Orbital signatures as a descriptor of regioselectivity and chemical reactivity of organic compounds

Abstract

The HOMO-LUMO formalism, developed more as an interpretation of reactivity, may not be suitable for describing the reactivity of certain chemical reactions. The concept of FERMO, based on chemical intuition and criteria of composition and localization for the correct determination of the frontier molecular orbital that regulates chemical reactions, can be understood as a complement to the HOMO-LUMO argument. The development of the FERMO (Frontier Effective for Reaction Molecular Orbital) concept describes the breaking and formation of new chemical bonds, providing important clues that modulate the chemical reactivity of atoms and molecules. Through this concept, it was possible to locate the frontier molecular orbitals involved in protonation reactions of substituted phenols, since protonation and deprotonation processes in aromatic molecules, such as phenols, are important in organic and biochemical chemistry. The analysis was performed using the MOLPROJ software to compare the computational results with experimental NMR data obtained from the literature. As a result, an accuracy rate of approximately 86% was achieved in the protonation sites. In another study, we analyzed the activation energies of the organic reaction between ethylene and nitrosoethylene, a Diels-Alder reaction, catalyzed by different types of catalysts: Lewis acid, oriented electric field, and electrondonating and electron-withdrawing substituents. All calculations were performed using Gaussian09 software. The compounds were fully optimized using the restricted Hartree-Fock method with the 6-31G basis set. Single-point energies were determined using the MP3 method and 6-31G basis set. It was observed that the lowest activation barrier was found when the electric field was applied, followed by the use of Lewis acid. It is worth noting that the field-catalysis field is still relatively new, and research is ongoing to explore its full potential and applicability in different chemical reactions. We can perceive that the FERMO idea is an innovative concept that has been pointed out as quite promising in the study of the importance of molecular orbitals for chemical reactivity and has been successfully applied to describe acid-base behavior, chemical reactions of organic compounds and inorganic complexes, pericyclic reactions, and biological systems. Understanding the behavior of molecular orbitals is essential for a better understanding of chemistry. Therefore, these investigations lead to new perspectives and new ideas about the reactivity of molecules.