New insights in chemical reactivity by means of electron pairing analysis

According to the Lewis model, valence electrons in closed-shell atoms and molecules can be arranged into pairs of electrons shared between bended atoms and lone pairs that belong to a single atom. Within this scheme, ionic bonding arises from the transfer of electrons between atoms, while covalent bonding is related to the sharing of electrons between atoms. Over the years, this simple model has proven to be extremely useful for the description of the bonding patterns in many molecules, and to describe the electronic rearrangements taking place during chemical reactions. However, a physically accurate description of the electron pairing in atoms and molecules has to be based on the electron-pair density. Within the theory of atoms in molecules, one can define atomic localization and delocalization indices which describe the intra- and interatomic distribution of the electron pairs in a molecule. Therefore, these indices can be considered as a physically sound and numerically accurate extension of the Lewis model. In this paper, we use localization and delocalization indices to study the electron-pair reorganization taking place in five different reactions: two intramolecular rearrangements, a nucleophilic substitution, an electrophilic addition, and a Diels-Alder cycloaddition. For each reaction, we perform a comparative analysis of the electron-pairing patterns in reactants, transition states, and products. The evolution of electron-pairing along the reaction path is also studied. In all cases, the use of localization and delocalization indices provides useful insights on the electronic rearrangements taking place during the reactions.