Contrasting Free Energies of Electron Transfer from - and Annulenes to Their Perdeuteriated and Per-(13)C Analogues

Article abstract of DOI:10.1021/jo00032a049

A very large equilibrium isotope effect (confirmed via physical, separation oof the isotopes involved) was observed via the EPR analysis of a mixture of benzene and perdeuteriated-per-(13)C-benzene competing for a deficient number of electrons in tetrahyydrofuran (THF) in the presence of 18-crown-6 (18C6).The K_eq for the reaction C6H6^.-, K⁺(18C6) + (13)C6D6 = C6H6 + (13)C6D6^.-, K⁺(18C6) is 0.096 +/- 0.008 at 100 deg C.An analogous competition for electrons exists between <8>annulene and perdeuteriated-per-(13)C-<8>annulene.In contrast to the benzene system the K_eq for the reaction C8H8^.-, Na⁺ + (13)C8D8 = C8H8 + (13)C8D8^.-,Na⁺ in liquid ammonia is 1.20 +/- 0.04 at -100 deg C.Similar contrasting results (but smaller in magnitude) were observed for the benzene and cyclooctatetraene perdeuteriated and per -(13)C systems.The results are interpreted in terms of the divergence of the <6>annulene system from aromatic character upon electron addition and the convergence of the <8>annulene system toward aromatic character upon electron addition.