Gas‐phase protonation and methyl cation transfer from methyl halide ions

The ion-molecule reactions between [CH3X]+˙ [CH3XH] +, [CH3XCH3]+ ions (X = F, Cl, Br, I) and a number of nucleophiles have been studied by ion cyclotron resonance techniques. Protonation of the nucleophiles is observed to occur from both the molecular ions [CH3]X+˙ and protonated species [CH3XH]+ whereas dimethylhalonium ions [CH3XCH3]+ react principally by methyl cation transfer. A notable exception occurs in methyl iodide where the molecular ions [CH3I]+˙ act both as proton and methyl cation donors, whereas dimethyliodonium ions are found unreactive. The results are discussed with reference to the use of alkyl halides as reagent gases in chemical ionization experiments.