A kinetic study of the CH2OO Criegee intermediate self-reaction, reaction with SO2 and unimolecular reaction using cavity ring-down spectroscopy

Abstract

Criegee intermediates are important species formed during the ozonolysis of alkenes. Reaction of stabilized Criegee intermediates with various species like SO₂ and NO₂ may contribute significantly to tropospheric chemistry. In the laboratory, self-reaction can be an important loss pathway for Criegee intermediates and thus needs to be characterized to obtain accurate bimolecular reaction rate coefficients. Cavity ring-down spectroscopy was used to perform kinetic measurements for various reactions of CH₂OO at 293 K and under low pressure (7 to 30 Torr) conditions. For the reaction CH₂OO + CH₂OO (8), a rate coefficient k₈ = (7.35 ± 0.63) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ was derived from the measured CH₂OO decay rates, using an absorption cross section value reported previously. A rate coefficient of k₄ = (3.80 ± 0.04) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ was obtained for the CH₂OO + SO₂ (4) reaction. An upper limit for the unimolecular CH₂OO loss rate coefficient of 11.6 ± 8.0 s⁻¹ was deduced from studies of reaction (4). SO₂ catalysed CH₂OO isomerization or intersystem crossing is proposed to occur with a rate coefficient of (3.53 ± 0.32) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹.