Electron delocalisation and stabilisation in heptatrienyl and polyenyl radicals

Heptatrienyl radicals were generated by hydrogen abstraction from hepta-1,3,6-triene with t-butoxyl radicals and observed by e.s.r. spectroscopy. The hyperfine splittings of heptatrienyl and other polyenyl radicals indicate that spin density is not uniformly distributed along the chain but more concentrated around the central region of the structure. Semi-empirical SCF MO calculations agree with this result and also show that most radical character is associated with the central region. The stabilisation energies (SE) of polyenyl radicals increase with chain length and are related to the hydrogen hyperfine splittings as in equation (i) where ∂(H1) is the average of the anti- and syn-hydrogen hyperfine splittings (G) of SEESA/kJ mol 1 =375 ±4 - (272 ±10)log10[a(H 1)/G] (i) the terminal methylene groups.