Nonstatistical Unimolecular Decay of the Singly Fluorinated Criegee Intermediate syn-FCHOO

The fluorinated Criegee intermediate, FCHOO, is produced from the ozonolysis of hydrofluoroolefins (HFOs) which are next-generation refrigerants. Unimolecular decay of syn-FCHOO proceeds via a 1,3 ring closure pathway to fluorodioxirane and subsequent rearrangement and/or dissociation to many products including hydroxyl (OH) radicals that are detected experimentally. Vibrational activation of jet-cooled syn-FCHOO with two quanta of CH stretch (17.70 kcal mol^-1) leads to unimolecular decay via quantum mechanical tunneling at an energy slightly below the transition state barrier of 18.03 ± 0.25 kcal mol^-1, determined utilizing the high accuracy composite HEAT-345(Q)_Λ method. The observed unimolecular decay rate of (5.4 ± 1.6) × 10⁷ s^-1 is an order of magnitude slower than that predicted by statistical unimolecular reaction theory with 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.