Direct Kinetic Measurements of Criegee Intermediate (CH2OO) Formed by Reaction of CH2I with O2

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Science  13 Jan 2012:
Vol. 335, Issue 6065, pp. 204-207
DOI: 10.1126/science.1213229

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  1. Fig. 1

    Time-resolved mass spectrum at 10.5 eV, acquired during photolysis of CH2I2 in the presence of O2. (Note the changing x-axis scale.) Signal levels before the arrival of the photolysis laser pulse have been subtracted, and only positive signals are shown. The formation of CH2OO is observed at m/z = 46, and CH2I (m/z =141) appears above a background from dissociative ionization of CH2I2. Other primary and secondary products (I, IO, HI, and HOI) are also evident.

  2. Fig. 2

    Photoionization spectrum of the mass = 46 product of the CH2I plus O2 reaction, integrated over the first 2 ms after photolysis. The calculated photoionization spectra of the Criegee intermediate CH2OO and dioxirane (13), the experimental photoionization spectrum for formic acid (17), as well as schematic chemical structures, are shown for reference.

  3. Fig. 3

    Time-dependent CH2OO signal for various concentrations of SO2. Solid lines represent fits to the data traces, including convolution with a measured instrument response function (SOM text), from which pseudo–first-order decay constants are derived.

  4. Fig. 4

    Dependence of the pseudo–first-order Criegee intermediate decay constant (reciprocal of the lifetime τ) on the concentration of SO2. Error bounds are 95% uncertainty estimates based on least-squares analysis of the unweighted fits as depicted in Fig. 3, which include >20 points across the relevant decay. The second-order rate coefficient is given by the slope of the linear fit.

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