Structure and Reactivity of Protonated alpha,alpha,alpha-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 C6H5- CF2+ 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.