Effects of water vapor on the reaction of CH2OO with NH3

Abstract

The reaction of the simplest Criegee intermediate, CH₂OO, with ammonia and water vapor has been investigated at 278–308 K and under 100–760 Torr by monitoring the strong UV absorption of CH₂OO. We found that the observed decay rate of CH₂OO becomes much larger when ammonia and water vapor are both present; the combinational effect of ammonia and water vapor is significantly greater than the sum of their individual contributions, revealing a strong synergic effect. The kinetic data are consistent with a termolecular process of CH₂OO + NH₃ + H₂O reaction, of which the reaction rate coefficient was determined to be k_NH3+H2O = (8.2 ± 1.2) × 10⁻³¹ cm⁶ s⁻¹ at 298 K with a negative activation energy, E_a = −8.0 ± 0.8 kcal mol⁻¹ [k_NH3+H2O(T) = 1.04 × 10⁻³⁶ exp(4047/T)]. Quantum chemistry calculation (at the QCISD(T)/aug-cc-pVTZ//B3LYP/6-311+G(2d,2p) level) found a low-energy reaction pathway, on which water accepts a hydrogen atom (or proton) from ammonia and releases another hydrogen atom to the terminal oxygen of CH₂OO. The predicted products are H₂NCH₂OOH and a new H₂O molecule, indicating water catalysis. This reaction is very fast and probably barrierless, which poses a theoretical challenge to modeling the related kinetics.