Radical Substituent Effects of α-Fluorine and α-Trifluoromethyl Groups

Homolytic bond dissociation enthalpies (BDEs) of the C−H bonds in 30 fluorinated hydrocarbons have been obtained from ab initio CBS-4 (complete basis set) model calculation at both 0 and 298 K. The calculated C−H BDEs agree with the available literature experimental results within ±2 kcal/mol. Introduction of one or two α-fluorine (α-F) substituent(s) into methane, ethane, or isopropane was found to weaken the sp3 C−H bond strength by 1−4 kcal/mol primarily due to the resonance delocalization of the unpaired electron of the radicals formed by removal of one hydrogen atom. In contrast, introduction of three α-F substituents in trifluoromethane (CHF3) was found to strengthen the sp3 C−H bond by about 1 kcal/mol. The 4−6 kcal/mol bond-strengthening effects of α-trifluoromethyl groups were attributed exclusively to their inductive effects. The substituent effects of fluorine and trifluoromethyl groups have also been examined on the strength of the ethylene sp2 C−H bonds and the acetylene sp C−H bonds. The ground-state effects of the polyfluorine substituents on the C−H BDE values have also been discussed.