Extent of cyclic pi-electron delocalization modification in exocyclically substituted fulvenes.

Fulvene derivatives, 20 mono- and 14 di-exocyclically substituted, were optimized at the B3LYP/6-311+G level of theory. Aromaticity indices include aromatic stabilization energy (ASE), Schleyer's Nucleus Independent Chemical Shift (NICS), (3)He chemical shifts, anisotropy and exaltation of magnetic susceptibility, and the Harmonic Oscillator Model of Aromaticity (HOMA), which is a geometry-based descriptor. These indices were used to estimate the extent of a cyclic pi-electron delocalization due to the substituent effect. A dramatic variation of these indices was found, indicating great sensitivity of the pi-electron structure of the ring. Except for anisotropy of magnetic susceptibility, all other indices exhibited perfect equivalence.