Atmospheric oxidation mechanism and kinetics of isoprene initiated by chlorine radicals: A computational study.

ABSTRACT

The reaction with chlorine radicals (·Cl) has been considered to be one of indispensable sinks for isoprene. However, the mechanism of ·Cl initiated isoprene reaction was not fully understood. Herein, the reaction of isoprene with ·Cl, and ensuing reactions of the resulting isoprene relevant radicals were investigated by combined quantum chemistry calculations and kinetics modeling. The results indicate that ·Cl addition to two terminal C-atoms of two double bonds of isoprene, forming IM1-1 and IM1-4, are more favorable than H-abstractions from isoprene. Interestingly, the predicted reaction rate constant for the direct H-abstraction pathway is much lower than that of the indirect one, clarifying a direct H-abstraction mechanism for previously experimental observation. The IM1-1 and IM1-4 have distinct fate in their subsequent transformation. The reaction of IM1-1 ends after the one-time O2 addition. However, IM1-4 can follow auto-oxidation mechanism with two times O2 addition to finally form highly oxidized multi-functional molecules (HOMs), C5H7ClO3 and ·OH. More importantly, the estimated contribution of ·Cl on HOMs (monomer only) formation from isoprene is lower than that of ·OH in addition pathway, implying overall HOMs yield from atmospheric isoprene oxidation could be overestimated if the role of ·Cl in transforming isoprene is ignored.