Dynamics of the reaction CH2I + O2 probed via infrared emission of CO, CO2, OH and H2CO

Abstract

The reaction CH₂I + O₂ has been widely employed recently for the production of the simplest Criegee intermediate CH₂OO in laboratories, but the detailed dynamics of this reaction have been little explored. Infrared emission of several products of this reaction, initiated on irradiation of CH₂I₂ and O₂ (∼8 Torr) in a flowing mixture at 308 or 248 nm, was recorded with a step-scan Fourier-transform spectrometer; possible routes of formation were identified according to the observed vibrational distribution of products and published theoretical potential-energy schemes. Upon irradiation at 308 nm, Boltzmann distributions of CO (v ≤ 5, J ≤ 19) with an average vibrational energy of 32 ± 3 kJ mol⁻¹ and OH (v ≤ 3, J ≤ 5.5) with an average vibrational energy of 29 ± 4 kJ mol⁻¹ were observed and assigned to the decomposition of HCOOH* to form CO + H₂O and OH + HCO, respectively. The broadband emission of CO₂ was simulated with two vibrational distributions of average energies (91 ± 4) and (147 ± 8) kJ mol⁻¹ and assigned to be produced from the decomposition of HCOOH* and methylene bis(oxy), respectively. Upon irradiation of samples at 248 nm, the emission of OH and CO₂ showed similar distributions with slightly greater energies, but the distribution of CO (v ≤ 11, J ≤ 19) became bimodal with average vibrational energies of (23 ± 4) and (107 ± 29) kJ mol⁻¹, and branching (56 ± 5) : (44 ± 5). The additional large-v component is assigned to be produced from a secondary reaction HCO + O₂ to form CO + HO₂; HCO is a coproduct of OH. The branching between CO and OH is (50 ± 5) : (50 ± 5) at 308 nm and (64 ± 5) : (36 ± 4) at 248 nm, consistent with the mechanism according to which an additional channel to produce CO opens at 248 nm. Highly internally excited H₂CO was also observed. With O₂ at 16 Torr, the extrapolated nascent internal distributions are similar to those with O₂ at 8 Torr except for a slight quenching effect.