The bond dissociation enthalpies (BDEs) of the C-Cl bonds were calculated by using six density functional theory (DFT) methods (B3LYP, B3P86, MPW1K, TPSS1KCIS, X3LYP, BMK). The results were compared with the corresponding experimental values to examine the reliability of these methods. It was found that although some new methods developed in recent years were found to be superior to B3LYP method, they were not the best density functional methods for the calculation of the BDEs of C-Cl bonds. However the B3P86/6-311++G(2df, 2p) level of theory was found to be the most accurate, with an mean absolute deviation of 6 .58 kJ/mol for 17 C-Cl bonds. Finally, armed with the theoretical tool the C-Cl BDEs of some persistent environmental pollutants were calculated. The substituent effects were discussed.

Key words: homolytic bond dissociation enthalpies, biodegradation, chlorobenzene, chlorophenol