Gas-Phase Radiolysis of Propane

The radiolysis of CH₃CD₂CH₃ and of equimolar C₃H₃—C₃D₈ mixtures in the presence of radical scavengers has been investigated as a function of pressure from 0.35 to 6000 mm and as a function of temperature from 40° to 220°C. In the presence of scavengers, hydrogen and methane are mainly produced by molecular elimination. Values for the ratio CH₃D/CH₄ were found to increase with pressure and with surface to volume ratio of the irradiation vessel. Ethane is mostly formed by a hydrogen transfer reaction such as $${\mathrm{C}}_2{\mathrm{H}}_5^{+}+{\mathrm{C}}_3{\mathrm{H}}_8\to {\mathrm{C}}_2{\mathrm{H}}_6+{\mathrm{C}}_3{\mathrm{H}}_7^{+}.$$ From the distribution of the isotopic ethanes formed in the radiolysis of CH₃CD₂CH₃, it can be concluded that the hydrogen atoms in the CH₃CD₂⁺ ion undergo a rapid randomization prior to reaction with propane. Ethylene can be accounted for by the occurrence of the following processes: (a) decomposition: C₃H₈^*→CH₄+C₂H₄, (b) ion molecule reactions, (c) decomposition of vibrationally excited propyl and/or ethyl radicals. From a comparison of the radiolysis results with the mass spectral cracking pattern of propane and recent photochemical studies, it can be deduced that the decomposition of propane ion and of neutral excited propane molecules are responsible for the products. The relative importance of these two modes of decomposition depends on the pressure and the size of the reaction vessel.