New insights into the mechanism and kinetics of the addition reaction of unsaturated Criegee intermediates to CF3COOH and tropospheric implications

Abstract

In this work, we have investigated the mechanism, thermochemistry and kinetics of the reaction of syn–cis-CH₂R_zCR_yCO⁺O⁻ (where R_z, R_y = H, CH₃–) unsaturated Criegee intermediates (CIs) with CF₃COOH 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 syn–cis-CH₂R_zCR_yCO⁺O⁻ conformation in which conjugated CC and CO double bonds are aligned with each other, we propose a new pathway for the unidirectional addition of an OC–OH molecule (CF₃COOH) to the CC double bond of syn–cis-CH₂R_zCR_yCO⁺O⁻. The rate coefficient for the 1,4-CC addition reaction at 298 K is ∼10⁻¹⁰ to 10⁻¹¹ cm³ s⁻¹, resulting in the formation of CF₃C(O)OCH₂CR_zR_yCOOH 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_zCH₂ vinyl carbon atoms.