Criegee intermediate (also called Criegee zwitterions or Criegee biradicals) are a class of carbonyl oxide molecules. These molecules may react with sulfur dioxide and nitrogen oxides in the earth's atmosphere, and are implicated in the formation of aerosols, which are an important factor in controlling global climate. Criegee intermediates are also an important source of OH radicals. OH radicals are the most important oxidant in the troposphere, and are important in controlling air quality and pollution.
The formation of this class of molecules of structure was first postulated in the 1950s by Rudolf Criegee, for whom it is named, but it was not until 2012 that direct detection of such chemicals was reported. Infrared spectroscopy suggests the electronic structure has more zwitterionic than biradical character.
Criegee intermediates are formed by the gas-phase reactions of alkenes and ozone in the earth's atmosphere, according to the scheme below. Ozone adds across the carbon-carbon double bond of the alkene to form a primary ozonide, which then decomposes to produce a carbonyl () and a carbonyl oxide. The latter is known as the Criegee intermediate.
The alkene ozonolysis reaction is extremely exothermic, releasing about 50 kcal/mol of excess energy. Therefore, the Criegee intermediates are formed with a large amount of internal energy.
When Criegee intermediates are formed, some portion of them will undergo prompt unimolecular decay, to produce OH radicals and other products.
Alternatively, Criegee intermediates may be collisionally stabilized via collisions with other molecules in the atmosphere. These stabilized Criegee intermediates may then undergo thermal unimolecular decay to OH radicals and other products, or may undergo bimolecular reactions with other atmospheric species.
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