New insights into the mechanism and kinetics of the addition reaction of unsaturated Criegee intermediates to CFCOOH and tropospheric implications.

In this work, we have investigated the mechanism, thermochemistry and kinetics of the reaction of --CHRCRCOO (where R, R = H, CH-) unsaturated Criegee intermediates (CIs) with CFCOOH using quantum chemical methods. The rate coefficients for the barrierless reactions were calculated using variable reaction coordinate variational transition state theory (VRC-VTST). For the --CHRCRCOO conformation in which conjugated CC and CO double bonds are aligned with each other, we propose a new pathway for the unidirectional addition of an OC-OH molecule (CFCOOH) to the CC double bond of --CHRCRCOO. The rate coefficient for the 1,4-CC addition reaction at 298 K is ∼10 to 10 cm s, resulting in the formation of CFC(O)OCHCRRCOOH trifluoroacetate alkyl allyl hydroperoxide (TFAAAH) as a new transitory adduct. It can act as a precursor for the formation of secondary organic aerosols (SOAs). This novel TFAAAH hydroperoxide was identified through a detailed quantum chemical study of the 1,4-addition mechanism and will provide new insights into the significance of the 1,4-addition reaction of unsaturated Cls with trace tropospheric gases on -CRCH vinyl carbon atoms.