Rotational Barrier for 1,1-Difluoroethane, 1,1,1,2-Tetrafluoroethane, Pentafluoroethane, and Hexafluoroethane: A Density Functional and ab Initio Molecular Orbital Study

The structures and vibrational frequencies for eclipsed and staggered conformers of CF 2 HCH 3, CF 3 CFH 2, C 2 F 5 H, and C 2 F 6 have been calculated by using B3LYP and MP2 ab initio methods. A reassignment of the experimental IR spectra of CF 3 CFH 2 and C 2 F 6 is suggested. Relative electronic energies are determined with a number of basis sets. Electron correlation is included by means of Møller−Plesset perturbation methods up to the fourth order. Scaled vibrational frequencies are used to account for zero-point energy and thermal contributions to electronic energies. Temperature dependence of the rotational barriers is studied at 20 K intervals from 0 to 300 K.