In this talk, we will aim to construct bridges connecting conceptual density functional (CDFT) and valence bond (VB) theory. We start by demonstrating how many common local reactivity descriptors stemming from CDFT, i.e., the spin density distribution for radical reactions and the Fukui function for electro-/nucleophilic reactions, can be re-interpreted as indirect measures of delocalization, i.e., resonance stabilization, present within the reactants, from a VB perspective. Furthermore, 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, it becomes possible to probe directly 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 in order to accurately and conclusively gauge the reactivity patterns.

Building bridges between conceptual density functional theory and valence bond theory

ACS Fall 2021

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