Ab initio study of bond strengths in chlorinated ethane molecules and ethyl radicals

All the bond energies in chlorinated ethane molecules and ethyl free radicals have been calculated by ab initio molecular orbital methods. The bond strengths were determined using a direct homolytic bond dissociation reaction. All structures of interest were fully optimized at the MP2/6-31G(d,p) level of theory and were at their global minimum. A single-point energy was calculated for all compounds at the MP4/6-311G(d,p) level of theory. The zero-point energy corrected energy values of the reaction species were used in the calculations. Several trends in bond energies were observed. The results of the calculations showed that the alpha-substituted Cl atom has an obvious tendency to increase C-beta-H bond strength and decrease the vicinal C-alpha-H bond strength. The Cl atom has a tendency to make other C-Cl bonds weaker, both in radicals and in molecules. The Cl atom strongly affects the thermal stability of the radicals: all beta-chlorinated ethyl radicals have a very weak C-Cl bond which causes them to decompose at low temperatures, whereas alpha-chlorinated radicals are relatively stable at elevated temperature. When a chlorinated ethyl radical decomposes, the most probable product is vinyl chloride.