Relevance of the Concentration Dependence of Methyl Radical Formation from Methyl Bromide Solutions to the Distribution of Ion-Pair Separation Distances in Irradiated Hydrocarbons

Abstract

A radiochromatographic technique for measuring the yields of methyl radicals formed on electron capture by methyl bromide in irradiated liquid hydrocarbons is described. $\mathrm{G(}{\mathrm{CH}}_3)$ has been measured in n‐hexane, cyclohexane, and iso‐octane from 3 × 10⁻⁶ to $\text{0.3M}$ CH₃Br. Below ${\text{3\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−4}}M$ the yields are described by the equation $\mathrm{G(}{\mathrm{CH}}_3{\mathrm{)\hspace{0.17em}=\hspace{0.17em}G}}_{\mathrm{fi}}\mathrm{\hspace{0.17em}+\hspace{0.17em}K(}{\mathrm{CH}}_3\mathrm{Br}{)}^{\text{1/2}}$ in agreement with theory. The values of $G_{\mathrm{fi}}$ (free‐ion yield) determined are 0.12, 0.13, and 0.36 for n‐hexane, cyclohexane, and isooctane, respectively. Over the entire concentration range studied, $\mathrm{G(}{\mathrm{CH}}_3)$ can be described by the empirical expression $$\mathrm{G(}{\mathrm{CH}}_3{\mathrm{)\hspace{0.17em}=\hspace{0.17em}G}}_{\mathrm{fi}}{\mathrm{\hspace{0.17em}+\hspace{0.17em}(G}}_{\mathrm{gi}}{\mathrm{(\alpha }}_{{\mathrm{CH}}_3\mathrm{Br}}[{\mathrm{CH}}_3\mathrm{Br}{\mathrm{])}}^{\text{1\hspace{0.17em}/\hspace{0.17em}2}}{\text{\hspace{0.17em}/\hspace{0.17em}{1\hspace{0.17em}+\hspace{0.17em}(α}}_{{\mathrm{CH}}_3\mathrm{Br}}[{\mathrm{CH}}_3\mathrm{Br}{\mathrm{])}}^{\text{1\hspace{0.17em}/\hspace{0.17em}2}}\mathrm{\}).}$$ A value of approximately 4 is found for the yield of geminately recombining ion pairs for the three hydrocarbons. It is argued from the high‐concentration data that the distribution of thermalization distances are closely similar for the different hydrocarbons, in contradiction to the predicted longer distances in iso‐octane required to explain the high free‐ion yield.