Covalent Chemical Binding
Klaus Ruedenberg
Department of Chemistry and Ames Laboratory USDOE,
Iowa State University, Ames, Iowa 5001
Covalent bonding was identified by Avogadro, Ampère and Dumas in the early 19th century. One hundred years later, the quantum mechanical laws were discovered from which these interatomic attractions follow. The new physics generated new insights into chemical bonding structures and, in the 1930s, led to the interdisciplinary field of quantum chemistry. But the results of quantum mechanical calculations are not as easily interpreted in terms of physical concepts as the motions in classical mechanics. Notably, two related non-trivial problems arose with respect to chemical bonding. One is the question of the “physical” origin of the bonding attractions. Another is the theoretical identification of “interacting atoms in molecules” because, in a molecule, an atom is different from the free atom. Various hypotheses were put forth attributing quantum mechanical bond formation to a variety of physical interactions, and various models were postulated for the modified atoms in molecules.
The present approach to these problems is based on the premise that the actual electronic wave function of a molecule can be cast in a form from which the answers to these questions can be recognized without involving additional models. The wave function is rigorously expressed in terms of molecular orbitals that differ only by modest deformations from atomic orbitals of the non-bonded constituent atoms. Thereby, the configurational expansion is obtained as a sum of terms that embody the non-bonded quasi-atoms in the molecule and of terms that embody the interactions between these quasi-atoms. The molecular energy resulting from this expansion exhibits a resolution into contributions representing conceptual physical interactions that constitute the bonding synergism in the context of the variation principle.
The results of molecular analyses made so far imply that covalent bonds are created by the drive of electron waves to lower their kinetic energy through expansion. The critical terms of the synergisms are the interatomic kinetic interference energies between oriented quasi-atomic orbitals, and these useful indicators are readily extracted from the molecular wave functions. In all cases examined so far, the sum total of all interatomic potential energy interactions has been antibonding. Widespread notions regarding the virial theorem have been shown to greatly overrate its role in covalent bonding.
Research Support
U.S. National Science Foundation,
U.S. Department of Energy,