Molecular Electron Density Theory: A Modern View of Reactivity in Organic Chemistry

Abstract

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

Understanding the mechanism of the decomposition reaction of nitroethyl benzoate through the Molecular Electron Density Theory

“…Very recently, Domingo proposed the Molecular Electron Density Theory (MEDT) [30], in which changes in the electron density, but not molecular orbital (MO) interactions such as the Frontier Molecular Orbital (FMO) theory proposed [31], are responsible for the reactivity in organic chemistry. Note that the electron density obtained from the wavefunction is the unique physically observable.…”

A Molecular Electron Density Theory Study of the Reactivity of Azomethine Imine in [3+2] Cycloaddition Reactions

“…The present theoretical study emphasises how MEDT studies are able to rationalise the reactivity of organic compounds based on a rigorous analysis of the changes of the electron density along organic reactions [12]. Conversely, Houk's DIEM only permits to establish a good relationship between the distortion energy and the activation energy in the series of non-polar 32CA reactions given in Scheme 2, i.e.…”

A molecular electron density theory study of the [3 + 2] cycloaddition reaction of nitrones with strained allenes

“…These different interpretations about the role of the strain in the reactivity of strained species prompted us to revisit the 32CA reaction of the strained CHDE 10 with nitrone 15 within the recently proposed Molecular Electron Density Theory (MEDT) 21 (see Scheme 3). The main purpose of the present theoretical study is to understand how the strain modies the reactivity of CHDE 10 with respect to the non-strained linear allene 18; this MEDT study provides a different explanation to the one previously reported.…”