Doubly protonated 1,3,5-trimethylenebenzene (C9H112+) and homologous C7H72+ and C8H92+ dications: Structures and unimolecular fragmentation patterns

Structures and fragmentation patterns of C7H72+, C8H92+, and C9H112+ dications generated from various precursors are studied by means of mass spectrometry in combination with density functional theory calculations. It is shown that double protonation of the prototype hydrocarbon triradical 1,3,5-trimethylenebenzene (C9H9), similar to single protonation, leads to relative stabilization of the doublet state so that it becomes the ground state of the radical dication. The preferred structures of the C9H112+ manifold are best described as primary, secondary, or tertiary alkyl cations attached to a benzene radical-cation core. The most abundant fragmentation corresponds to dehydrogenation. The also observed charge-separation fragmentations of the dications can be rationalized on the basis of the preferred molecular structures. In addition, a brief comparison of the dicationic systems C7H72+, C8H92+, and C9H112+ reveals that while the larger aromatic dications do indeed bear a structural memory, the lower homologues C7H72+ and C8H92+ completely lose the structural identity of their neutral precursors.