Radical Ions of a Bishomobinaphthylene Containing two 1,6‐methano[10]annulene moieties: An ESR and ENDOR study

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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 {article}{empty}{document}3+ ̇{document} is dominated by a triplet splitting from the outer pair of methano β‐protons (Ho). In contrast, the ESR spectrum of {article}{empty}{document}3- ̇{document} is poorly resolved with the largest coupling constants arising from perimeter α‐protons. The different hyperfine features of {article}{empty}{document}3+ ̇{document} and {article}{empty}{document}3- ̇{document} are rationalized by MO models. The SOMO of {article}{empty}{document}3+ ̇{document} ψSA(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 {article}{empty}{document}3- ̇{document} , ψSS(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 {article}{empty}{document}3- ̇{document} are attributed to the near‐degeneracy of the lowest antibonding MO's. Due to their similar nodal properties, the SOMO's of {article}{empty}{document}3- ̇{document} and the radical anions of binaphthylene (4), 1,6‐methano[10]annulene (1), and naphthalene (2) are interrelated. Moreover, because the cyclic π‐systems in 3 and 1 deviate in the same way from planarity, the effect of such distortions on the coupling constants, aHμ, of the perimeter α‐protons in {article}{empty}{document}3- ̇{document} and {article}{empty}{document}1- ̇{document} should be comparable. Indeed, on going from {article}{empty}{document}4- ̇{document} to {article}{empty}{document}3- ̇{document} , the |aHμ| values are reduced exactaly by half as much as the corresponding values on passing from {article}{empty}{document}2- ̇{document} to {article}{empty}{document}3- ̇{document} , of which the cyclic π‐systems are twice contained in {article}{empty}{document}4- ̇{document} and {article}{empty}{document}3- ̇{document} respectively.