Ab Initio Density Functional vs Hartree Fock Predictions for the Structure of [18]Annulene: Evidence for Bond Localization and Diminished Ring Currents in Bicycloannelated [18]Annulenes

摘要

On the basis of steric arguments, Mislow predicted in 1952 that higher annulenes should not be planar and therefore should not enjoy "aromatic" stabilization (Figure 1). This plausible prediction came from weighing the known high energetic costs' fora molecule to form intramolecular contacts at less than van der Waals distances and extended angles against what at that time was a more vague estimation of the aromatic stabilization energy. Indeed, only through clever subversions, such as Vogel's methano[10]- and -[14]annulenes and Boekelheide and Mitchell's dirhethyldihydropyrenes, have we gained insight into the stable members of Huckel's (4n + 2) hydrocarbons with n = 2 and 3. Although the highly reactive [10]- and [14]annulenes could be prepared,it was Sondheimer's synthesis and characterization of the relatively stable [18]annulene (1) that provided unambiguous evidence that "aromatic" character could outweigh steric effects in simple annulenes.