Editorial [Hot Topic: Electron Delocalization in Organic Chemistry (Guest Editors: Dr. Eduard Matito and Prof. Sola)]

The wave picture of the electron implies an always present electron delocalization. This electronic delocalization is not an observable, and therefore there is not an experimental property that allows measuring it directly. However, electronic delocalization reveals itself in several chemical phenomena that can be experimentally interrogated to have a measure of the degree of electron delocalization in our system. From a theoretical point of view, the fact that electron localization/delocalization is not an observable implies that there is not a unique and generally accepted measure of this property. A number of ways of putting this concept on a theoretically sound quantum mechanical basis have been developed, many of them based in one way or another on the concept of the Fermi hole. Tools such as the electron localization function (ELF), which provides a picture of the regions where the electron localization is high, or the quantum theory of atoms in molecules (QTAIM), which provides with a wealth of resources to tackle the electron structure in organic species, are nowadays routinely used in organic chemistry to rationalize a number of observable facts such as the molecular structure, stability, magnetic properties, and chemical reactivity of many molecules, in particular, π-conjugated molecules. Electron localization is also essential to know where local groups of electrons like core or valence electrons, electron pairs, bonding pairs, unpaired electrons, or π-electron subsystems are placed and, therefore, to discuss chemistry with the familiar organic way of thinking. In this issue devoted to “Electron Delocalization in Organic Chemistry”, we have gathered seven contributions showing several applications and developments of theoretical methods that account for electron delocalization. It is shown how these methods help to solve daily problems encountered in organic chemistry. Thus, Arrieta, de Cozar, and Cossío perform a critical review on the application of different geometric, energetic, magnetic, and electronic criteria to analyze the cyclic electron delocalization in an important group of pericyclic reactions. García-Revilla and Rocha-Rinza discuss the use of the ELF and delocalization indicators based on the QTAIM to quantify the relevance of electronic delocalization in electrophilic aromatic substitution reactions. Silva and de Lera make an exhaustive appraisal of the use of the bond ellipticity in the framework of the QTAIM to account for electron delocalization in structure and reactivity. Silvi and Reinhardt review the present situation of the electron localization function and show different examples in which this function can be helpful for quantum organic chemists. Fuentealba and Santos assess the application of this ELF and its decomposition into the π and σ parts as a measure of electron delocalization and aromaticity, while Andres, Berski, Domingo, Polo, and Silvi evaluate the use of the ELF to visualize the bond breaking/forming processes, forming/annihilation of lone pairs, and pair rearrangements along a reaction coordinate, thus providing a way to analyze the reaction mechanism at a molecular level. Finally, Angyan paves the way for future tools to measure electron delocalization as he suggests the use of a linear response function of the molecular charge density to an external perturbation as a new measure of electron delocalization in molecules. The present issue reveals many of the state-of-the-art methods used to account for electron delocalization in organic chemistry and, at the same time, we hope it will convince the reader about the importance of theoretical studies on electron localization/delocalization and how they significantly improve our understanding of many chemical processes.