The photochemistry of the rhodizonate dianion in aqueous solution

Abstract

The thermal and photochemical reactions of rhodizonate dianion have been studied in aqueous solution in the presence of various oxidizing agents. Both reactions are initiated by electron transfer to an acceptor which is a sufficiently strong oxidizing agent. With hydrogen peroxide and ferricyanide a square root dependence of the rate on the concentration of additive was observed whereas with tetracyanoethylene the rate was first order with respect to additive. This difference in behaviour is explained on the basis of the rate of separation of ions from the initial charge transfer complex. In most cases the main product was croconic acid. NMR analysis of other products indicated some formation of C—H bonds and possibly incorporation of an oxygen atom into the ring. A maximum quantum yield of 0.04 for disappearance of rhodizonate was observed in the presence of tetracyanoethylene. Key words: photochemistry, rhodizonic acid, dianion, electron transfer.