Collision‐induced fragmentation reactions of doubly charged ions of polycyclic aromatic hydrocarbons

Unimolecular and collision‐induced reactions of molecular dications M 2 + of polycyclic aromatic hydrocarbons (PAHs) have been investigated. For larger PAHs (three or more rings) charge‐separation reactions, and both dissociative and non‐dissociative electron‐capture collisions, are less important. In both low energy (0‐400 eV) and high energy (16 keV) collision regimes, the dominant processes are expulsion of hydrogen atoms and of neutral acetylene molecules. In the high‐energy collision regime, neutral free radicals are also expelled from these M 2 + precursors. Some degree of isomer differentiation is possible via relative intensities of doubly charged fragment ions in the low‐energy collision regime, but not all isomers can be thus distinguished and it seems unlikely that such a scheme could be made predictive. The CID spectra are largely independent of precursor size or structure at lower masses ( m / z ⩽ 70). These findings are reviewed in the light of literature data on expulsion of small neutral species from aromatic dications. It is proposed that the charge‐separation reactions characteristic of smaller aromatic systems are charge‐site‐initiated cleavages, while expulsion of small neutral species such as acetylene is a radical‐site‐initiated reaction from M 2 + ions possessing considerable diradical character. In all cases an important driving force is the avoidance of anti‐aromatic character for a benzene ring from which two electrons have been removed. The similarity of the CID spectra at low masses is analogous to that discussed recently (S. J. Pachuta et al., J. Am. Chem. Soc. 110, 657 (1988)) for fragmentations of the corresponding singly charged molecular radical cations.