Determination of branching ratios for the reactions of H3O+ with ethylbenzenes as a function of relative kinetic energy

The proton-transfer reactions of H3O+ with ethylbenzene and deuterium-labeled ethylbenzenes were systematically investigated by means of proton-transfer reaction mass spectrometry. The branching ratios for the formation of protonated ethylbenzenes and for fragmentation reactions involving elimination of ethylene from the protonated molecules, accompanied by hydrogen migration from the α position or the β position of the ethyl group, were determined as a function of the mean relative center-of-mass kinetic energies of the reactants (Kcm). The branching ratio for the intact protonated molecule decreased with increasing relative kinetic energy: the branching ratio decreased from 0.87 at Kcm = 0.19 eV to 0.05 at Kcm = 0.36 eV, and in turn the ratio for ethylene elimination increased from 0.13 to 0.95. The branching ratios determined for partially deuterium-labeled ethylbenzenes were consistent with one another as well as with the ratios for unlabeled ethylbenzene. The fraction of H migration from the α position increased with increasing Kcm (from 0.13 ± 0.01 at Kcm = 0.19 eV to 0.24 ± 0.01 at Kcm = 0.36 eV) and appeared to saturate at ∼0.24 at Kcm values above 0.28 eV.