Temperature and pressure dependence of ion–molecule association and dissociation reactions: the N2+ + N2 (+ M) ⇔ N4+ (+ M) reaction

Abstract

Thermal dissociation and the reverse association reactions of molecular ions can be represented within the same framework as unimolecular reactions of neutral molecules. However, because of different intra- and intermolecular interaction potentials, some differences between ionic and neutral systems should also be expected. Starting from these generalities, the present article analyzes the temperature and pressure dependences of dissociation/association rates of ion–molecule reaction systems. The reaction N₂⁺ + N₂ (+ M) ⇔ N₄⁺ (+ M) is considered in particular and falloff curves for this reaction are constructed over the temperature range 5–600 K. Falloff corrections are applied to earlier experiments which allows for a representation of the rate coefficients in terms of a limiting low-pressure rate coefficient k_ass,0/[N₂] cm⁶ molecule⁻² s⁻¹ = 6.8 × 10⁻²⁹(300 K/T)^2.23 − 5.6 × 10⁻³¹(300 K/T)^3.12, a limiting high-pressure rate coefficient k_ass,∞/cm³ molecule⁻¹ s⁻¹ = 8.33 × 10⁻¹⁰[1 + 0.069(300 K/T)^0.33] and a center broadening factor of the falloff curve F_c = 0.53 ± 0.05 nearly independent of the temperature. Other examples such as the reactions NH₄⁺ + NH₃ ⇔ N₂H₇⁺, C₇H₇⁺ + CH₃ ⇔ C₈H₁₀⁺, and C₇H₇⁺ + C₂H₅ ⇔ C₉H₁₂⁺ are illustrated for comparison.