Oxidation of alkyl ions, C_nH_2n+1⁺ (n = 1-5), in reactions with O₂ and O₃ in the gas phase

Rate constants and product ion branching fractions are reported for the reactions of CH₃⁺, C₂H₅⁺, s-C₃H₇⁺, s-C₄H₉⁺, t-C₄H₉⁺, and t-C₅H₁₁⁺ with O₂ and O₃ at 300 K in a variable-temperature selected-ion flow tube (VT-SIFT). The reaction rate constant for CH₃⁺ with O₃ is large and approximately equal to the thermal energy capture rate constant given by the Su-Chesnavich equation. The C₂H₅⁺, s-C₃H₇⁺, and s-C₄H₉⁺ ions are somewhat less reactive, reacting at approximately 7-46% of the thermal capture rate. The HCO⁺ and C₂H₃O⁺ ions are the major products in these reactions. The t-C₄H₉⁺ and t-C₅H₁₁⁺ ions are found to be unreactive, with rate constants < 5 × 10^-12 cm³ s^-1, which is the present detection limit of our apparatus using this ozone source. Ozone is a singlet in its ground state, and ab initio calculations at the B3LYP/6-31G(d) level of theory indicate that reactant complexes can be formed, decreasing in stability with the size of alkyl chains attached to the cationic carbon atom. The decreasing reactivity of the alkyl ions with increasing order of the carbocation is attributed to a greatly reduced O₃ binding energy. The ions listed above do not undergo two-body reactions with O₂, k < 5 × 10^-13 cm³ s^-1, despite the availability of reaction channels with exothermicities of several hudnred kilojoules per mole. Ab initio calculations at the B3LYP/6-31G(d) level of theory indicate that the O₂ reaction systems form weak complexes with large C-O bond distances (repulsive at smaller distances) on the lowest energy triplet potential energy surface. Access to the singlet surface is required for bond formation; however, this surface is not accessible at thermal energies.