title: The role of quantum-mechanical interference and quasi-classical effects in conjugated hydrocarbons.
abstract: 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.
authors.affiliation: Instituto de Química, Universidade Federal do Rio de Janeiro, Cidade Universitária, CT, Bloco A, sala 412. Rio de Janeiro, RJ, 21949-900, Brazil.
authors.forename: Felipe
authors.initials: F
authors.lastname: Fantuzzi
authors.forename: Thiago Messias
authors.initials: TM
authors.lastname: Cardozo
authors.forename: Marco Antonio Chaer
authors.initials: MA
authors.lastname: Nascimento
mesh_terms: D006838:Hydrocarbons / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D006838:Hydrocarbons