The Concerted or Electron Transfer Mechanism

These seemingly easily distinguished mechanisms are considered together as an alternative to the diradical mechanism for reasons which will become apparent. Electron transfer is particularly important in the more electron deficient, cyclic per-ester dioxetanones. Dioxetans in general do not respond to activators -fluorescent compounds of low ionisation potential (see p. 40) almost certainly because they are poorer oxidants than dioxetanones. There is however a hint that simple dioxetans, if sufficiently strained may accept electrons from an activator [36]. The light emitted from the cyclic compound (S) shows a linear dependence on DP A concentration, and the ionisation potential of several fluorescent compounds. However, true electron exchange luminescence cannot be 

In spite of this evidence, there are calculations which show that symmetry forbidden reactions of this sort are likely to involve crossing to triplet surfaces [38]. Another suggestion is that in the cleavage of the 0-0 bond rotation about the C-O bond enhances the spin-orbit coupling which increases the rate of the necessary intersystem crossing from the singlet to triplet state [11]. 

The evidence presented for almost total 0-0 cleavage before the C-C bond breaks seems incontrovertible. If a concerted mechanism is to be considered, it can probably only be done in the context of a very unsymmetrical transition state. The lifetime of the diradical is certainly extremely short, and whether it has a discrete existence in all cases of simple dioxetan cleavage may never be established. Experiments in the gas phase intensify this difficulty. Absorption of infra- 

Although the various calculations referred to should, in principle, give an answer to the problem of radical versus concerted pathways, the several correlation diagrams published [4] provide an effective meeting point for the two mechanisms, even if less rigorous. 

C=carbonyl products S=symmetric excited state A=antisymmetric excited state 

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