A p riori falloff analysis for OH + NO 2

The pressure‐dependent rate constants for the recombination of hydroxyl radical and nitrogen dioxide to form nitric acid $(\rm OH+NO_{2}{\buildrel \mit k_{1}\rm\over \longrightarrow}HONO_{2})$ and peroxynitrous acid $(\rm OH+NO_{2}{\buildrel \mit k_{2}\rm\over \longrightarrow}HOONO)$ 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 k 1 . The estimate of k 1 at ambient conditions was not very sensitive to uncertainties in the barrier height (dissociation limit), the average collisional energy transferred 〈ΔE down 〉, the high‐pressure limit k 1 ∞ , or to treatments of the OH torsions as stiff vibrators, hindered rotors, or free rotors. Estimates of k 1 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 k 1 or k 2 depends on improvements to the treatment of the J mode. We calculate the equilibrium constant for the formation of HOONO from OH and NO 2 , treating the OH moiety as a hindered rotor and using a direct‐count density of states algorithm, to be K   OH+NO   2 →HOONO= 1.3 × 10 10 [cm 3 /mol/s] at 300 K. This work suggests published high‐pressure measurements of k 1 (pressures > 10,000 torr) are likely convolved with HOONO production. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 245–262, 2000