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 C_Β-C_γ 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.     

Spin trapping of the short-lived free radicals formed in γ-irradiated alcohols

Short-lived free radicals formed in γ-irradiated methanol, ethanol and 2-propanol have been detected by E.S.R. combined with spin trapping method using 2,4,6-tri-t-buthylnitrosobenzene (BNB) as a spin trap. The observed spectra could be identified clearly by the help of deuterated alcohols. The hydrogen spin adduct, which had not been clearly identified in earlier works, was observed together with alkoxy radicals and other radicals. It was revealed that a part of the hydrogen adduct could come from proton transfer to the anion radical of BNB, which was produced by electron capture during irradiation. From the change of ¹CH₂OH/CH₃O¹ ratio with that of added spin trap concentration one may conclude that the methoxy radical and hydrogen atom are primary radicals in radiolysis of liquid phase methanol. This conclusion is also supported by both the change of the ratio with irradiation temperature and the isotope effect on the spin adduct yields.     

Deposition of radiation energy in solids as visualized by the distribution,structure and properties of alkyl radicals in γ-irradiated n-alkane single crystals

This paper summarizes results obtained earlier from E.S.R. studies of γ-irradiated n-alkane single crystals. It also contains some new experimental results that serve to give a more complete picture of the deposition of radiation energy in solid alkanes. The experiments performed with solid n-alkanes have thus far provided structural data that permit the nature and even the conformation of alkyl radicals to be clearly understood. Two types of radical exist, namely, one where the unpaired electron is located next to the end methyl group and one with the unpaired electron in the interior of the chain. The first type has a conformation which differs from that of the undamaged molecule. Microwave saturation data show that there is a difference in relaxation properties of these radicals which can be understood in terms of a difference in mobility. Relative yield measurements give the distribution of isomeric alkyl, the result differing from that obtained using product analysis in liquids. For protiated n-alkanes n-alkyl is lacking and the 2-alkyl concentration is higher than expected. For deuterated n-alkanes the E.S.R. spectrum is mainly that of radicals with the unpaired electron located in the interior of the carbon chain. This isotope effect is again contrary to observations in liquid n-alkanes.The broad lines observed in protiated alkanes irradiated at 77 K and deuterated alkanes irradiated at 4.2 K are not believed to arise from strong spin-spin interactions. They are thought instead to arise from distorted crystal and radical structures relating to the damage regions of the crystals.Radical pairs exist with different stability, yield and structure. Our estimate, in deuterated alkanes, that as much as 40% of the radicals are formed in pairs or clusters at 4.2 K shows that radical pair formation is an important radiolytic process in solid n-alkanes. Energy transfer from deuterated to protiated molecules has also been experimentally verified. This energy transfer is temperature dependent and occurs efficiently at 77 K, but less efficiently at 4.2 and 273 K. A relationship also exists for deuterated alkanes between the amount of radical pairs formed at 4 K and the long-range energy transfer at 77 K. This can be readily accounted for by an exciton transfer mechanism analogous to that in aromatic crystals. It might also be qualitatively described by a hydrogen abstraction process. In this case, however, the properties of the deuterium atoms are not understood.     

The role of oxidizing radicals in neptunium speciation in γ-irradiated nitric acid

The irradiation of aqueous nitric acid solutions generates transient, reactive species that are known to oxidize neptunium. However, nitrous acid is also a long-lived product of nitric acid irradiation, which reduces neptunium. When we irradiated nitric acid solutions of neptunium and measured its speciation by UV/Vis spectroscopy, we found that at short irradiation times, oxidation of Np(V) to Np(VI) occurred due to reactions with radicals such as ^?OH, ^?NO₃ and ^?NO₂. However, at higher absorbed doses and after a sufficient amount of nitrous acid was produced, reduction of Np(VI) to Np(V) began to occur, eventually reaching an equilibrium distribution of these species depending on nitric acid concentration. Neptunium(IV) was not produced.     

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.     

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 k ₂₇₀ 2.7 × 10²l mol^–1cm^–1and k ₂₉₀ 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 300 nm, can belong to the cyanide ions (CN^–) and H₂C=N⁺cations (the molar absorption coefficients of the ions are k _ion= (0.4–1.0) × 10²l mol^–1cm^–1). In the photobleaching of -irradiated HCN ( = 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.     

Conformational changes of DNA γ-irradiated in the presence of aliphatic alcohols in solution

The conformational changes of DNA in aqueous solutions containing ethanol and iso- and n-propanol having concentrations of 0–23 wt.% and ionic strength μ = 0.003 NaCl and exposed to γ radiation at doses from 0 Gy to 30 Gy were studied by low-gradient viscosimetry and circular dichroism. The dependence of the specific volume (V _sp) of DNA on the radiation dose changed at different contents of alcohol. There is a critical concentration (C _cr) at which the structure of the mixed solvent is rearranged. The V _sp of DNA decreased as the radiation dose increased at C _alc = 12 wt.% < C _cr and increased at C _alc = 23 wt.% > C _cr. In both cases, the largest V _sp of DNA was observed in water ethanol solutions. The secondary structure of the macromolecule in the systems corresponds to the form. It was shown that the observed conformational changes in irradiated DNA at C _alc < C _cr and C _alc > C _cr could not be explained only by the ability of alcohols to intercept the radicals formed in radiolysis of water.     

ESR study of the free radicals formed in the photolysis of α-ketofluorosulfates

ESR spectroscopy was used to establish that the UV irradiation of -ketofluorosulfates containing an OSO ₂ F group at a tertiary carbon atom gives radicals formed upon cleavage of the C-OSO ₂ F bond. If the OSO ₂ F group is attached to a primary or secondary carbon atom, we observe ordinary photodecomposition with cleavage of the acyl—carbon bond. The ESR spectra of the new radicals were studied. Replacement of fluorine by the OSO ₂ F group in the trifluoromethyl radical leads to compression of the radical site. Branched -oxoperfluoroalkyl radicals are not heteroallylic and the unpaired electron is localized largely on the carbon atom. The [(CF ₃ ) ₂ CF] ₂ C(O)CF ₃ radical is stable in the air.A. N. Nesmeyanov Institute of Heteroorganic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 757–761, March, 1992.     

Photoconductivity in γ-irradiated alkali halides

γ-irradiated alkali halides undergo bleaching under prolonged light illumination, hence the photocurrent decreases with time. We studied the kinetics of photocurrent decay of γ-irradiated alkali halides under steady-state illumination, and the spectral response of the photocurrent for various alkali halides. From the data on the variation of the photocurrent with applied electric field and intensity of F light employed for illumination, the value of the product of quantum efficiency and mean electron range per unit field is calculated at room temperature.     

Molecular-topology of powder of the γ-irradiated tetrafluoroethylene/ethylene copolymer

The molecular-topological structure of γ-irradiated commercial copolymers of tetrafluoroethylene with ethylene (CTE) were explored in the coaxial mode, which has the vectors of CTE loading and compression pressure in the same plane, and in the perpendicular mode with the two vectors perpendicular to each other. Instead of the amorphous-crystal structure observed with the coaxial orientation of the vectors, the CTEs are completely amorphous in their perpendicular orientation. Radiolysis with a dose of 140 kGy leads to the formation of a crosslinked structure for the CTEs. On irradiation, the crystals are transformed to an amorphous state and CTE can form a fully amorphous structure after 600 kGy of irradiation.     

Molecular-topological structure of γ-irradiated polytetrafluoroethylene

The molecular-topological structure of polytetrafluoroethylene (PTFE) has been studied in the range of ?100 to +450°C by thermomechanical spectrometry. Revealed in this temperature range is a fourblock topological structure composed of one amorphous (T _g = 16°C) and three crystalline (low-melting (T _m = 315°C), intermediate (T _m ¹ = 355°C), and high-melting (T _m ² = 388°C)) polymorphs. At a dose of 1 kGy, the long-range orientation of chains in the intermediate and high-melting crystalline blocks of PTFE is replaced by short-range orientation of the cluster association structure. At doses of 100?C500 kGy, the latter structure transitions to the amorphous state and the irradiated samples acquire a semicrystalline structure of the two-block type. The molecular-mass distribution function of interjunction chains of the pseudo-network of the amorphous block is bimodal in character and its maxima are noticeable shifted toward lower masses with an increase in the radiation dose. As the dose increases, the crystallinity decreases and the molecular mobility of amorphized chains is enhanced. As a result, both the glass transition and the molecular flow onset temperatures of the polymer are reduced.     

Molecular-topological structure of γ-irradiated ftorlon polytetrafluoroethylene

Six topological structures (an amorphous and five crystalline blocks) have been detected in a polytetrafluoroethylene film with a pseudo-network structure. During the Γ-irradiation of the polymer in air, the crystalline fractions degrade and gradually convert into amorphous and cluster states with the increasing radiation dose. After irradiation at a dose of 90 kGy, the polymer loses its capability for crystallization and forms a completely amorphous topological structure. However, regardless of dose in the range of 3–90 kGy, the topological structure of the polymer irradiated in a vacuum remains unchanged and consists of amorphous, cluster, and crystalline blocks.     

Isothermal decomposition of γ-irradiated indium acetate

The isothermal decomposition of un-irradiated (pristine) and pre-γ-irradiated indium acetate was studied in the temperature range (298–1273 K) and in air using isothermal thermogravimetric technique. The data were analyzed using various solid-state reaction models. The results showed that the kinetic of isothermal decomposition of indium acetate was governed by random nucleation reaction (Erof’ev equation A₃). The kinetics and thermodynamic parameters of the main decomposition process for un-irradiated and γ-irradiated samples were calculated and evaluated.     

Phototransformations of methylsubstituted oxiranes’ radical cations in freonic matrices at 77 K

It has been established that, upon X-ray irradiation of various methyloxiranes in freonic matrices at 77 K, both open and cyclic (with the elongated C-C bond) forms of radical cations are stabilized. It has been shown that observed reversible photoinduced transformations of 2,3-dimethyloxirane and methyloxirane radical cations are related to the conversion between the open and cyclic forms of the radical cations with high quantum yields (0.02?C0.39, depending on the oxirane and the matrix). For the trimethyloxirane radical cation the action of light on the trans-isomer of the open form results in its photoinduced transformation into a C-centered radical with low quantum efficiency (??4 × 10^?3). Tetramethyloxirane radical cations, stabilized in their open form, are resistant to the action of light. Probable causes of the observed effects are discussed. Upon the X-ray irradiation of 2,2-dimethyloxirane in freonic matrices at 77 K, a cyclic form of the radical cation is stabilized (presumably, as part of a complex with matrix molecules) which transforms into a distonic C-centered radical cation under the action of light with the quantum yield of ??10^?3.     

Luminescence of toluene and deuterium-substituted toluenes at 77°K in polycrystalline methylcyclohexane

The luminescence spectra of C₆H₅CH₃, o-, m- and p-DC₆H₄CH₃, C₆H₅CD₃, C₆D₅CH₃ and C₆D₅CD₃ in polycrystalline methylcyclohexane at 77°K have been studied. The vibrational analyses of the phosphorescences and fluorescences of these isotopically related molecules are reported. For the phosphorescences, progressions in a totally symmetric ring carbon—carbon stretching frequency, based mainly on one quantum of a non-totally symmetric vibration of e_2g parentage in benzene, are interpreted to signify a planar, non-regular hexagon ring structure for the lowest triplet state of the toluenes. In the fluorescences, progressions in the ring breathing mode, with a different e_2g mode of benzene serving as a false origin, are suggestive of an expanded regular hexagon ring geometry of the lowest excited singlet state.     

A preliminary study of isopropyl alcohol matrix effect and correction in ICP-MS

Isopropyl alcohol matrix effect was found to be element specific by using the defined matrix effect factor in ICP-MS,which could not be corrected by using the conventional internal reference method.Unlike the conventional internal reference method,the presented method allows for the analyte to behave differently from the internal reference under the influence of the matrix.     

A critical study of the hydrogen position in the crystal lattice of boehmite, γ-AlO(OH)

The relative intensities of i.r. absorption bands due to proton vibrations in boehmite samples of different origin are analysed and interpreted in terms of slight modifications of the hydrogen positions in the crystal lattice. There seems to be a linear relation between two angles characterizing the geometry of the hydrogen bond. The structural deformations are correlated with impurity content of boehmite samples.