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) cm(3) 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) cm(3) 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) cm(3) molecule(-1) s(-1).
Highlights
IntroductionIn 1949 Rudolph Criegee[1] proposed that an intermediate (later to be called a Criegee intermediate) was formed during the ozonolysis of alkenes
In 1949 Rudolph Criegee[1] proposed that an intermediate was formed during the ozonolysis of alkenes
In the low SO2 pressure regime, our model indicates that the pseudo first order rate coefficient should be the sum of contributions from bimolecular reaction and catalysed isomerization/intersystem crossing (ISC) by SO2, justification for which is provided in the Electronic supplementary information (ESI).† A value of (3.53 Æ 0.32) Â 10À11 cm[3] moleculeÀ1 sÀ1 was obtained for the catalysed isomerization/ISC rate coefficient by subtraction and propagation of errors of the slope values obtained from Fit 1 and Fit 2
Summary
In 1949 Rudolph Criegee[1] proposed that an intermediate (later to be called a Criegee intermediate) was formed during the ozonolysis of alkenes. Recent work has shown that the rate coefficient for the selfreaction of CH2OO (reaction (8)) is very large.[35] This reaction has no atmospheric relevance, it could be important in laboratory studies that probe the kinetics and mechanisms of alkene ozonolysis.[36] In this paper we report measurements of k4, k6 and k8 at room temperature over a range of pressure, using near UV cavity ring-down spectroscopy (CRDS) to detect CH2OO. Typical initial CH2OO concentrations of 2.5–5.0 Â 1012 molecule cmÀ3 were generated Under such conditions, the self-reaction can contribute. Assuming the fast self-reaction to be the dominant loss mechanism, the decay traces of CH2OO were fitted to an integrated second order decay expression Further justification for this fitting procedure is provided later. Characterization of the overlap length is presented in the ESI.† Combining eqn (15) and (16) gives
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