Molecular geometry, bond energy and reactivity of conjugated hydrocarbons IV fulvenes

Abstract

The LCAO-MO-SCF-PPP (π + σ) method developed previously a–c is found to depend fundamentally on the valence state sp 2 carbon ionization potential, and this dependence is discussed. The method is then applied to the fulvenes, dibenzofulvenes, and heptafulvenes, and the calculated bond lengths are analysed systematically in terms of substituent effect, π-electron density distribution, dipolar character and molecular stability. The reactivities of the fulvenes are related to the polarity of the exocyclic double bond. Six reactions of the fulvenes are discussed. Reaction sites are predicted using the SCF π-electron densities of reactants and the calculated energies of possible products. Good agreements are obtained. An Analysis of Component Stabilization is developed to estimate the contribution of composite structural units to the total stabilization as measured by the Empirical Resonance Energy. The Heats of Hydrogenation are calculated in three cross-conjugated fulvenes. Annelation in these 5- and 7-membered ring compounds is found to result in the loss of fulvenic character.