Insights into the Reactions of Hydroxyl Radical with Diolefins from Atmospheric to Combustion Environments.

Hydroxyl radicals and olefins are quite important from combustion and atmospheric chemistry standpoint. Large amounts of olefinic compounds are emitted into the earth's atmosphere from both biogenic and anthropogenic sources. Olefins make a significant share in the tranportation fuels (e.g., up to 20% by volume in gasoline), and they appear as important intermediates during hydrocarbon oxidation. As olefins inhibit low-temperature heat release, they have caught some attention for their applicability in future advanced combustion engine technology. Despite their importance, the literature data for the reactions of olefins are quite scarce. In this work, we have measured the rate coefficients for the reaction of hydroxyl radicals (OH) with several diolefins, namely 1,3-butadiene, cis-1,3-pentadiene, trans-1,3-pentadiene, and 1,4-pentadiene, over a wide range of experimental conditions (T = 294 - 468 K and p ~ 53 mbar; T = 881 - 1348 K and p ~ 1 - 2.5 bar). We obtained the low-T data in a flow reactor using laser flash photolysis and laser induced fluorescence (LPFR/LIF), and the high-T data were obtained with a shock tube and UV laser-absorption (ST/LA). At low temperatures, we observed differences in the reactivity of cis- and trans-1,3-pentadiene, but these molecules exhibited similar reactivity at high temperatures. Similar to monoolefins + OH reactions, we observed negative temperature dependence for dienes + OH reactions at low temperatures - revealing that OH-addition channels prevail at low temperatures. Except for 1,4-pentadiene + OH reaction, which shows evidence of significant H-abstraction reactions even at low-temperatures, other diolefins studied here almost exclusively undergo addition reaction with OH radicals at the low-temperature end of our experiments; whereas the reactions mainly switch to hydrogen abstraction at high temperatures. These reactions show complex Arrhenius behaviour as a result of many possible chemical pathways in such a convoluted system.