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 , for the radical anion III⊖. correlate fairly well with the π‐charge populations ϵ_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 ϵ_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.