Estudo de barreiras rotacionais internas do Etano, e derivados substituídos por Bromo, Cloro, Flúor e Iodo utilizando a teoria Atoms in Molecules – aim

Abstract

The presence of internal rotation at carbon-carbon bonds (C-C) is essential for conformational analysis of organic molecules and their understanding is of great relevance in chemistry as in several other areas. However, for ethane substituted by chlorine, C2H5Cl, C2H4 Cl2 and C2H3Cl3, the height of the rotational barrier increases with the added of the chlorines and the experimental values are: 3.53; 4.26 and 5.20 kcal mol-1, respectively. Differently, the height of the rotational barriers decreases for the ethanes substituted by fluorine, C2H5F, C2H4F2, C2H3F3, following the values: 3.35; 3.32 and 3.13 kcal mol-1. For compounds substituted by bromine and iodine, C2H5Br, C2H4Br2, C2H3Br3, C2H5I, C2H4I2 and C2H3I3, there are no experimental results. Since the atoms are halogens and the opposite convergence of the rotational barriers of the fluorine and chlorine compounds, as the lack of adequate explanation and also lack of experimental results, the work studied the compounds using the methods HF, B3LYP, MP2, MP3, MP4, QCISD (T), CCSD (T), and the composite theories G3 and G3CEP. The origin of the barriers was investigated through calculations of the quantum theory of atoms in molecules (QTAIM). The results obtained for the chlorines and fluorine ethane show that the most accurate methods were G3CEP and G3, with small mean absolute deviations of 0.10 and 0.12 kcal mol-1, respectively. The results of the methods for the compounds replaced by Br and I show that the rotational barrier increases with the number of atoms added following the same order that the compounds of chlorine. By the QTAIM analysis it was possible to show that the height of the rotational barrier of chlorine ethanes occurs by the high electronic density of the chlorine atom, which causes a greater repulsion between the atoms. As for fluorine-substituted compounds, the height of the rotational barrier decreases, a result of the prevalence of the hyperconjugation effect, since the fluorine atom presents a small electronic density and it is easier for the electrons pass to an orbital bonding for an antibonding orbital with adequate symmetry. The QTAIM results for the bromine and iodine substituted compounds are similar to those of the chloro substituted compounds, wherein the increase of the torsion barriers is due to the strong steric repulsion, because the bromine and iodine atoms are more bulky than the chloro.