Nonstatistical Unimolecular Decay of the CH₂OO Criegee Intermediate in the Tunneling Regime

Unimolecular decay of the formaldehyde oxide (CH₂OO) Criegee intermediate proceeds via a 1,3 ring-closure pathway to dioxirane and subsequent rearrangement and/or dissociation to many products including hydroxyl (OH) radicals that are detected. Vibrational activation of jet-cooled CH₂OO with two quanta of CH stretch (17-18 kcal mol^-1) leads to unimolecular decay at an energy significantly below the transition state barrier of 19.46 ± 0.25 kcal mol^-1, refined utilizing a high-level electronic structure method HEAT-345(Q)_Λ. The observed unimolecular decay rate of 1.6 ± 0.4 × 10⁶ s^-1 is 2 orders of magnitude slower than that predicted by statistical unimolecular reaction theory using several different models for quantum mechanical tunneling. The nonstatistical behavior originates from excitation of a CH stretch vibration that is orthogonal to the heavy atom motions along the reaction coordinate and slow intramolecular vibrational energy redistribution due to the sparse density of states.