Higher sulfur oxides

Higher sulfur oxides are a group of chemical compounds with the formula SO_3+x where x lies between 0 and 1. They contain peroxo (O-O) groups and the oxidation state of sulfur is +6 as in SO₃.
Monomeric SO₄ can be isolated at low temperatures(below 78°K) following the reaction of SO₃ and atomic oxygen or photolysis of SO₃ /ozone mixtures. The favoured structure is :

Additional recommended knowledge

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Colourless polymeric condensates are formed in the reaction of gaseous SO₃ or SO₂ with O₂ in a silent electric discharge. The structure of the polymers is based on β-SO₃ ( one of the three forms of solid SO₃) with oxide bridges (−O−) replaced randomly by peroxide bridges(−O−O−). As such these compounds are non stoichiometric.

Footnotes

Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.