New theoretical and experimental proton affinities for methyl halides and diazomethane: A revision of the methyl cation affinity scale

High-level ab initio calculations and variable-temperature proton-transfer equilibrium constant measurements have been used to obtain new thermochemical data for protonated halogenomethanes (CH₃XH⁺, X = F, Cl, Br, and I) and protonated diazomethane (CH₃NN⁺). Proton affinities of CH₃X and CH₂NN and methyl cation affinities of HX and N₂ have been derived. The theoretical and experimental results are in good agreement with one another but in several cases are in conflict with currently accepted experimental proton and methyl cation affinities. Experimental and theoretical methyl cation affinities are presented for a variety of molecules, leading to the proposal of a new methyl cation affinity scale.