Quantum Chemical Interpretation of the Photoinitiated Autoxidation of Sulphite Catalysed by Ferric Ions

Autoxidation of sulphite is the first well known chain reaction proceeding through free radicals in solutions. Dark and photochemical autoxidation of sulphite were stated to take place by the same mechanism except in the initial steps. It was found that the formation of sulphite radicals in photoinitiated autoxidation of sulphite which is a metal catalysed reaction as well as the classical thermal reaction is due to the absorption of photons by ferrisulphite complexes rather than sulphite itself. Confirmation of this finding was achieved spectrophotometrically as well as by the fact that such complexes are specified by very high stability constants. The present work is a further evidence for the presence of such complexes and to cover the gap present before in the literature data. Quantum mechanical calculations using the extended Hückel molecular orbital method has proved that the range of absorption by sulphite lies in a region at which sulphite alone does absorb at all, and the absorption process is thus due to the formation of ferrisulphite complexes.