Radical Ions in the Pentalene Series [1]. Part II. Dibenzo [b,f]pentalene and its 5, 10-dimethyl derivative

Proton hyperfine data have been determined for the radical anions and cations of dibenzo b,f]pentalene (III) and its 5, 10-dimethyl derivative (IV) . The assignment of the coupling constants to pairs of equivalent protons follows from a simple MO model, the use of which enables one to reproduce satisfactorily the experimental values. The proton hyperfine data, a_Hμ

1 The meaning of ^aH_μis ^aH? C(x),H? C(y),whereas only x and y are given in the particular cases.

, for the radical anion III?. correlate fairly well with the π-charge populations ?_μ derived from ¹H-NMR. spectra for the carbon centres μ in the dianion III^2?. The analogous correlation is less good in the case of the radical cation III⊕. and the dication III^2⊕., presumably due to the rough approximations involved in the evaluation of the numbers ?_μ for the latter species. The coupling constants a_H5,10 for III?. and III⊕. are very close to the corresponding values a_H4,6 for the radical ions of 1,3,5-tri-t-butylpentalene (II), in accord with the prediction of the MO model. A similarity is also found between the proton hyperfine data for III?. and those for the radical anions of structurally related 1,4-diphenyl-1,3-butadiene (V). On the other hand, there are striking changes in the coupling constants of the analogous protons on passing from III?. to the radical anions of dibenzo a,e] cyclooctene (VI) and 16] annulene (VII), as a consequence of raising the symmetry from C_2h to D_2h and D_4h, respectively.