Kinetics of the Reaction Between the Criegee Intermediate CH2OO and NO2: Experimental Measurements and Comparison with Theory.

Abstract

Kinetics of the gas phase reaction between the stabilized Criegee intermediate formaldehyde oxide (CH₂OO) and nitrogen dioxide (NO₂) have been measured using laser flash photolysis of CH₂I₂/O₂/N₂/NO₂ mixtures coupled with time-resolved broadband ultraviolet absorption spectroscopy. Experiments were performed in N₂ under pseudo-first-order conditions at temperatures between 242 and 353 K and pressures in the range 25 to 300 Torr. The kinetics of CH₂OO + NO₂ are independent of pressure, with a mean rate coefficient of k₁ = (1.24 ± 0.22) × 10^-12 cm³ s^-1 at 298 K, where the uncertainty represents a combination of the 1σ statistical error and the systematic errors resulting from uncertainties in gas flow rates and in the concentration of NO₂. Measurements indicate upper limits of <5% for production of NO₃ and <5% for production of NO, and further studies of product yields are warranted. In contrast to expectations from theory, the kinetics of CH₂OO + NO₂ display a negative temperature dependence that can be described by k₁ = (1.07 ± 0.02) × 10^-12 × (T/298)^-(2.9±0.2) cm³ s^-1. Analysis using the Master Equation Solver for Multi-Energy well Reactions is able to reproduce a negative temperature dependence for the reaction if significant changes to barrier heights are made, but the overall agreement between the experiment and theory remains poor. This work highlights the challenges associated with calculations for systems with significant multi-reference character.