The Radical Anion of 2,7-Diazapyrene,a Change in Orbital Sequence on Protonation

ESR.-spectra are reported for the radical anion I · Θ of 2,7-diazapyrene (I), along with those for the radical cations I(2H) · ⊕ and I(2 CH₃) · ⊕ of 2,7-dihydro-2,7-diazapyrene and its 2,7-dimethyl-derivative, respectively. In contrast to the analogous radical ions of 4,4′-bipyridyl (II) and other previously studied diazaaromatic compounds, there is a striking change in the ¹⁴N and proton coupling constants on going from the radical anion I · Θ to the radical cations I(2H) · ⊕ and I(2 CH₃) · ⊕. This change can be rationalized in terms of the HMO model of the pyrene π-system. A reversal in the energy sequence of the lowest antibonding orbitals is predicted upon an increase in the absolute value of the Coulomb integral for the azasubstituted π-centres, such an increase simulating the enhanced electronegativity of the azanitrogen atoms 2 and 7 on protonation.