Radical Ions of a Bishomobinaphthylene Containing two 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 \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{+ \atop \dot{}} $\end{document} is dominated by a triplet splitting from the outer pair of methano β‐protons (H o ). In contrast, the ESR spectrum of \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{- \atop \dot{}} $\end{document} is poorly resolved with the largest coupling constants arising from perimeter α‐protons. The different hyperfine features of \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{+ \atop \dot{}} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{- \atop \dot{}} $\end{document} are rationalized by MO models. The SOMO of \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{+ \atop \dot{}} $\end{document} ψ SA (b 1 ), has substantial LCAO coefficients of the same sign at the bridged atoms C(1), C(6), C(11), and C(16), whereas in the SOMO of \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{- \atop \dot{}} $\end{document} , ψ SS (a 1 ), the four atoms lie in the vertical nodal planes. The large width and the reluctance to saturation of the lines in the ESR spectrum of \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{- \atop \dot{}} $\end{document} are attributed to the near‐degeneracy of the lowest antibonding MO's. Due to their similar nodal properties, the SOMO's of \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{- \atop \dot{}} $\end{document} and the radical anions of binaphthylene ( 4 ), 1,6‐methano[10]annulene ( 1 ), and naphthalene ( 2 ) are interrelated. Moreover, because the cyclic π‐systems in 3 and 1 deviate in the same way from planarity, the effect of such distortions on the coupling constants, a Hμ , of the perimeter α‐protons in \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{- \atop \dot{}} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ 1^{- \atop \dot{}} $\end{document} should be comparable. Indeed, on going from \documentclass{article}\pagestyle{empty}\begin{document}$ 4^{- \atop \dot{}} $\end{document} to \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{- \atop \dot{}} $\end{document} , the |a Hμ | values are reduced exactaly by half as much as the corresponding values on passing from \documentclass{article}\pagestyle{empty}\begin{document}$ 2^{- \atop \dot{}} $\end{document} to \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{- \atop \dot{}} $\end{document} , of which the cyclic π‐systems are twice contained in \documentclass{article}\pagestyle{empty}\begin{document}$ 4^{- \atop \dot{}} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ 3^{- \atop \dot{}} $\end{document} respectively.