Potential role of stabilized criegee intermediates in the reactivity of isocyanic acid

Isocyanic acid (HNCO) is a toxic atmospheric constituent emitted by biomass burning and catalytic converters in car engines. The major sinks for HNCO were thought to be heterogeneous loss processes and dry deposition in the free troposphere. Based on the rigorous electronic structure calculations and kinetic simulations, here we show that HNCO is highly reactive to the stabilized Criegee intermediates (sCIs, e.g., CH₂OO and syn-CH₃CHOO) in the atmosphere. The energetically most preferable reaction route refers to a concerted mechanism by which the H atom is transferred from N of HNCO to the terminal O site of sCIs with simultaneous addition of N or O in HNCO to the other CH₂ (resp. CHCH₃) end, leading to the highly exothermic HOOCH₂NCO and HOOCH₂OCN (resp. HOOCH(CH₃)NCO and HOOCH(CH₃)OCN). The precursor complexes are stabilized via H-bond/p-π interactions and the barriers are submerged below reactants. The reaction of HNCO with CH₂OO occurs with an average rate coefficient of 8×10⁻¹³ cm³molecule⁻¹s⁻¹ at 275 K and 760 Torr, which is a factor of 10³ faster than the HNCO + OH reaction. The total rate coefficients exhibit negative temperature dependence under the tropospheric conditions. sCIs might be one of the potential sinks for the budget of HNCO in the atmosphere.