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Infrared-driven unimolecular reaction of CH3CHOO Criegee intermediates to OH radical products

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Science  26 Sep 2014:
Vol. 345, Issue 6204, pp. 1596-1598
DOI: 10.1126/science.1257158

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Breaking down a Criegee intermediate

Ozone's damaging role in the upper atmosphere is well known, but ozone is also quite active closer down to where we live. In particular, ozone's run-ins with airborne unsaturated hydrocarbons, from natural or anthropogenic sources, produce even more-reactive OH radicals. Liu et al. used vibrational spectroscopy to study how OH emerges from a so-called Criegee intermediate formed when ozone attacks 2-butene. The results suggest that OH production is easier than current theory predicts.

Science, this issue p. 1596

Abstract

Ozonolysis of alkenes, an important nonphotolytic source of hydroxyl (OH) radicals in the troposphere, proceeds through energized Criegee intermediates that undergo unimolecular decay to produce OH radicals. Here, we used infrared (IR) activation of cold CH3CHOO Criegee intermediates to drive hydrogen transfer from the methyl group to the terminal oxygen, followed by dissociation to OH radicals. State-selective excitation of CH3CHOO in the CH stretch overtone region combined with sensitive OH detection revealed the IR spectrum of CH3CHOO, effective barrier height for the critical hydrogen transfer step, and rapid decay dynamics to OH products. Complementary theory provides insights on the IR overtone spectrum, as well as vibrational excitations, structural changes, and energy required to move from the minimum-energy configuration of CH3CHOO to the transition state for the hydrogen transfer reaction.

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