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 CF₃C₆H₅ and deprotonation of [CF₃C₆H₅]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 C₆H₅CF₂⁺ and HF formation at a gas phase basicity of B equal to 172 kcal mol^-1. Stronger acids, CH₅⁺, SO₂H⁺ and C₂H₅⁺ yield persistent [CF₃C₆H₅]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 C₆H₅CF₂⁺/HF binding energy of 10.3 kcal mol^-1. The kinetics of proton transfer from [CF₃C₆H₅]H⁺ to benzene and to (C₂H₅O)₂CO, a fairly strong base in the gas phase, show similar rate ratios in FT-ICR and in radiolytic systems at atmospheric pressure.