Charge-separated diradical

C-P-Q is converted to the charge separated diradical state C h-P -Q-. Then an electron is transferred from Q to the lipid-soluble 2,5-diphenylbenzoquinone Qs 133b yielding Qs. In the third step uncharged semiquinone QsH is formed when the latter accepts a proton from the external aqueous solution. The basic function of a proton shuttle is carried out when the semiquinone radical diffuses through the membrane in the fourth step. It is then oxidized to Qs + by the carotenoid radical cation... 

Electronically excited phthalimides can act as good electron acceptors and carboxylic acids are documented to serve as electron donors in photoinduced SET reactions. It is likely then, that in the excited state, an electron transfer process between the phthalimide system and the carboxylic acid would occur, leading to charge-separated diradical 6. Proton transfer from the carboxylic acid function (iT" is an electrofugal group) would form a carboxy radical 7, which could undergo rapid decarboxylation to azomethyne ylide 8. This reactive species is transformed into the final decarboxylated product 3 by intramolecular proton transfer (Scheme 16.4). 

To justify the experimental result we could follow the stepwise mechanism discussed in the previous section. The phthalimide system is a good electron acceptor in the excited state and the alkylsilane can be an electron donor, so a photoinduced electron transfer process could lead to the charged separated diradical 13. Transfer of the electrofugal TMS group to the oxyanion carbonyl oxygen would form azo-methyne ylide 14 that can be trapped by the mefliyl acrylate to yield the endo-reaction product 12 (Scheme 16.9). 

The mechanism of the reaction has generally been discussed in terms of a thermally allowed concerted 1,3-dipoIar cycloaddition process, in which control is realized by interaction between the highest occupied molecular orbital (HOMO) of the dipole (diazoalkane) and the lowest unoccupied molecular orbital (LUMO) of the dipolarophile (alkyne).76 In some cases unequal bond formation has been indicated in the transition state, giving a degree of charge separation. Compelling evidence has also been presented for a two-step diradical mechanism for the cycloaddition77 but this issue has yet to be resolved. 

No charge and spin separations appear in the case of AN because of closed-shell character, but spin separation (SS) ( 1) is significant for FR case, although the charge separation (CS) ( ) is rather weak, as in the case of homolytic diradical. On the other hand, the zwitterionic ( ) CS is remarkable for FN case, and both SS and CS ( +, 1) become important for AR case such as electron-transfer diradical reactions. In recent papers, we have performed the symmetry-stability analyses of chemi- and bio-luminescence reactions from these theoretical view points.4,5... 

Sodamide also reacts with bromoarenes by elimination of HBr (Figure 10.37) to give benzyne, a very reactive and unstable intermediate. Mechanistically, the elimination may be E2 or ElcB or somewhere in between. Although it is common, and quite conventional, to write the structure of benzyne with a triple bond, this is not a triple bond of the same type as we find in alkynes. The extra bond is formed by weak overlap of two sp orbitals, each with one electron in it (10.20). It is probably more correct to think of it as a resonance hybrid of the triply bonded form, diradicals, and charge-separated species. The IR spectrum of isolated benzyne in an inert gas matrix at 8 K has been studied although assignments have been a matter of discussion, the value of 1846 cm for the C-C triple bond stretch is at least approximately correct. This suggests that the triple bond is considerably weaker than that in a conventional alkyne where a frequency of 2200 cm would be typical. 

Definition A distonic ion is a positive radical ion, which would formally arise by ionization of a zwitterion or a diradical, by isomerization or fragmentation of a classical molecular ion, or by ion-molecule reactions. Consequently, distonic ions have charge and radical at separate atoms in a conventional valence bond description. [42,43]... 

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