Data for the reactions between OH and NO2 have been modeled using a multiwell, multichannel master equation approach. In this work, new ab initio quantum chemical results for cis−cis- and trans−perp-HOONO at the QCISD(T)/cc-pVDZ level are used with the multiple-well, multiple-channel master equation approach in order to model the data between 220 and 430 K in both He and N2. The results are in good agreement with the experimental data over the entire ranges of temperature and pressure. The contribution from HOONO is evaluated for the experimental conditions. It is also evaluated for the conditions described by the U.S. Standard Atmosphere (1976). Although the HONO2 pathway dominates over all atmospheric conditions, up to ∼20% of the reaction is predicted to yield HOONO near the tropopause. If the atmospheric fate of HOONO is different than that of HONO2, this can affect atmospheric chemistry models.

Alkyl nitrates (RONO 2 ) are minor products formed in the atmospheric reactions of alkyl peroxy radicals (RO 2 •) with nitric oxide. The major products are alkoxy radicals (RO•) and NO 2 . The alkyl nitrate channel is important in the troposphere because RONO 2 formation results in removal of NO x and trapping of free radicals; both effects reduce the rate of ozone production. We have used electronic structure calculations at the G3 and B3LYP/6-311++G** levels to calculate the geometries, energies, and vibrational frequencies for major stationary points on the potential energy surfaces for R ) H, CH 3 , C 2 H 5 , n-C 3 H 7 , i-C 3 H 7 , and 2-C 5 H 11 . Selected calculations have been made at the G2 and QCISD(T)/cc-pVTZ levels. Reaction energies are found to be rather insensitive to the size of the alkyl group. Corrections to the reaction energies are estimated, and a generic set of reaction energies are suggested. The B3LYP/6-311++G** barriers for the isomerization of ROONO to RONO 2 are found to be much too high to account for observed nitrate formation.

Master equation calculations are used to model gas-phase literature experimental data for alkyl nitrate formation via the following reaction system of reversible reactions:  (1) RO2 + NO ↔ ROONO, (2) ROONO ↔ RO + NO2, (3) ROONO ↔ RONO2, and (4) RONO2 ↔ RO + NO2 for R = CH3, i-C3H7, and 2-C5H11. The structures and thermochemistry of the stable species are based on electronic structure calculations described in the preceding companion paper in this issue (Lohr et al. J. Phys. Chem. A 2003, 107, xxx−xxx). Literature data for recombination rate constants are used to constrain the model calculations. Several transition state models and a range of energy transfer parameters are investigated. The results for R = CH3 show that a wide variety of plausible transition state models for k - 4 gives good agreement with experiment for reaction (−4), because changes in assumed energy transfer parameters can compensate for differences between the transition state models. It is concluded that recombination reactions are good sources of absolute energy transfer parameters only when transition state properties are known with great accuracy. Although satisfactory models are obtained for the individual systems, the parameters cannot be transferred reliably from one system to another. Master equation models can be made to reproduce the experimental 2-pentyl nitrate yields from the title reaction as long as 〈Δ E〉down, the average energy transferred in deactivating collisions, is assumed to be surprisingly, and perhaps unphysically, small (∼25 cm-1), regardless of assumptions about the barrier to isomerization reaction 3. Several critical assumptions in the master equation models are examined, but none of them accounts for the small value of 〈Δ E〉down. It is concluded that new experiments should be carried out to verify or possibly revise the pressure-dependent alkyl nitrate yield data currently available in the literature.

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