Gas-Phase Photolysis of Cyclohexane in the Far Ultraviolet: Modes of Decomposition of the Neutral Excited Cyclohexane Molecule and Reactions of the Parent Cyclohexane Ion

Abstract

The photolysis of cyclohexane—cyclohexane‐d₁₂ (1:1) mixtures and of cyclohexane‐1,1,2,2,3,3‐d₆ was investigated using xenon (8.4 eV) and krypton (10, 10.6 eV) resonance radiation. It is demonstrated that the formation of hydrogen by molecular detachment is the major primary process $$\mathrm{cyclo}-{\mathrm{C}}_6{\mathrm{H}}_{12}^{*}\to {\mathrm{C}}_6{\mathrm{H}}_{10}+{\mathrm{H}}_2.$$ In this process, the hydrogen is mainly eliminated from a single carbon atom. The resulting C₆H₁₀ diradical rearranges mainly to cyclohexene, a fraction of which decomposes to form 1,3‐butadiene and ethylene. It is shown that, because of collisional deactivation, the probability of the latter mode of decomposition decreases with an increase in pressure or an increase in wavelength. Other primary processes such as $$\mathrm{cyclo}-{\mathrm{C}}_6{\mathrm{H}}_{12}^{*}\text{→3}{\mathrm{C}}_2{\mathrm{H}}_4$$ and, to a lesser extent, $$\mathrm{cyclo}-{\mathrm{C}}_6{\mathrm{H}}_{12}^{*}\text{→2}{\mathrm{C}}_3{\mathrm{H}}_6$$ as well as processes leading to the formation of H atoms and CH₃ radicals are also shown to occur. In the photolysis at 1236 and 1165 Å, C₆H₁₂⁺ ions are formed which undergo H₂‐transfer reactions such as $$\begin{array}{c}{\mathrm{C}}_6{\mathrm{H}}_{12}^{+}+{\mathrm{CD}}_3{\mathrm{CDCD}}_2\to {\mathrm{C}}_6{\mathrm{H}}_{10}^{+}+{\mathrm{CD}}_3{\mathrm{CDHCD}}_2\mathrm{H}\\ {\mathrm{C}}_6{\mathrm{H}}_{12}^{+}\mathrm{+(}{\mathrm{CD}}_2{)}_3\to {\mathrm{C}}_6{\mathrm{H}}_{10}^{+}+{\mathrm{CD}}_2{\mathrm{HCD}}_2{\mathrm{CD}}_2\mathrm{H}.\end{array}$$ The efficiency of these ion—molecule reactions does not depend on the concentration of C₃D₆ or oxygen, but is reduced by NO because of the occurrence of a charge transfer from the cyclohexane ion to NO. The same processes have been reported to occur in the gas‐phase photolysis of similar hydrocarbon mixtures.