Theoretical reactivity indices based on the conceptual Density Functional Theory (DFT) have become a powerful tool for the semiquantitative study of organic reactivity. A large number of reactivity indices have been proposed in the literature. Herein, global quantities like the electronic chemical potential µ, the electrophilicity ω and the nucleophilicity N indices, and local condensed indices like the electrophilic Pk and nucleophilic Pḱ Parr functions, as the most relevant indices for the study of organic reactivity, are discussed.

Pseudodiradical structures and GEDT involved in the C–C single bond formation in non-polar, polar and ionic organic reactions.

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The electrophilic/nucleophilic character of a series of captodative (CD) ethylenes involved in polar cycloaddition reactions has been studied using DFT methods at the B3LYP/6-31G(d) level of theory. The transition state structures for the electrophilic/nucleophilic interactions of two CD ethylenes toward a nucleophilically activated ethylene, 2-methylene-1,3-dioxolane, and an electrophilically activated ethylene, 1,1-dicyanoethyelene, have been studied, and their electronic structures have been characterized using both NBO and ELF methods. Analysis of the reactivity indexes of the CD ethylenes explains the reactivity of these species. While the electrophilicity of the molecules accounts for the reactivity toward nucleophiles, it is shown that a simple index chosen for the nucleophilicity, Nu, based on the HOMO energy is useful explaining the reactivity of these CD ethylenes toward electrophiles.

Building upon our recent studies devoted to the bonding changes in polar reactions [RSC Advances, 2012, 2, 1334 and Org. Biomol. Chem., 2012, we propose herein two new electrophilic, P þ k , and nucleophilic, P { k , Parr functions based on the spin density distribution at the radical anion and at the radical cation of a neutral molecule. These local functions allow for the characterisation of the most electrophilic and nucleophilic centres of molecules, and for the establishment of the regio-and chemoselectivity in polar reactions. The proposed Parr functions are compared with both, the Parr-Yang Fukui functions [

A good correlation between the activation energy and the polar character of Diels-Alder reactions measured as the charge transfer at the transition state structure has been found. This electronic parameter controls the reaction rate to an even greater extent than other recognized structural features. The proposed polar mechanism, which is characterized by the electrophilic/nucleophilic interactions at the transition state structure, can be easily predicted by analyzing the electrophilicity/nucleophilicity indices defined within the conceptual density functional theory. Due to the significance of the polarity of the reaction, Diels-Alder reactions should be classified as non-polar (N), polar (P), and ionic (I).

Abstract:A new theory for the study of the reactivity in Organic Chemistry, named Molecular Electron Density Theory (MEDT), is proposed herein. MEDT is based on the idea that while the electron density distribution at the ground state is responsible for physical and chemical molecular properties, as proposed by the Density Functional Theory (DFT), the capability for changes in electron density is responsible for molecular reactivity. Within MEDT, the reactivity in Organic Chemistry is studied through a rigorous quantum chemical analysis of the changes of the electron density as well as the energies associated with these changes along the reaction path in order to understand experimental outcomes. Studies performed using MEDT allow establishing a modern rationalisation and to gain insight into molecular mechanisms and reactivity in Organic Chemistry.Keywords: molecular electron density theory; DFT reactivity indices; electron localisation function; non-covalent interactions; electron density; molecular mechanisms; chemical reactivity Structure and Reactivity in Organic Chemistry Based on Quantum Chemical ModelsWhy and how do reactions take place are two fundamental questions in Organic Chemistry. Many chemical concepts have been developed through a large number of experiments, however, understanding how molecules change is by no means experimentally easy because changes take place in a very short time. Based on experiments, some important theories, such as the transition state theory [1], have been developed in kinetic chemistry, which have permitted to establish fundamental concepts used in the study of molecular mechanisms. Within this theory, the concept of the activation complex or transition state structure (TS) enabled the establishment of a relationship between the experimental activation energy [2] and the energy of the TS associated with an organic reaction.Since the introduction of the chemical bond concept by Lewis in 1916 [3], two different quantum mechanics theories to explain chemical bonding, namely, the valence bond theory [4-6] (VBT) and the molecular orbital theory [7] (MOT), were established. The two theories were developed at about the same time, but quickly diverged into two different schools that have established a complete interpretation of chemistry [8]. Both theories are based on the resolution of Schrödinger's equation [9] (see Equation (1)). The information obtained from the resolution of Schrödinger's equation is a wavefunction Ψ whose square describes the electron density distribution around the nuclei, and the total electronic energy E associated with this wavefunction Ψ. The square of the wavefunction Ψ is related with the electron density, which is a physically observable [10].Until the mid-1950s, chemistry was dominated by the classical VBT, which expresses the molecular wavefunction Ψ as a combination of explicit covalent and ionic structures based on pure atomic orbitals (AOs) or hybrid atomic orbitals. However, the computational effort required to perform ab initio

After Huisgen's and Firestone's mechanistic proposals made in the 1960s based on experiments, several theories were proposed during the last century to explain [3+2] cycloaddition (32CA) reactions, most of them still prevailing today. Recent molecular electron density theory (MEDT) studies of 32CA reactions involving representative three‐atom components have allowed characterising at least four different electronic structures, which experience a dissimilar chemical reactivity. In this review, the four simplest reactivity models are presented, providing a modern rationalisation of 32CA reactions based on MEDT.