A computational study of C-X (X=H, C, F, Cl) bond dissociation enthalpies (BDEs) in polyhalogenated methanes and ethanes

摘要

Bond dissociation enthalpies (BDEs) play a significant role in the photolysis of Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which lead to the depletion of stratospheric ozone. In this work, we estimate the performance of Density Functional Theory (DFT) methods in calculating BDEs of CFCs and HCFCs, and find that DFTs are unreliable for this system. The reasons for the unreliability of DFT methods in this system are also concluded. Furthermore, composite ab initio methods G3 and G3B3 are demonstrated to accurately estimate BDEs of polyhalogenated lower alkanes. Eighty two experimental values from Comprehensive Handbook of Chemical Bond Energies (2007, 2nd edition) are re-evaluated. Eight of them are doubted as having a deviation exceeding 20.0 kJ/mol between the theoretical and experimental values. We also systematically predict the BDEs in polyhalogenated methanes and ethanes. A further study is conducted on their relationships of structures and properties. 12 DFT methods including B3LYP, B3P86, B3PW91, BHandHLYP, MPW1PW91, MPW1K, MPW1B95, MPW3LYP, BMK, X3LYP, PBE1KCIS, TPSS1KCIS are evaluated by comparing with the experimental bond dissociation enthalpies (BDEs) of polyhalogenated alkanes. It is found that DFTs are unreliable for the C-X(X=H,C,F,Cl) BDE calculations of polyhalogenated alkanes and the reasons are concluded. Composite ab initio methods G3 and G3B3 are applied and they are found to be reliable for this system. Based on our calculation, the relationship between their structures and properties are further discussed.