The role of quantum-mechanical interference and quasi-classical effects in conjugated hydrocarbons

The nature of the chemical bond in conjugated hydrocarbons is analyzed through the generalized product function energy partitioning (GPF-EP) method, which allows the calculation of the quantum-mechanical interference and quasi-classical contributions to the energy. The method is applied to investigate the differences between the chemical bonding in conjugated and non-conjugated hydrocarbon isomers and to evaluate the contribution from the energy components to the stabilization of the molecules. It is shown that in all cases quantum-mechanical interference has the effect of concentrating π electron density between the two carbon atoms directly involved in the (C–C)π bonds. For the conjugated isomers, this effect is accompanied by a substantial reduction of electron density in the π space of the neighbouring (C–C)σ bond. On the other hand, quasi-classical effects are shown to be responsible for the extra stabilization of the conjugated isomers, in which a decrease of the π space kinetic reference energy seems to play an important role. Finally, it is shown that the polarization of p-like orbitals in compounds with alternating single and double bonds ultimately increases electron density in the π space of the neighbouring (C–C)σ bond. Therefore, quasi-classical effects, rather than covalent ones, seem to be responsible for several properties of conjugated molecules, such as thermodynamic stability, planarity and (C–C)σ bond shortening. The shortcomings of the delocalization concept are discussed.