7t-bonding delocalized

In attempting to approximate (CNDO/2) the nature of the 1,2,5-thiadiazole ring (2a vs. 2b), a nontraditional view has been proposed <83IJC(B)802> with two C=N bonds and one three-center two-electron 7t-bond delocalized over the N—S—N system. This would account for the electron-rich N—S—N nature of the molecule. Classical structures (2a, 2b) are viewed as unlikely versus a hybrid structure (2c). 

Heptalene, (16) a 12ic system, was reduced to the dianion by Mullen and Vogel94). The resulting (4n + 2)7c-system, i.e., 162 is obtained from 16 by lithium metal reduction. The radical anion of heptalene appears from its ESR spectroscopic data, to have a 7t-bond-delocalized ground state 95). The number of experimental ESR coupling constants fits a C2y or a D2h symmetry 95>. 

A particularly attractive group of substrates are the annulenes, since electron transfer allows an interconversion of (4n + 2)7t- and (4n)rr-systems, and thus a switch between 7t-bond delocalization and n-bond localization (Mullen, 1984). 

Monosubstituted benzothiadiazinone tautomerism was also investigated. The HF/6-31G level calculations showed a clear preference for the keto tautomers 31 and 33 in the N-1- and N-3-monosubstituted compounds over the respective hydroxy tautomers 32 and 34. The 0-alkyl derivative tautomer 35 is preferred over 36 because of better 7t-bond delocalization in the fused benzene ring <1999T12405>. 

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