Visualizing the evolution of charge density in fulvene bond torsion: a bond bundle case study

Abstract

The chemical bond is a central concept of many sciences, but there is no unified consensus as to the physical representation of a bond, or how this representation relates to a bond’s properties. A variety of bonding models have been proposed, each different in its explanation and prediction of chemical properties. Quantum theory of atoms in molecules (QTAIM) aims to encompass a well-rounded approach applicable to many areas of molecular studies. The QTAIM bonding model uses the topology of the electron charge density (ρ(r)) and defines bonding interactions as one-dimensional ridges of (r)—known as bond paths. As with any bonding model, there are instances where its predictions do not provide a full picture. For example, a 1D bond path does not accurately describe properties of interest, such as an accounting of the energy barrier to bond torsion. A bond bundle analysis case study presented in this thesis analyzes the π-bond rotation in a computational benchmark molecule, fulvene. This methodology is an extension of QTAIM and illustrates the applicability of charge density based methods to bonding. We demonstrate the bond bundle can capture the same type of chemical information as valence bond or molecular orbital theories. The bond bundle accurately represents the transition from double to single bond character in fulvene by qualitative and quantitative observation of the evolution in size and shape of the bond bundle.