RADICAL IONS OF A BISHOMOBINAPHTHYLENE CONTAINING 2 1,6-METHANO[10]ANNULENE MOIETIES - AN ESR AND ENDOR STUDY

The radical cation and the radical anion of 'syn'-cyclobuta[1,2-c:3,4-c]di-1,6-methano[10]annulene ('syn'-4a,12a:6a,10a-bishomobinaphthylene; 3) have been characterized by their hyperfine data. The highly resolved ESR spectrum of 3.+ is dominated by a triplet splitting from the outer pair of methano beta-protons (H(o)). In contrast, the ESR spectrum of 3.- is poorly resolved with the largest coupling constants arising from perimeter alpha-protons. The different hyperfine features of 3.+ and 3.- are rationalized by MO models. The SOMO of 3.+ psi(SA)(b1), has substantial LCAO coefficients of the same sign at the bridged atoms C(1), C(6), C(11), and C(16), whereas in the SOMO of 3.-, psi(SS)(a1), the four atoms lie in the vertical nodal planes. The large width and the reluctance to saturation of the lines in the ESR spectrum of 3.- are attributed to the near-degeneracy of the lowest antibonding MO's. Due to their similar nodal properties, the SOMO's of 3.- and the radical anions of binaphthylene (4), 1,6-methano[10]annulene (1), and naphthalene (2) are interrelated, Moreover, because the cyclic pi-systems in 3 and 1 deviate in the same way from planarity, the effect of such distortions on the coupling constants, a(Hmu) of the perimeter alpha-protons in 3.- and 1.- should be comparable. Indeed, on going from 4.- to 3.-, the \a(Hmu)\ values are reduced exactly by half as much as the corresponding values on passing from 2.- to 1.-, of which the cyclic pi-systems are twice contained in 4.- and 3.-, respectively.