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.