Valence-State Atoms in Molecules. 6. Universal Ionic−Covalent Potential Energy Curves

Abstract

A semiempirical approach for constructing a universal ionic−covalent (UIC) potential energy curve is presented, and two related UIC functions are discussed. In the vicinity of the equilibrium bond length, the attraction between the atoms in the molecule (AIM) is modeled as purely Coulombic, −C/R, as implied by the asymptotic reference to the promoted valence-state energy of partially charged atoms [Gardner, D.; Szentpály, L. v. J. Phys. Chem. A 1999, 103, 9313]. The partial charge is calculated by electronegativity equalization. Along the dissociation coordinate R, we model the decreasing contribution of “ionic structures” as a “soft” Coulson−Fischer transition: the composite UIC function is generated by continuously reducing the weight of the valence-state potential energy function by the admixture of a modified Morse function. Average unsigned errors of 1.42% and 1.16% of De are obtained by comparing our five-parameter UIC and UICα curves with the full Rydberg−Klein−Rees, or ab initio, curves of 42 covalent or polar diatomic molecules (from H2 to NaCl). The evaluation of the rotation−vibration coupling constant, αe, requires only three parameters and yields an average unsigned error of 6.37% for 50 molecules.