A priori falloff analysis for OH + NO2

The pressure-dependent rate constants for the recombination of hydroxyl radical and nitrogen dioxide to form nitric acid and peroxynitrous acid are calculated a priori using two inverse Laplace transform approaches and both the modified strong collision and the master equation treatments of collisional energy transfer, at a variety of temperatures and pressures. Accuracy within a factor of 3 is demonstrated for the most accurate a priori predictions of k1. The estimate of k1 at ambient conditions was not very sensitive to uncertainties in the barrier height (dissociation limit), the average collisional energy transferred 〈ΔEdown〉, the high-pressure limit k1∞, or to treatments of the OH torsions as stiff vibrators, hindered rotors, or free rotors. Estimates of k1 were sensitive to coupling between the reaction coordinate and the external rotational mode, or J mode, of the adduct. Increased accuracy in any further a priori calculation of k1 or k2 depends on improvements to the treatment of the J mode. We calculate the equilibrium constant for the formation of HOONO from OH and NO2, treating the OH moiety as a hindered rotor and using a direct-count density of states algorithm, to be K = 1.3 × 1010 [cm3/mol/s] at 300 K. This work suggests published high-pressure measurements of k1 (pressures > 10,000 torr) are likely convolved with HOONO production.