CONFORMATIONAL-ANALYSIS OF 1,2-DIHALOETHANES - A COMPARISON OF THEORETICAL METHODS

The geometries of the gauche and trans conformers and the two transition states connecting the conformers for the 1,2-dihaloethanes have been calculated. The methods used in this study are the AM-1 and PM-3 parametrizations of the MNDO Hamiltonian, ab initio molecular orbital (MO) SCF and MP-2 with large basis sets and local density functional (LDF). The ab initio MO and LDF results are in good agreement with each other and with the available experimental data for the energetics and structures. The semiempirical methods show much poorer agreement with both experiment and the other calculations and are often qualitatively wrong. The gauche structure is calculated to be less stable than the trans by 1.78, 1.78, and 2.32 kcal/mol for X = Cl, Br, and I at the highest level of calculation in contrast to the result found for X = F where the gauche is more stable than the trans. The rotation barrier between the two gauche structures ranges from 9.55 to 9.93 kcal/mol, and the rotation barrier between the trans and the gauche ranges from 5.06 to 5.77 kcal/mol for X Cl, Br and I at the highest level of calculation.