90% of the ·CF3 radicals formed recombine at 300 K. The remaining radicals add to the HFPT molecules generating the long-lived radicals [(CF3)2CF]3C·. Unlike the ·CF3 radicals produced by the photodecomposition of radicals 1, the ·CF3 radicals formed during radiolysis of HFPT are not stabilized in the glassy HFPT matrix at 77 K.