Estrutura eletrônica e ligações químicas em mecanismos de reações de substituição nucleofílica: análises topológica da densidade eletrônica total e de recobrimento

Abstract

Understanding the key aspects of chemical reaction mechanisms is usually focused on the profile energetic surface (PES) energetic and structural. However, bond analysis methodologies can add information about the reaction mechanisms. An attractive reaction to apply bond analysis methods is the bimolecular nucleophilic substitution (SN2). A peculiar feature of this reaction in the gas phase is the PES dependence on the atomic center and/or substituent group nature, generally attributed to steric effects. The central goal of this work is to apply the QTAIM, overlap properties (OP), and local vibrational mode (LVM) theory models to study the chemical bonds in stationary points of the Cl– + XR3Cl (with A = C and Si) and C– – + BR2Cl (with B = P and N) reactions, with R = H, F, Cl, Me, e Et. On the one hand, for the SN2@C and SN2@N reactions, the reactants and products are separated by a pentacoordinate(@C) and tetracoordinate (@N) transition state (TS), corresponding to a saddle point in the PES. On the other hand, for SN2@Si and SN2@P reactions a stable pentacoordinate(@Si) and tetracoordinate (@P) transition complex is observed. The chemical bond descriptors ∇ 2𝜌𝐵𝐶𝑃 and 𝐻𝐵𝐶𝑃, overlap properties (𝐽𝑂𝑃 𝑖𝑛𝑡𝑟𝑎) and LVM force constants (𝑘𝑛 𝑎 ) were obtained for the studied systems. QTAIM analysis indicated that the C–R and N–R bonds are indicated to be more covalent than the Si–R and P–R bonds, due more negative 𝐻𝐵𝐶𝑃 and ∇ 2𝜌𝐵𝐶𝑃 values. These results agree with the overlap properties to C–R and N–R bonds, indicating a high density concentration as compared to Si–R and P-R bonds. Besides, the LVM analysis (from the force constants 𝑘𝑛 𝑎 values) also reveal that the C–R and N–R bonds are more stronger than Si–R and P–R ones. Lastly, the overlap Coulomb repulsion energy (𝐽𝑂𝑃 𝑖𝑛𝑡𝑟𝑎) is generally greater in C–R than in Si–R in XR3Cl systems, suggesting that the steric hindrance experienced by Cl– in SN2@C reactions is probably associated with the greater covalent nature of C–R bonds, that concentrate density along the chemical bond more efficiently than in Si–R. We conclude that in the case SN2@C and @Si the properties of the chemical bonds are predominantly influenced by the nature of the central atoms, for SN2@N and @P there is greater dependence on the type of the substituent group.