Unraveling electronic effects that influence the bimolecular chemistry of fluorinated Criegee intermediates

Ozonolysis is important for the removal of unsaturated volatile organic compounds emitted into the Earth's atmosphere. The alkene + ozone reaction (alkene ozonolysis) forms a carbonyl oxide – a Criegee intermediate (CI). CIs are interesting from an atmospheric chemistry perspective. They are implicated in enhancing the oxidizing capacity of the troposphere and in secondary organic aerosol formation. CIs may participate in bimolecular reactions with trace atmospheric vapors like water and methanol. Here, the chemistry of the two isomers of CFHOO and of CF3CHOO with water, water dimer, and methanol is explored. The present study confirms that the syn-conformer is the minimum energy structure for both CIs and that the reactions of anti-CFHOO display slightly lower activation energies cf. syn-CFHOO, thus promoting faster reaction. For both syn- and anti-CFHOO, the energy barrier in the pathway to adding water, water dimer, or methanol is lower—and the overall reaction exoergicity greater—than for the corresponding reactions involving the nonfluorinated analogue, CH2OO. The corresponding bimolecular reaction rates are thus predicted to be faster for CFHOO than CH2OO. Similar reactivity trends are found when comparing the same reactions of syn-CF3CHOO, cf. syn-CH3CHOO, but not when comparing the energetics of the reactions involving anti-CF3CHOO cf. anti-CH3CHOO.