Gaseous [M - H]+ ions of alpha,omega-diphenylalkanes: cyclization to [M + H]+ type ions of benzocycloalkanes as recognized by chain-length dependent proton exchange

Kuck D. Gaseous [M - H]+ ions of alpha,omega-diphenylalkanes: cyclization to [M + H]+ type ions of benzocycloalkanes as recognized by chain-length dependent proton exchange. International Journal of Mass Spectrometry and Ion Processes. 1992;117(1-3):441-455.Metastable [M - H]+ ions of alpha,omega-diphenylalkanes C6H5(CH2)xC6H5 where x = 3-6 (structures 3-6 respectively), generated by hydride abstraction in the chemical ionization (i-butane) source, eliminate benzene after proton exchange between the aromatic rings. The proton exchange is slow for ions [3 - H]+ and [4 - H]+, but fast and apparently complete for ions [5 - H]+ and [6 - H]+. These observations, combined with collision activation experiments, suggest the cyclization of the [M - H]+ ions to isomeric protonated 1-phenylbenzocycloalkane and 1-benzylbenzocycloalkane derivatives, i.e. to [M1 + H]+ type ions, with a preference for protonated tetralin structures. Hydrogen exchange between the aliphatic chain and the rings is absent or negligible for [M - H]+ ions of 3-5 but is significant for ions [6 - H]+