High-Pressure Photoionization Mass Spectrometry. Photoionization of Propane at 11.6–11.8 eV. Formation and Reactivity of the (C3H8)2+ Dimer Ion

The major reaction path of the propane molecular ion with propane was found to be the formation of the dimer ion (C₃H₈)₂⁺ via a termolecular mechanism, $${\mathrm{C}}_3{\mathrm{H}}_8^{+}+{\mathrm{C}}_3{\mathrm{H}}_8\to {\displaystyle {lim}^{{\mathrm{C}}_3{\mathrm{H}}_8}}({\mathrm{C}}_3{\mathrm{H}}_8{)}_2^{+}+{\mathrm{C}}_3{\mathrm{H}}_8.$$ In addition, C₃H₆⁺ and C₃H₇⁺ were also found as minor reaction products at lower pressures. The reactions of the dimeric ions with ethylene and NO were also investigated. The charge exchange reaction, (C₃H₈)₂⁺+NO→NO⁺+ 2C₃H₈, was found in propane–NO mixtures, suggesting a recombination energy in excess of 9.24 eV. The formation of C₃H₈NO⁺ was also detected at higher total pressures. The dimeric ion was also found to transfer H₂ to ethylene without affecting the structural integrity of the carbon skeleton, (C₃H₈)₂⁺+C₂H₄→C₆H₁₄⁺+C₂H₆, indicating that this species exhibits the chemical behavior of a saturated hydrocarbon ion.