Temperature and pressure dependence of ion-molecule association and dissociation reactions: the N-2(+)+N-2(+M)double left right arrow N-4(+)(+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 N-2(+) + N-2 (+ M) double left right arrow N-4(+) (+ 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)/[N-2] cm(6) 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)/cm(3) 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 double left right arrow N2H7+, C7H7+ + CH3 double left right arrow C8H10+, and C7H7+ + C2H5 double left right arrow C9H12+ are illustrated for comparison.