Temperature and pressure dependence of ion-molecule association and dissociation reactions: the N2(+) + N2 (+ M) <--> N4+ (+ M) reaction.

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 x 10(-29) (300 K/T) 2.23 - 5.6 x 10(-31) (300 K/T)3.12, a limiting high-pressure rate coefficient k(ass,infinity)/cm3 molecule(-1) s(-1) = 8.33 x 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.