Bernard Tilquin

Comparison of gamma -radiolysis of hydrocarbons in glassy and polycrystalline states. I. Products of high molecular weight

The effects of the glassy and polycrystalline states of methylcyclohexane (MCH), isopentane (IP), isohexane (IH) and 2,3-dimethylbutane (DMB) on the yields of the heavy products from the gamma -radiolysis has been investigated. Total G values are the same in glassy and polycrystalline solids for MCH and IP, but G values for IH and DMB are larger in the polycrystalline states than those in the glassy forms. Comparison of the relative distribution of stable products always shows matrix effects. Differences in ESR spectra between either states have only been observed for irradiated IH and DMB. The results from radical scavenging methods are interpreted in terms of different probabilities of radical processes in the glassy and polycrystalline states.

γ-Radiolysis of solid branched hydrocarbons: 2, 2-dimethylbutane

Abstract 2, 2-Dimethylbutane was γ-irradiated in the quenched and annealed solid states at 77 K, G-yields for intermediate and heavy compounds were measured by capillary gas chromatography. Post-irradiation scavenging of trapped radicals by dissolving the polycrystalline irradiated alkane in a propane-oxygen mixture lowered the product yields to an extent (50%, intermediate product; 30%, dimer isomers; 60%, unsaturated heavy products) corresponding to the radical contribution to the product formation (delayed mechanism). The alkene gases and the congruent olefin produced during radiolysis at 77 K may play a major role in the reaction pathway.

Radiation-chemistry of Solid Branched-chain Alkanes

Abstract The yields of intermediate molecular weight products (PM = C7 to C11 = middle molecular weight product) have been determined for the radiolysis of 2-methylpentane and 3-methylpentane. A GC/MS system was used for the products identification. The product distribution determined by capillary column gas chromatography (GC2) reflects a marked structural effect in the radiolysis processes. In the solid radiolysis, the results suggest that the PM products do not originate from the combination of fragment and parent radicals. A chemical activation transfer to light olefinic products enclosed in the solid during the radiolysis plays a prominent role in the formation of the PM products.

Radiolysis of solid neopentane at 4 and 77K as studied by capillary gas chromatography and ESR spectrometry

Final products in the radiolysis of pure neopentane at 4 and 77K have been analyzed by capillary gas chromatography. The results are inconsistent with those obtained by ESR spectroscopy. The ratio C-C to C-H bond rupture in the radiolysis of neopentane is 0.3-0.5 from the product analysis at 4 and 77K, while ESR spectroscopy at 4K indicates an extremely low ratio (<0.01).

Radiolysis of vitreous cyclohexane-neohexane mixtures at 77K

Vitreous mixtures of cyclohexane and neohexane were irradiated by gamma rays at 77K. The relative radical concentrations were computed from ESR spectra of the irradiated mixtures. A chemical activation transfer from the neohexane to the cyclohexane can explain the excess of cyclohexyl radicals with respect to the electron fraction. A hydrogen transfer from neohexane to cyclohexyl radicals occurs at 120K.

Gamma radiolysis of 3-methylpentane. Effect of added olefins on the formation of C12-olefins

Abstract Contributions of congruent (parent derived) olefins to the formation of unsaturated heavy products (C 12 H 24 ) are investigated. Effects of dose or of olefinic additives on the G yield values are studied by capillary gas chromatography. The dose dependence is explained by transfer of positive charge to congruent olefins (C 6 H 12 ) even though their concentrations build up linearly with dose. γ Irradiation of 3-methylpentane containing pentenes or butenes (0.2–2 mol%) provides good examples of transfer of energy, however the results are unexpected. C-H scission in the radiolysis of solid 3-methylpentane is also reviewed.

Energy transfer in γ-irradiated solid pentane-2 methylpentane mixtures

Pentane-2 methylpentane mixtures have been irradiated in the solid phase at 77K. The trapped radicals are studied by ESR measurements and the dehydrodimers are analysed by capillary gas chromatography. A model has been developed for the quantitative evaluation of the competing growth of final products from each component. The results do not exhibit any transfer of chemical activation between the constituents of the mixture. The physical properties of the solid exert a subtle effect upon the processes initiated by the gamma -radiation.

Evidence for dose rate effect in gamma-radiolysis—II. Methylmethoxyacetate in 2,3-dimethylbutane and in cyclohexane

Solutions of 2,3-dimethylbutane (DMB) and cyclohexane containing a captodative solute, methylmethoxyacetate (MMA), were irradiated (continuous wave). Alkane dimeric products were analysed by capillary gas chromatography. G-yields were estimated as a function of the concentration of the captodative solute. By reaction with alkyl radicals, the captodative solute gives the free radical stabilized by the captodative effect. Combination reaction of this radical with itself or with alkyl radicals gives solute or mixed dimer which were also analysed. A large intensity effect was noticed, as was seen in the solutions of n-hexane MMA. Dose effect was definitively rejected.

Evidence for a dose rate effect in gamma-radiolysis—I. Methylmethoxyacetate in aliphatic linear hydrocarbons

Abstract Alkanes (C6, C16, C17) containing a captodative solute, methylmethoxyacetate (MMA), were irradiated with 60Co. Dimeric products were analysed by capillary gas chromatography. G-values were measured as a function of the concentration of the captodative solute. By reaction with alkyl radicals, the captodative solute gives a free radical stabilized by the captodative effect. The combination reaction of this radical with itself or with alkyl radicals gives solute or mixed dimers which were also analysed. A large dose rate effect was seen, when the solute was not able to scavenge most of the alkyl radicals. Very good purification of the starting materials was needed to achieve reliable results. This may explain some discrepancies which appear in the literature.

Irradiation temperature effect upon crosslinks induced by gamma-irradiations of polycrystalline n-alkanes, n-undecane to n-heptadecane

Abstract Even-odd alternation of dimer isomers distribution of a series of irradiated normal alkanes starting from n -undecane is found to be due to prominent crosslinks at the ends of monomers in odd members of the series irradiated at 77 K. However, penultimate sites are also favoured in opposition to ESR results. By increasing the temperature of the irradiation to 195 K, crosslinks in the central part of chains (H-links) are enhanced particularly for even homologues. The temperature dependence explains that none of the free radicals are found at the terminal position after irradiation of n -eicosane at room temperature.