Irradiation of Cyclohexene with 8.4, 10.0, and 11.6–11.8-eV Photons. Dissociation of Neutral Excited Cyclohexene and Reactions of C6H10+

The photolysis of cyclohexene has been investigated at photon energies above (10.0, 10.6–11.8 eV) and below (8.4 eV) its ionization potential (8.9 eV). The ionic processes were studied in a mass spectrometer equipped with a photon source. The parent C₆H₁₀⁺ ion was seen to undergo the following reactions with cyclohexene: $${\mathrm{C}}_6{\mathrm{H}}_{10}^{+}+\mathrm{cyclo}‐{\mathrm{C}}_6{\mathrm{H}}_{10}\to \mathrm{cyclo}‐{\mathrm{C}}_6{\mathrm{H}}_{12}+{\mathrm{C}}_6{\mathrm{H}}_8^{+}{\text{, k\hspace{0.17em}=\hspace{0.17em}0.7\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−10}}{\mathrm{cm}}^3/\mathrm{molecule}·\sec$$ $$\to {\mathrm{C}}_{12}{\mathrm{H}}_{20}^{+}{\text{, k\hspace{0.17em}=\hspace{0.17em}4.1\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−10}}{\mathrm{cm}}^3/\mathrm{molecule}·\sec .$$ Analysis of the yields of cyclohexane formed in the first of these reactions in the photolysis at 10.0 eV over the pressure range 1–10 torr indicates that in this pressure range the relative importance of the two reactions is the same as in the ion source of the mass spectrometer at a pressure of 10⁻² torr. At 11.6–11.8 eV the parent ion also dissociates, mainly to form C₅H₇⁺ and CH₃. Evidence is presented which indicates that at these energies, the majority of the C₆H₁₀⁺ ions retain a cyclic structure. The neutral, electronically excited cyclohexene molecule dissociates mainly as follows: $${\mathrm{C}}_6{\mathrm{H}}_{10}\text{*→1,3‐}{\mathrm{C}}_4{\mathrm{H}}_6+{\mathrm{C}}_2{\mathrm{H}}_4,\mathrm{at}\text{8.4}\mathrm{eV}\text{, Φ\hspace{0.17em}=\hspace{0.17em}0.82}$$ $$\to {\mathrm{C}}_5{\mathrm{H}}_7+{\mathrm{CH}}_3,\mathrm{at}\text{8.4}\mathrm{eV}\text{, Φ\hspace{0.17em}=\hspace{0.17em}0.08.}$$ The photolysis of 3,3,6,6‐d₄‐cyclohexene demonstrated that the first of these processes occurs predominantly by the “symmetrical” cleavage of the 3–4 and 5–6 C–C bonds, at all energies.