Structure and Reactivity of Protonated α,α,α-Trifluorotoluene in the Gas Phase. A Combined FT-ICR, Radiolytic, and ab Initio MO Study

The protonation of CF 3 C 6 H 5 and deprotonation of [CF 3 C 6 H 5 ]H + ions have been studied by FT-ICR with the kinetic bracketing technique. The protonation by weak BH + acids is dissociative, with an onset for C 6 H 5 CF 2 + and HF formation at a gas phase basicity of B equal to 172 kcal mol -1 . Stronger acids, CH 5 +, SO 2 H + and C 2 H 5 + yield persistent [CF 3 C 6 H 5 ]H + ions. These results have been interpreted with the aid of ab initio MO calculations showing that the fluorine atoms have a higher proton affinity than the ring carbons. The dissociative proton transfer appears to be entropically driven, surmounting a C 6 H 5 CF 2 + /HF binding energy of 10.3 kcal mol -1 . The kinetics of proton transfer from [CF 3 C 6 H 5 ]H + to benzene and to (C 2 H 5 O) 2 CO, a fairly strong base in the gas phase, show similar rate ratios in FT-ICR and in radiolytic systems at atmospheric pressure.