Abstract
The barrier heights of the internal rotations for ethyl halides calculated by ab initio methods differ from those of experiments by more than 0.2 kcal/mol. The use of basis sets larger than the $6-31G^{\ast}$ set and the inclusion of correlation do not improve the agreement between the calculated and experimental values. The zero-point vibration corrections are substantial in the HF calculations with $6-31G^{\ast}$ basis sets, but become negligible in the MP2 calculations with $6-311G^{{\ast}{\ast}}$ basis sets for $C_2H_5F\;and\;C_2H_5Cl$. It is shown that the rigid rotor approximation and the assumed shape of the potential curve as a cos2${\theta}$ curve could also be the sources of discrepancies between calculated and experimental values. Higher order perturbation corrections narrow the gap between experimental and theoretical values, but there still remains about 10% overestimate of 0.3 kcal/mol. Optimized geometries from the HF and MP2 calculations are in good agreement with those from experiments. Dipole moments calculated from the MP2 densities show slightly better agreement with experiments than those from the HF densities.
