Radical Anion of Azuleno[5.6.7-cd]phenalene, a Failure of the Simple HMO Model

Hyperfine proton coupling constants are reported for the radical anions of azuleno‐[5.6.7‐cd]phenalene (I), 6‐phenylazulene (II) and the corresponding 1,3‐dideuterio‐derivatives (I‐d2 and II‐d2). The singly occupied orbitals of both I⊖ and II⊖ are found to be symmetric with respect to the mirror plane perpendicular to the plane of the molecule. In the case of I, surprisingly, such an orbital corresponds not to the first, but to the second lowest antibonding HMO. A correlation diagram for the relevant orbitals of I and II indicates that the correct energy order in the HMO model of I can be achieved by a decrease in the absolute value of the parameter βμν for the bonds 4a–5, 6–6a, 9a–10 and 11–11a.