Modeling the Organic Nitrate Yields in the Reaction of Alkyl Peroxy Radicals with Nitric Oxide. 1. Electronic Structure Calculations and Thermochemistry

Alkyl nitrates (RONO2) are minor products formed in the atmospheric reactions of alkyl peroxy radicals (RO2•) with nitric oxide. The major products are alkoxy radicals (RO•) and NO2. The alkyl nitrate channel is important in the troposphere because RONO2 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, CH3, C2H5, n-C3H7, i-C3H7, and 2-C5H11. 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 RONO2 are found to be much too high to account for observed nitrate formation.