Valence orbital behaviour during changes in intermolecular separation. Comparison of models based on the empirical united atom theory and on the semi-empirical CNDO/BW theory

Abstract

In the united atom theory, the molecular group orbitals are degenerate at the separated atom limit, but at the equilibrium molecular structure these degenerate molecular group orbitals become non-degenerate, the least stable molecular group orbital being that with highest group symmetry. This behaviour is at variance with the predictions of overlap and ligand field theories, and it is examined in detail using the semi-empirical CNDO/BW theory. Using NH₃ as a specific example, it is shown that within the CNDO/BW theory, the calculated orbital energies at large internuclear distances predict the orbital of highest symmetry to be most stable; as the equilibrium internuclear distance is approached an inversion of the orbital multiplet occurs, making the highest symmetry orbital least stable at equilibrium. The destabilization of the highest symmetry orbital is discussed in terms of the one-electron anisotropic potential destabilization of the empirical united atom theory and the two-electron repulsion of molecular orbital theory, which are shown to become critically important due to the build up of interference density. This change in orbital sequence between large internuclear distances and the equilibrium internuclear distance is accompanied by changes in molecular geometry along the lowest energy reaction coordinate and is shown to be an important aspect of the quantum mechanical prediction of activation energies.