Fluorination effect on the structural properties of selected benzocyclopropenes

Abstract

Structures and electronic aspects of mono- and poly-annelated benzocyclopropenes and their gem-disubstituted fluorine derivatives are examined theoretically at the HF/6–31G* and MP2/6–31G* levels of theory. The computed geometries of benzocyclopropene and gem-difluorinated benzocyclopropene are in good qualitative agreement with experimental information. It appears that geminal difluorination leads to a reversed or anti-Mills–Nixon (MN) distortion of the aromatic nucleus (i.e. lengthening of the adjacent or ortho bonds), in contrast to the pure hydrocarbon systems, where a regular MN bond fixation pattern occurs. The tris-annelated perfluoro-benzocyclopropene shows the most pronounced anti-MN effect with ortho bonds 0.022 Å longer than in free benzene. This finding is of interest since anti-MN systems are more the exception than the rule. The origin of the MN and anti-MN distortions are interpreted in terms of rehybridization, partial π-electron localization, hyperconjugative interaction with the CF₂-groups and intramolecular charge transfer caused by electronegativity differences. Energetic consequences of fusion of small rings are briefly discussed employing homodesmic reactions.