OOO!

In an organic molecule, how long can a chain of oxygen atoms be? No chain-like polyoxides are known that have more than three oxygen atoms in a row; only a few trioxides are known. But to this collection, Holger Pernice et al. now add bis(fluoroformyl)trioxide, or FC(O)OOOC(O)F (Angew. Chem. Int. Edn 43, 2843–2846; 2004).

Bis(fluoroformyl)trioxide is an acyl compound, characterized by a C=O acid-group derivative. It is the second known example of a chain-like acyl trioxide (after the more complex CF₃OC(O)OOOC(O)OCF₃) — but the first to have its twisting, chiral structure determined.

Starting from a potentially explosive mixture of carbon monoxide, fluorine and oxygen, the reaction proceeds to produce the acyl trioxide and the peroxide FC(O)OOC(O)F. The spectrum of infrared radiation absorbed by the sample has bands at around 800 and 900 cm⁻¹ that are typical of O–O–O stretching vibrations. Similarly, data from ultraviolet, nuclear magnetic resonance and X-ray diffraction studies confirm the formation of the trioxide.

For the determination of its structure, Pernice et al. allowed a trioxide sample to crystallize at a temperature of −78 °C in a stream of nitrogen gas. Diffraction data show that the molecule has a trans-syn-syn structure — that is, the C–O bonds are in the trans (opposite) conformation with respect to the plane of the three bonded oxygen atoms, and the two C=O bonds are in the syn (same-side) conformation. The left-handed version of this structure is shown here (oxygen in red, carbon in blue, fluorine in green).

Theory predicts that another structure, or ‘rotamer’, should exist: trans-syn-anti, in which, instead of curling round, the molecule twists back on itself about the central oxygen atom. Pernice et al. say that their infrared data indeed suggest the presence of a second rotamer.