Towards non-vertical radical cations: Quantum chemical studies on pentafulvene and triafulvene radical cations

摘要

Fulvene radical cations are examined by semiempirical AM1 method and by ab initio calculations at HF/6-31G, MP2/6-31G and B3LYP/6-31G levels. The perpendicular form of fulvene radical cations is predicted to be higher in energy than the planar form. Frequency calculations at the optimized semiempirical and ab initio geometries show that the planar arrangements correspond to stable minima while the perpendicular forms are transition states. Further the exocyclic C-C bond, which is a double bond in neutral fulvene molecule, becomes a partial double bond in the planar form and a single bond in the perpendicular form. These observations are also in agreement with previous experimental results. DFT calculations predict that the perpendicular form of fulvene cation sustains ring current while aromaticity is lost in the 6,6-difluoro derivatives. The AM1 calculations repeated, with the three membered ring system triafulvene, with the same set of substituents, also give the same results.