Empirical LCAO parameters for molecular orbitals in planar organic molecules

A novel parametrization within a simplified LCAO model (a type of Huckel model) is presented for the description of molecular orbitals in organic molecules containing -bonds between carbon, nitrogen, or oxygen atoms with sp2 hybridization. It is shown that the model is quite accurate in predicting the energy of the highest occupied orbital and the first -* transition energy for a large set of organic compounds. Four empirical parameter values are provided for the diagonal matrix elements of the LCAO description, corresponding to atoms of carbon, nitrogen with one pz electron, nitrogen with two pz electrons, and oxygen. The bond-length dependent formula of Harrison (proportional to 1/d 2) is used for the non-diagonal matrix elements between neighbouring atoms. The predictions of our calculations have been tested against available experimental results in more than sixty organic molecules, including benzene and its derivatives, polyacenes, aromatic hydrocarbons of various geometries, polyenes, ketones, aldehydes, azabenzenes, nucleic acid bases and others. The comparison is rather successful, taking into account the small number of parameters and the simplicity of the LCAO method, involving only pz atomic-like orbitals, which leads even to analytical calculations in some cases.