Energy diagram for the protonation of ketene. Total energies relative to those of the separated reactants, ketene and the proton donor X, are given in the enclosed table in kJ mol⁻¹. C* is the TS energy for the interconversion between CH₂COH⁺ and CH₃CO⁺ isomers. Y** is the energy difference between the reactants and the interconversion TS; a negative value indicates that the TS is submerged below the reactant energy, and a positive value implies that the TS lies above the reactant energy.