Staggered and eclipsed conformations of C₂F₆: A systematic ab initio study

Molecular geometries, energies, and vibrational frequencies for eclipsed and staggered conformers of perfluoroethane were determined by ab initio calculations. Basis set effects were investigated systematically with progressively larger basis sets, up to triple zeta, d and f polarization functions plus diffuse functions on all atoms. Electron correlation was included by means of Møller-Plesset perturbation calculations to the third order. Vibrational frequencies, calculated at both Hartree-Fock (HF) and second order Møller-Plesset perturbation (MP2) methods, were used to make zero-point energy and thermal corrections to electronic energies. Some semi-empirical methods and molecular mechanics force fields were also employed for comparison. The rotational barrier (3.81 kcal mol^-1) predicted from the HF/6-311+G(3df) method is close to the measured electron diffraction value of 4.40 kcal mol^-1 (-0.90,+1.10). The MP2/6-311G(d) level gives a lower barrier of 3.33 kcal mol^-1, and the best MP3 calculation, MP3/6-311+G(3df)//MP2/6-311G(d), a value of 3.55 kcal mol^-1.