Ab initio chemical kinetics of Isopropyl acetate oxidation with OH radicals

Global reactivity descriptors of Isopropyl acetate (IPA) and thermo-kinetics aspects of its oxidation via the ground state (X2Π) and the first excited state (A2Σ+) •OH radicals have been studied computationally using the moderate ab initio composite method, restricted open-shell complete basis setquadratic Becke3 (ROCBSQB3), the accurate thermo-kinetic density functional method (DFT) M06-2X/cc-pVTZ and the time-dependent density functional TDDFT-M06-2X/cc-pVTZ//M06-2X/cc-pVTZ levels. Ten oxidation pathways have been investigated all of them are exothermic. The potential energy diagram has been sketched using different pre- and post-reactive complexes for all reactions. Rate constants calculations were obtained directly by connecting the separated reactants with different transition states and via an effective approach using the unimolecular Rice-Ramsperger-Kassel-Marcus (RRKM) and the transition state (TST) theories. The results indicate that the reaction of IPA with •OH radicals occurs in the ground state rather than the excited state and the rate constants obtained directly and from the effective approach are the same which confirmed the accuracy of the estimated pre-reactive complexes and the reaction mechanism. Rate constants and branching ratios show that H- atom abstraction from iso C-H (C2 atom) bond is the most kinetically preferable route up to 1000 K, while at higher temperature H-atom abstraction from the out-plane CH3 group (C3 atom) became the most dominant route with the high competition with that of in-plane CH3 group (C4 atom).