A theoretical study of hydrogen and chlorine transfer reactions from fluorine- and chlorine-substituted methanes by F3C˙ radicals

Abstract

The minimum energy path for the hydrogen and chlorine abstraction reactions from CX_4−nH_n (n = 1, 2 or 3; X = Cl or F) and CCl_4−nF_n (n = 0, 1 or 2) series by trifluoromethyl radicals (F₃C˙) has been determined using ab initio molecular orbital calculations. All the structural parameters (geometry and vibrational frequencies) were computed at the B3LYP/6-31G(d) level of theory. The characteristics of the transition states for the hydrogen atom transfer reactions from the halomethane series, CX_4−nH_n (n = 1, 2 or 3; X = Cl or F), were also determined using the MP2/6-31G(d,p) level of theory. This database was then used to calculate the kinetic parameters by means of the transition-state theory. The results indicated that the density functional theory (DFT) approach can provide energies that are comparable to most of the experimentally derived values. However, to obtain a better reference and to test the reliability of the activation barriers, we have also carried out computations using the MP4(SDQ)(fc)/6-31+G(d,(f),d,p)//MP2/6-31G(d,p) approach and the CBS-RAD(B3LYP-B3LYP) procedure on the reactions involving the CX_4−nH_n (n = 1 or 2; X = Cl or F) halomethanes. In addition, the reactivity trends of these species were interpreted in terms of a balance between the relative strength of the bonds being broken and formed and the polarity variations at the transition state.