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 and conditions and the MIKE spectra of their [ M + H ] ' ions CI(CH 4 ) CI(iC 4 H 10 ) were measured. Loss of from the parent ion [ 1 + H ] ' and almost exclusive loss of from the methyl CH 4 C 2 H 4 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 nalkylcycloheptatrienes undergo skeletal isomerization to the corresponding ""chain-elongatedΓΓ (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 from the constituents of the seven-C 2
H 4 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 a position of the side chain and the ring.