The radiation chemistry of liquid methanol. I. The oxidizing radical

Abstract

Methanolic solutions of I$^{-}$, Br$^{-}$, Cl$^{-}$, SCN$^{-}$ and TMPD have been investigated by pulse radiolysis. Except for Cl$^{-}$ solutions oxidation of the solutes occurs and the transient products I$_{2}^{-}$, Br$_{2}^{-}$ (SCN)$_{2}^{-}$ and TMPD$^{+}$ have been identified by their absorption spectra $(\lambda _{\max}$ = 380, 360, 470 and 565 nm respectively). For the Br$^{-}$ and SCN$^{-}$ cases the oxidation occurred only in acidified solution. These results are attributed to the occurrence of the reactions X$^{-}$ + CH$_{3}$O $\rightarrow $ CH$_{3}$O$^{-}$ + X, X + X$^{-}\rightleftharpoons $ X$_{2}^{-}$, where X$^{-}$= I$^{-}$, Br$^{-}$, SCN$^{-}$ or TMPD. In pure methanol the methoxy radicals react to form the CH$_{2}$OH radical. The dependences of the yields of I$_{2}^{-}$ and TMPD$^{+}$ on the concentration of I$^{-}$ and TMPD respectively indicate that the yield of scavengable methoxy radicals $G$(CH$_{3}$O) = 2.0 and that $k$(CH$_{3}$O + X$^{-}$)/$k$(CH$_{3}$O + CH$_{3}$OH) = $(1.4\pm 0.1)\times 10^{4}$. The presence of 0.1 mol HClO$_{4}$/l or saturation of the solutions with N$_{2}$O increases $G$(CH$_{3}$O) by 0.5, an effect which is attributed to methoxy radicals which would otherwise react with electrons within the spurs. Solutions saturated with CO$_{2}$ do not show the increased yield presumably because of the occurrence of the back reaction: CO$_{2}^{-}$ + CH$_{3}$O $\rightarrow $ + CH$_{3}$O$^{-}$ + CO$_{2}$. The yield of ethoxy radicals, $G$(C$_{2}$H$_{5}$O) derived from measurement of I$_{2}^{-}$ formed in ethanolic KI solutions is estimated to be 1.5 $\pm $ 0.1. Formate ion was found to have no effect on the yield of I$_{2}^{-}$ from methanolic solutions of iodide and it is concluded that the reaction between methoxy radicals and formate ion is slow. The similar lack of effect of nitrite ion and iodide ion on the TMPD system is attributed to the reactions TMPD + NO$_{2}$ $\rightarrow $ TMPD$^{+}$ + NO$_{2}^{-}$, TMPD + I$_{2}^{-}$ $\rightarrow $ TMPD$^{+}$ + 2I$^{-}$.