New interpretation of the valence tautomerism of 1,6-methano[10]annulenes and its application to fullerene derivatives

Relaxed potential energy surface scans of 1,6-methano[10]annulene and its derivatives have been calculated using Hartree-Fock (HF), density functional (DFT), and second-order Møller-Plesset perturbation theory (MP2) in the pursuit of nonclassical structures corresponding to an extremely long C₁-C₆ bond (∼1.8 Å). Substituents affect the relative stabilities between the aromatic and the bisnorcaradine forms, but their effects on the minima positions are moderate. We attribute the absence of a static C-C bond length in the vicinity of 1.8 Å to the well-defined inherent minima of cyclopropane in its singlet state (∼1.5 Å) and trimethylene in its triplet state (∼2.6 Å), which serve as the basis of our fragment analysis. The 1,6 bonding in these molecules is associated with a shallow potential well, in agreement with the Woodward-Hoffmann rules rather than to the existence of a static intermediate structure with a very long C-C bond. These results are in qualitative agreement with recent temperature-dependent ¹³C NMR experiments of Dorn et al. The trends in the C₁-C₆ bridgehead bonding are well-reproduced by the correlated calculations and to a lesser extent even by the HF calculations. However, simple electron donation/withdrawal arguments fail for this series. Due to the similarity in the delocalization energy of 1,6-methano[10]annulenes and disubstituted methanobuckyballs predictions are made concerning the structural preferences for some disubstituted methanofullerenes and -fulleroids. In contrast to general expectations open [6,6] methanofullerenes may exist. Major differences between methano[10]annulenes and methanofullerenes are derived from the constraints concerning the bookfolding angle that are strong in the latter and moderate in the former.