How does substitution affect the unimolecular reaction rates of Criegee intermediates?

Abstract

To gain an understanding of the substitution effect on the unimolecular reaction rate coefficients for Criegee intermediates (CIs), we performed ab initio calculations for CH₂OO, CH₃CHOO, (CH₃)₂COO, CH₃CH₂CHOO, CH₂CHCHOO and CHCCHOO. The energies of the CIs, products and transition states were calculated with QCISD(T)/CBS//B3LYP/6-311+G(2d,2p), while the rate coefficients were calculated with anharmonic vibrational correction by using second order vibrational perturbation theory. It was found that for single bonded substitutions, the hydrogen transfer reaction dominates for the syn-conformers, while the OO bending reaction dominates for the anti-conformers. However once a double bond or a triple bond is added, the OO bending reaction dominates for both syn and anti-conformers. The rate coefficients for OO bending reaction show a significant increase when adding a methyl group or ethyl group. On the other hand, the addition of unsaturated vinyl and acetylene groups usually results in a slower thermal decomposition compared to the substitution with saturated carbon groups. Interestingly, for syn_Syn-CH₂CHCHOO, a special five member ring closure reaction forming dioxole was calculated to have an extremely fast rate coefficient of 9312 s⁻¹ at room temperature.