Bond energy, π-density, molecular geometry, and dipolar resonance of conjugated hydrocarbons. Part V. Fulvalenes (biarylidenes)

Abstract

The LCAO–MO–SCF (π+σ) method has been applied to some fulvalenes (biarylidenes) and their di-ions. The gain in the stabilization energy per C–C bond, defined for the tropylium ring unit in the sesquifulvalene (cyclopentadienylidenecycloheptatriene) derivatives, is linearly related to the loss of π-electrons from this ring. Benzo-annelation increases the stability of these molecules. The ‘4n+ 2’ rule is found to govern the calculated geometries of the neutral molecules and their di-ions. The relative stability of three isomers of 11,12 : 13,14-dibenzo-3,8-methanopentahendecafulvalene (1,6-methanocycloundecapentaenylidenefluorene) are calculated.

A method is developed whereby the dipolar resonance contribution to the ground state of these cross-conjugated hydrocarbons is calculated directly from energy considerations.