Torsion potentials and electronic structure of trifluoromethoxy- and trifluoromethylthiobenzene: an ab initio study

Potential functions of internal rotation about the C_sp²X bonds in molecules C₆H₅XCF₃ (X=O, S) were calculated at the second-order Møller-Plesset perturbation level of theory with 6-31G(d) basis set. The profile of the potential function and the rotation barrier (ΔE^#=3.0 kJ/mol) found for C₆H₅OCF₃ suggest that, depending on experimental conditions, there can be either free rotation about the C_sp²O bond or the conformational equilibrium is shifted to the side of the orthogonal form. The rotational barrier for C₆H₅SCF₃ is 14.7 kJ/mol and the molecule exists in the stable orthogonal conformation. The nature of hybridization, energy and population of lone electron pairs (LPs) on the oxygen and sulfur atoms were considered by using the Natural Bond Orbital (NBO) method. The energy of interactions of the LPs with antibonding π^∗-orbitals of the aromatic moiety were estimated for different conformations. The distribution of electron density in the molecules was discussed. The results were compared with analogous calculations on the molecules C₆H₅XCH₃.