Elucidação do mecanismo de reação dos radicais metil halogenados com o sulfeto de hidrogênio

Abstract

This study aims to evaluate the mechanism of reactions between halogenated methyl radicals and hydrogen sulfide (H2S), which can have a great impact on atmospheric processes. Among the halogenated methyl radicals, the structures CH3-nXn (X=F, Cl, Br; n=1-3) will be evaluated. The main focus of the study was the hydrogen abstraction reaction of H2S by CH3-nXn radicals (X=F, Cl, Br; n=1-3), however several analyzes were performed among the electronic properties of the entrance and exit complexes presented in the reaction mechanisms. Initially, an analysis was performed among methods based on Density Functional Theory (DFT) which was applied to optimize the geometry of the chemical structures in each chemical reaction and the transition states that characterize the barrier energies. These analyzes were based on the characterization of molecular geometry, electric dipole moment, enthalpy variation and vibrational frequencies using experimental values in literature devoted to the molecular structures in the hydrogen abstraction mechanisms. A second step was the analysis of the methodology applied in the refinement of electronic energies. Thus, based on the calculations of the enthalpy and the Gibbs free energy variation at 298.15K, the hydrogen abstraction reactions are exothermic and spontaneous, respectively. Furthermore, different correlation analyzes were carried out based on the interaction energies of the complexes formed between the reactant and product molecules using the vibrational frequencies, atomic charge and different reactivity parameters values. These correlations analyze were also employed in understanding the energy barrier values among each chemical process. An additional study was directed to the inverse reaction involving the mercapto radical and the halomethanes, where the transition states associated with the halogen and hydrogen elimination reactions were considered.