Endothermic proton transfer reactions from three [C6H6]+˙ isomers

A triple quadrupole mass spectrometer was used to establish the proton affinities of phenyl, CH₃C?CC?CCH₃ and HC?CCH₂C?C?CH radicals as 870±29, 824±25, and 757±21 kJmol^?1, respectively, from the kinetic energy of benzene, 2,4-hexadiyne, and 1.5-hexadiyne molecular ions at which the onset of proton transfer to less basic species occurs in the second rod assembly. These values were confirmed by other triple quadrupole experiments involving bracketing of exothermic proton transfers.