Charge-transfer complexes in the scavenging by iodine of radicals formed on pulse radiolysis of cyclo hexane

Abstract

Pulse radiolysis of solutions of iodine in cyclohexane resulted in the formation of a transient absorption with wavelength maximum at 3300 $^\overset{\circ}{\mathrm A}$. This transient spectrum was assigned to the C$_6$H$_{12}\ldots$ I$^\dot$ charge-transfer complex. The spectrum and extinction coefficients of the complex over the wavelength range 2500 to 6500 $^\overset{\circ}{\mathrm A}$ were obtained, the coefficient at 3300 $^\overset{\circ}{\mathrm A}$ being 2025 l. mole$^{-1}$ cm$^{-1}$. Hydrogen iodide in cyclohexane gave rise to the same transient absorption. Pulse radiolysis of cyclohexane and cyclohexyl iodide alone gave rise to different transient spectra attributed to the cyclohexyl radical, absorbing in the ultraviolet, extinction coefficient 340l. mole$^{-1}$cm$^{-1}$ at 2550 $^\overset{\circ}{\mathrm A}$ and the C$_6$H$_{11}$I$\ldots$I$^\dot$ charge-transfer complex, absorption maximum 3900 $^\overset{\circ}{\mathrm A}$A, extinction coefficient~ $\sim$ 10$^3$ to 10~$^4$ l. mole$^{-1}$ cm$^{-1}$, respectively. During the course of the investigations rate constants were obtained for the following reactions: \begin{equation*}\mathrm{C}_6\mathrm{H}^\cdot_{11} + \mathrm{I}_2 \rightarrow \mathrm{H}_{11}\mathrm{I} + \mathrm{I}, \quad k = 7 x 10^9 l\ldot \text{mole}^{-1} s^{-1}, \\ 2\mathrm{C}_6\mathrm{H}_{11}\ldots \mathrm{I}^\cdot _{11} \mathrm{C}_6\mathrm{H}^\cdot_{11} + \mathrm{I}_2 \rightarrow \mathrm{H}_{11}\mathrm{I} + \mathrm{I}, \quad k = 7 x 10^9 l\ldot \text{mole}^{-1} s^{-1}, \\ 2\mathrm{I}^\cdot \xrightarrow \mathrm{I}_2, \\ 2\mathrm{C}_6\mathrm{H}_{11}^{\nearrow a\mathrm{C}_6\mathrm{H}_{10} +\mathrm{C}_6\mathrm{H}_{12}_{\searrow b\mathrm{C}_{12}\mathrm{H}_{22} k_{IIa} + k_{IIb} = 2\cdot5 x 10^9 l\ldot \text{mole}^{-1} s^{-1}.\end{equation*} Dilute solutions of iodine (10$^{-4}$M) in cyclohexane yielded experimental evidence for neutral radical intermediates only. There were, however, indications that ionic radical intermediates occur in more concentrated solutions (>10$^{-3}$M).