Electric and Magnetic Properties Computed for Valence Bond Structures: Is There a Link between Pauling Resonance Energy and Ring Current?

The making of ring currents

The π-electron diatropic ring current emerging throughout the trimerization of acetylene to benzene.

The non-covalent nature of the molecular structure of the benzene molecule

The benzene molecule is one of the most emblematic systems in chemistry, with its structural features being present in numerous different compounds. We have carried out an analysis of the influence of quantum mechanical interference on the geometric features of the benzene molecule, showing that many of the characteristics of its equilibrium geometry are a consequence of non-covalent contributions to the energy. This result implies that quasi-classical reasoning should be sufficient to predict the defining aspects of the benzene structure such as its planarity and equivalence of its bond lengths.

On the interpretation of valence bond wavefunctions

Valence bond wavefunctions are naturally geared to the chemist's idea of chemical bonding. In a structure one may distinguish different electron pair bonds and possible radical character. A structure may correspond to a covalent bond, where all electrons are equally divided over the atoms, or may describe an excess charge on a discrete part of the molecule, which indicates ionic bond character. From the weights of the structure in a variationally optimised multi-structure valence bond wavefunction one may derive the importance of the different bonding types. The individual structures could be considered to represent the different physical situations. We explore this concept for simple diatomic molecules and for polyatomics, and we discuss the relation to Lewis structures. We show that the assumption of individual properties for the individual structures leads to inconsistencies.