Theoretical study of the internal rotational barriers of fluorine, chlorine, bromine, and iodine-substituted ethanes

Physical and chemical characteristics of flexible compounds are extremely dependent of internal rotation barriers. On the other hand, halides of organic compounds are extremely common, and the nature of their rotation barriers are still poorly understood. As a simple example, the experimental height of the internal rotational barrier decreases with the number of fluorine-substituted in ethane, C2H5F, C2H4F2, and C2H3F3. For chlorine-substituted ethane C2H5Cl, C2H4Cl2 and C2H3Cl3 the internal rotational barrier converges in an opposite manner. No experimental results are available for the bromine- and iodine-substituted compounds C2H5Br, C2H4Br2, C2H3Br3, C2H5I, C2H4I2, and C2H3I3. In light of lack of both an adequate explanation for this phenomenon and experimental results for Br and I, the present work studied these compounds using different levels of theory: MP2, MP3, MP4, QCISD(T) and CCSD(T) methods, and the G3 and G3CEP composite theories. The results showed that the G3 and G3CEP theories were the most accurate calculations for the F- and Cl-substituted compounds as a result of the additive contributions of the energy values. From Natural Bond Orbitals (NBO), Quantum Theory of Atoms in Molecules (QTAIM) and Energy Decomposition Analysis (EDA) it was verified that the internal rotational barriers decrease with the addition of Fluorine atoms for fluorine-containing ethanes and can be explained by: π character in the CF bond and delocalization indexes (DI(A)) by QTAIM analysis; the interaction of the ligand and anti-ligand orbitals by NBO analysis and for the descriptors by EDA. On the other hand, the enlargement of the barriers with the addition of Cl, Br and I atoms for substituted ethanes, are dominated by the large electronic cloud of these halogens, which generates significant steric repulsion.