Barrierless Reactions with Loose Transition States Govern the Yields and Lifetimes of Organic Nitrates Derived from Isoprene.

The chemical reaction mechanism of NO addition to two β and δ isoprene hydroxy-peroxy radical isomers is examined in detail using density functional theory, coupled cluster methods, and the energy resolved master equation formalism to provide estimates of rate constants and organic nitrate yields. At the M06-2x/aug-cc-pVTZ level, the potential energy surfaces of NO reacting with β-(1,2)-HO-IsopOO• and δ-Z-(1,4)-HO-IsopOO• possess barrierless reactions that produce alkoxy radicals/NO2 and organic nitrates. The nudged elastic band method was used to discover a loosely bound van der Waals (vdW) complex between NO2 and the alkoxy radical that is present in both exit reaction channels. Semiempirical master equation calculations show that the β organic nitrate yield is 8.5 ± 3.7%. Additionally, a relatively low barrier to C-C bond scission was discovered in the β-vdW complex that leads to direct HONO formation in the gas phase with a yield of 3.1 ± 1.3%. The δ isomer produces a looser vdW complex with a smaller dissociation barrier and a larger isomerization barrier, giving a 2.4 ± 0.8% organic nitrate yield that is relatively pressure and temperature insensitive. By considering all of these pathways, the first-generation NOx recycling efficiency from isoprene organic nitrates is estimated to be 21% and is expected to increase with decreasing NOx concentration.