The question of cyclic versus acyclic ions: The structure of [C₁₀H₁₂]⁺˙ gas phase ions

The extent of isomerization of acyclic and cyclic gas phase radical cations of composition [C₁₀H₁₂]⁺˙ has been investigated by using collisionally activated dissociation spectroscopy. Both electron and charge exchange ionizaiton were employed to form the ions with various internal energies. The [C₁₀H₁₂]⁺˙ ions investigated consisted of ionized phenylbutenes, ring‐substituted methyl derivatives of allylbenzene and phenylpropene, 1‐methyl‐2‐isopropenylbenzene, benzylcyclopropane, phenylcyclobutane, tetralin and 1‐methylindan. The 1‐methylindan and tetralin radical cations are the most stable of the C₁₀H₁₂ isomeric radical ions. The [C₁₀H₁₂]⁺˙ formed from acyclic olefins having the double bond in conjugation with the aromatic ring retain the initial structure to a significant extent. However, ions derived from olefins with the double bond out of conjugation with the benzene ring preferentially cyclize to stable five‐ and six‐membered cyclic ions. Ring opening of small‐ring cyclic ions, such as ionized benzylcyclopropane and phenylcyclobutane, occurs, followed by ring closure to the tetralin radical cation.