The reaction of Criegee intermediates with acids and enols

The reaction of CH₂OO, the smallest carbonyl oxide (Criegee intermediate, CI), with several acids was investigated using the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ quantum chemical method, as well as microvariational transition state theory and RRKM master equation theoretical kinetic methodologies. For oxoacids HNO₃ and HCOOH, a 1,4-insertion mechanism allows for barrierless reactions with high rate coefficients, in agreement with literature experimental data. This mechanism relies on the presence of a double bond in the α-position to the acidic OH group. We predict that reactions of CI with enols will likewise have high rate coefficients, proceeding through a similar mechanism. The hydracid HCl was found to react through a less favorable 1,2-insertion reaction, leading to lower rate coefficients, again in good agreement with the literature. We conclude that the reaction mechanism is the main indicator for the reaction rate for CH₂OO + acid reactions, with acidity only of secondary influence. At room temperature and 1 atm the main product for all reactions was found to be the thermalized hydroperoxide initial adduct, with minor yields of fragmentation products. One of the product channels characterized is a novel reaction path involving intramolecular H-abstraction after a roaming reaction in the OH + product radical complex formed by the dissociation of the hydroperoxide adduct; this channel is the lowest fragmentation route for some of the reactions studied.