Rate coefficients for 1,2-dimethyl-allyl + HO₂/O₂ and the implications for 2-methyl-2-butene combustion

A detail investigation is presented on the reaction of 1,2-dimethyl-allyl with HO₂, including bimolecular reactions of aC₅H₉-c with HO₂, thermal decomposition reactions of aC₅H₉OOH, and the subsequent decomposition and isomerization reactions of C₅H₉O based on theoretical work. In addition, the reaction of aC₅H₉-c with O₂ and the resulting peroxy-radical chemistry is presented. The majority of the stationary points are obtained at CCSD(T)-F12/cc-pVTZ-F12//M06–2X/6–311G(d,p) level, with barrierless reaction and some transition states using multireference method. Microcanonical rate theory and the master equation are used to determine the temperature- and pressure-dependent rate coefficients, as implemented in a RRKM/ME code. Uncertainty analysis for energy barrier and ∆E_down were conducted. The newly developed submechanisms for the reactions of 1,2-dimethyl-allyl with HO₂ and O₂ are added to a previous literature model for 2-methy-butene. The results show that at low pressure/high temperature, the reaction aC₅H₉-c with HO₂ would directly decompose to aC₅H₉O + OH, which, though technically a chain propagating reaction, and significantly increases the reactivity of the system. The updated mechanism is in much better agreement with the available experimental data.