ESTUDO COMPUTACIONAL DOS ADUTOS ÁCIDO-BASE DE LEWIS

Abstract

Experimental studies point out that the strength of Lewis acids in boron tri-halides, with respect to a strong Lewis base, increase in the order BF3 < BCl3 < BBr3. However, this acidity order of the BX3 compounds, with respect to the halogen variation is the opposite of that expected considering the electronegativity order F > Cl > Br. Computational studies commonly use methodologies of chemical bond analysis to study the interactions between atoms in these systems, pointing out that the chemical bond covalent character is related to the experimentally observed trends. The objective of this work is the application of a method based on the analysis of properties of chemical bonds from the point of view of the overlap model and compare it with the use of the Quantum Theory of Atoms in Molecules. The methodology applied to understand these aspects of the chemical bonds of the Lewis acid-base adducts, involved calculations based on the DFT, with the functional wB97X-D, which presents long-range correction and dispersion and the method of locating the orbitals employed is the Pypek Mezey. The combinations used for the acid involves hydrogen and halide (F, Cl and Br) for boron and the base ligands (Y1, Y2, Y3) will be varied in 7 combinations (X3B-NH3; X3B-NH2CH3; X3B-NH CH3) 2; X3B-N (CH3) 3; X3B-NH2et; X3B-NH (et) 2; X3B-N (et) 3) for each acid (BX) and for the base with the oxygen binding atom, will have 2 combinations (X3B-Oet2; X3B-O (CH3) 2) for each acid. As a result of this, interesting results were obtained, the tendencies for the BH3 systems were remarkable, as donor bases were added the values of αOP, λ1 and λ2 increased proportionally, giving a common tendency between both methods, the covalence of the binding. For the other systems the effects caused by the addition of halogen to the acid, like the electronegativity, when the steric hindrance, were noticed in the values of the αOP. Thus, for the theoretical understanding of the chemical bonds involved in the acid-base Lewis adduct, the BOPP model presents a relative sensitivity in determining the covalent character of the bonds, as well as a correlation with the QTAIM parameters.