New insights in chemical reactivity by means of electron pairing analysis

According to the Lewis model, valence electrons in closed-shell atoms and m olecules can be arranged into pairs of electrons shared between bended atom s and lone pairs that belong to a single atom. Within this scheme, ionic bo nding arises from the transfer of electrons between atoms, while covalent b onding is related to the sharing of electrons between atoms. Over the years , this simple model has proven to be extremely useful for the description o f the bonding patterns in many molecules, and to describe the electronic re arrangements 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 molecu les, one can define atomic localization and delocalization indices which de scribe the intra- and interatomic distribution of the electron pairs in a m olecule. Therefore, these indices can be considered as a physically sound a nd numerically accurate extension of the Lewis model. In this paper, we use localization and delocalization indices to study the electron-pair reorgan ization taking place in five different reactions: two intramolecular rearra ngements, a nucleophilic substitution, an electrophilic addition, and a Die ls-Alder cycloaddition. For each reaction, we perform a comparative analysi s of the electron-pairing patterns in reactants, transition states, and pro ducts. The evolution of electron-pairing along the reaction path is also st udied. In all cases, the use of localization and delocalization indices pro vides useful insights on the electronic rearrangements taking place during the reactions.