Kinetics of ion-ion mutual neutralization: halide anions with polyatomic cations.

The binary mutual neutralization (MN) of a series of 17 cations (O₂⁺, NO(+), NO₂⁺, CO(+), CO₂⁺, Cl(+), Cl₂⁺, SO₂⁺, CF₃⁺, C₂F₅⁺, NH₃⁺, H₃⁺, D₃⁺, H2O(+), H3O(+), ArH(+), ArD(+)) with 3 halide anions (Cl(-), Br(-), I(-)) has been investigated in a flowing afterglow-Langmuir probe apparatus using the variable electron and neutral density attachment mass spectrometry technique. The MN rate constants of atom-atom reactions are dominated by the chemical nature of the system (i.e., the specific locations of curve crossings). As the number of atoms in the system increases, the MN rate constants become dominated instead by the physical nature of the system (e.g., the relative velocity of the reactants). For systems involving 4 or more atoms, the 300 K MN rate constants are well described by 2.7 × 10(-7) μ(-0.5), where the reduced mass is in Da and the resulting rate constants in cm(3) s(-1). An upper limit to the MN rate constants appears well described by the complex potential model described by Hickman assuming a cross-section to neutralization of 11,000 Å(2) at 300 K, equivalent to 3.5 × 10(-7) μ(-0.5).