Quantum chemical valence indices from the one-determinantal difference approach

Abstract

The recently introduced quadratic (two-electron) valence indices, ionic and covalent, derived from the Hartree–Fock finite-difference approach, are applied to selected organic and inorganic molecules to demonstrate their utility in monitoring chemical bonding patterns in molecular systems. The indices are defined in terms of differerences between simultaneous probabilities of finding two electrons on specified atoms, calculated from the molecular and separated-atom-limit (SAL) wave functions, respectively, in the UHF approximation. The total quadratic valence number represents the overall number of chemical bonds in the system under consideration; it is interpreted as the molecular expectation value of the difference operator of the molecular and SAL density operators. This interpretation leads to a new set of ionic atomic and diatomic valence components; these modified valence numbers are discussed using the two-orbital model in the UHF scheme. A new procedure is proposed for dividing the one-center contributions to the bond valences; it generates effective bond orders in qood agreement with chemical expectations. The new valence quantities are tested on selected typical molecules and prototype hydrogen-bonded dimers. A more extensive study has been carried out on small-ring propellanes, to examine changes in bond valences between bridgehead atoms in selected systems. Key words: chemical valence: UHF difference approach; chemical bond: two-electron model; bond multiplicities; ionic/covalent bond components; propellanes: valence study.