Bridging Conceptual Density Functional and Valence Bond Theories

The goal of this chapter is to build bridges connecting conceptual density functional (CDFT) and valence bond (VB) theory. First, we demonstrate how many common (orbital-based) local reactivity descriptors stemming from CDFT, i.e. the spin density distribution for radical reactions and the Fukui function for electrophilic/nucleophilic reactions, can be re-interpreted as indirect measures of delocalization, i.e. resonance stabilization, present within the reactants, from a VB perspective. Second, we show that VB theory enables a straightforward resolution of soft-soft, i.e. orbital-based, and hard-hard, i.e. electrostatic, interactions. As such, one can probe the predictive power of different local reactivity descriptors to gauge the magnitude of both of these interaction types. Unsurprisingly, we find that electrostatic potential maps and/or partial charges describe electrostatic interactions rather well and that spin density/Fukui function distributions do the same for spin-pairing interactions. However, we also observe at the same time that in reaction types that were previously assumed to be dominated by hard-hard interactions (e.g. protonation reactions), the spin-pairing interactions are generally far from negligible, indicating that a clear-cut hard-soft dichotomy is not always warranted. Consequently, a hybrid approach is required in such a case, in which descriptors of both interaction types are combined to accurately and conclusively gauge the reactivity patterns. Overall, this chapter demonstrates that a more profound understanding and enhanced insights can be achieved by combining CDFT and VB theory.