Chemiluminescence biradical mechanism

Unimolecular peroxide decomposition chemiluminescence, 1227-31 asynchronous concerted mechanism, 1230 biradical mechanism, 1181-2, 1227-31 concerted mechanism, 1227, 1228-9, 1230... 

The degradation process has a free radical mechanism. It is initiated by free radicals P that appear due to, for example, hydroperoxide decomposition induced thermally or by trace amounts of metal ions present in the polysaccharide. One cannot exclude even direct interaction of the polysaccharide with oxygen in its ground triplet state with biradical character. Hydroperoxidic and/or peracid moieties are easily formed by oxidation of semiacetal chain end groups. The sequence of reactions on carbon 6 of polysaccharide structural unit that ultimately may lead to chemiluminescence is shown in Scheme 11. 

Figure 8 Chemiluminescent mechanism for 1,2-dioxetanes (A) a concerted decomposition process (B) a two-step biradical process.

Fig. 32. Possible mechanisms for the thermal decomposition of dioxetanes [adapted from Hummelen et al. (H21), with permission). In (a), a concerted bond cleavage leads directly to two carbonyl products, one of which is in the excited state and emits light (M15, M16). The substituents R1-R4 can be simple alkyl or alicyclic groups. In (b), homolytic bond cleavage leads to a biradical that exists as an equilibrium mixture of singlet-state (f ) and triplet-state (t t) forms. As before, chemiluminescence emission probably occurs via the excited singlet-state carbonyl product arising from the homolytic bond cleavage of the intermediate biradical (R5, R6). The substituents R and Ar can include uncharged alkyl, alicyclic, and aromatic groups.

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