Tunneling-Dominated Unimolecular Decay of syn-Methyl Vinyl Ketone Oxide to Hydroperoxide: A Dual-Level VTST/MT Study

We present dual-level variational transition state theory with multidimensional tunneling (VTST/MT) calculations to characterize the unimolecular decay of syn-methyl vinyl ketone oxide (syn-MVKO) to 2-hydroperoxybuta-1,3-diene (HPBD) over the 150-600 K temperature range. High-level energetics were computed using CCSD-(T)/aug-cc-pVTZ, while the reaction path information was obtained at the M06-2X/6-31+G-(d,p) level. The decay dynamics are strongly influenced by quantum mechanical tunneling, which increases the rate constants by approximately 2 orders of magnitude at 300 K and by more than 6 orders of magnitude at 200 K relative to conventional transition state theory. The resulting thermal rate constants for syn-MVKO reproduce the magnitude and temperature dependence of the available experimental rate constants, highlighting the importance of multidimensional tunneling treatments for quantitatively meaningful kinetic predictions across a wide temperature range. Furthermore, deuterium substitution results in exceptionally large kinetic isotope effects (KIE ≥ 50 at 300 K and increasing rapidly at lower temperatures), arising from hydrogen-atom transfer along a strongly curved reaction path. These results demonstrate that multidimensional tunneling is essential for accurately describing the unimolecular decay kinetics of syn-MVKO over a broad temperature range and provide quantitative benchmarks for future experimental and theoretical studies of Criegee intermediate kinetics.