Protonated 1,3,5-cycloheptatriene and 7-alkyl-1,3,5-cycloheptatrienes in the gas phase: ring contraction to the isomeric alkylbenzenium ions

1,3,5-Cycloheptatriene (1) and various 7-alkyl-1,3,5-cycloheptatrienes (3, 6, 9, 13, and 16–19) were subjected to gas-phase protonation under CI(CH4) and CI(iC 4H10) conditions and the MIKE spectra of their [M+H]+ ions were measured. Loss of CH4 from the parent ion [1+H]+ and almost exclusive loss of C2H4 from the methyl derivative [3+H]+ indicate ring contraction of the dihydrotopylium ions to protonated toluene (toluenium ions) and protonated ethylbenzene (ethylbenzenium ions), respectively, prior to fragmentation. With increased exothermicity of protonation, ions [3+H]+ also isomerize to xylenium ions. Similarly, higher protonated n -alkylcycloheptatrienes undergo skeletal isomerization to the corresponding ‘‘chain-elongated’’ (n+1)-alkylbenzenium and to the corresponding n-alkyltoluennium ions. Starting with ethyldihydrotropylium ions, a competing isomerization channel is opened giving rise to expulsion of C2H4 from the constituents of the seven-membered ring, as evidenced by deuterium labelling and an unusually high kinetic energy release. Isoalkyl analogues behave in a similar manner with increased hydrogen exchange between the α position of the side chain and the ring.