Gaseous [M - HI+ ions of a,wdiphenylalkanes: cyclization to [M + H]+ type ionsof bknzocycloalkanes as recognized by chain-length dependent proton exchange*

Metastable [M H]+ ions of a,o-diphenylalkanes C,H,(CH,),C,H, 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 HI+, but fast and apparently complete for ions [S H]+ and [6 H]+ These observations, combined with collision activation experiments, suggest the cyclization of the [M H]+ ions to isomeric protonated I-phenylbenzocycloalkane and I-benzylbenzocycloalkane derivatives, i.e. to [M’ + 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 -HI+.