Benzocyclopropenyl Anion:  A Stable 8π-Electron Species

The conjugate base of benzocyclopropene has been generated in the gas phase. Its reactivity and thermodynamic stability were explored. The measured acidity is ΔH°acid(benzocyclopropene) = 386 ± 3 kcal/mol, and the electron affinity of benzocyclopropenyl radical is 0.51 eV < EA < 1.11 eV. Ab initio calculations satisfactorily reproduce the experimental results and provide additional insights. Benzocyclopropene is found to be 34.5 kcal/mol more acidic than the allylic position of cyclopropene and only 4 ± 3 kcal/mol less acidic than toluene. These findings are explained in terms of the structure and electronic properties of benzocyclopropenyl anion.