(315c) Rate Coefficients for RH + NO2 and Subsequent Decomposition Kinetics on the HNO2 Potential Energy Surface

Alkyl nitrates are commonly added to hydrocarbon fuels to improve the combustion characteristics. The basic mechanism for RO-NO₂ decomposition is relatively straightforward: The weak RO-NO₂ bond cleaves, resulting in an alkyloxy radical and NO₂. Although much of the improved ignition performance is due to the alkyloxy radical, the NO₂ radical also participates in the oxidation of the fuel. RH + NO₂ has three possible products:

Despite the prevalence of 2-ethylhexyl nitrate as a fuel additive (e.g. 100,000 tons/year), little is known about the kinetics of NO₂ in the low-temperature oxidation regime (e.g. 600 to 1000 K for most transportation fuels). In this talk, we will present high-accuracy electronic structure calculations for the RH + NO₂ potential energy surfaces (PES) for RH = H₂, CH₄, and C₂H₆. These benchmark calculations will be used to test compound methods for accurate determination of the PES for larger alkanes for each of the three possible product channels. Variational Transition State Theory calculations will be presented to predict the branching fraction between trans-HONO, cis-HONO, and HNO₂. Contra thermodynamics, although trans-HONO is the most stable HNO₂ isomer, it has the highest barrier to formation. We will also present calculations for the subsequent isomerization between the HNO₂ isomers, as well as decomposition to H + NO₂ and OH + NO.