Radiolysis of Ethylene. IV. Unimolecular Dissociation of Intermediate Ion—Molecule Complexes

Abstract

Kinetic analysis of free‐radical reactions in the radiolysis of ethylene demonstrates that the relative extent of radical addition to ethylene can be held constant by maintaining the ratio of accelerator beam current to ethylene pressure invariant. The pressure dependence of the methane and methyl radical yields when this ratio is constant reveals that the mechanism of their formation includes competition between unimolecular dissociation of an intermediate ion and its participation in an ion—molecule reaction. Consideration of known ion—molecule reactions suggests that the species involved in the competing steps are [C₆H₁₁⁺] (methane formation) and [C₆H₁₂⁺] or [C₄H₈⁺] (methyl radical formation). Rate constants of 4.2×10⁸ and 1.0×10⁹ sec⁻¹, respectively, are calculated for the dissociation processes on the basis of collision efficiency for the competing ion—molecule reactions. About one‐fourth of the methane and one‐half of the methyl radicals are formed by reactions which are independent of pressure below 1 atm. This may be ascribed to rate constants larger than 10¹⁰ sec⁻¹ for dissociation of [C₈H₁₅⁺] and [C₈H₁₆⁺] ion, respectively, or to the existence of at least two discrete energy levels for the dissociating species. Relative dissociation probabilities of intermediate ion—molecule complexes, determined by mass spectrometry, are used to estimate G(C₂H₄⁺) ≈ 1.5 and G(C₂H₃⁺) ≈ 1.0 ions/100 eV at approximately atmospheric pressure, in good agreement with values of 1.51 and 0.96, respectively, which may be calculated from W and fragmentation patterns for 70‐eV electrons.