TOPOLOGICAL PROPERTIES OF ELECTRON-DENSITY IN SEARCH OF STERIC INTERACTIONS IN MOLECULES - ELECTRONIC-STRUCTURE CALCULATIONS ON ORTHO-SUBSTITUTED BIPHENYLS

The bond critical points in the electron density are not necessarily associated with bonding interactions. When the distance between two atoms is smaller than their contact interatomic separation (CIS), an attractor interaction line passing through the corresponding bond critical point appears, indicating a nonbonding repulsive interaction. Sterically crowded molecules are defined as those possessing such interaction lines at their equilibrium geometries. The appearance (or disappearance) of the interaction lines along the reaction paths allows for classification of the barriers to rotation into the categories of those sterically hindered, sterically facilitated, and sterically neutral. The values of contact interatomic separations provide a measure of the steric crowding in molecules that is, due to its mathematical rigor, superior to the concept of the van der Waals radii. The HF/6-31G** electronic structure calculations on biphenyl and its ortho-substituted derivatives illustrate the applications of the aforementioned theoretical concepts.