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

Abstract

Unimolecular and collision-induced reactions of molecular dications M2 + of polycyclic aromatic hydrocarbons (PAHs) have been investigated. For larger PAHs (three or more rings) charge-separation reactions, and both dissociative and non-dissociative electroncapture 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 M2+ 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 radicalsite-initiated reaction from M3+ 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.

Bond-formation versuselectron transfer: C–C-coupling reactions of hydrocarbon dications with benzene

“…In addition to the C 7 H n 2+ dications (n = 6-8) derived from toluene, 20,21 the reactions of some larger hydrocarbon dications were investigated. 22,23 As a precursor for C 8 H n 2+ dications, only ethylbenzene has been investigated because C 8 H n 2+ ions (n = 8-10) generated from a variety of precursors appear to rearrange to a common structure (or the same mixture of ions). 24 As an example for probing the reactions of C 9 H n 2+ dications, indene was used as a precursor which easily affords C 9 H n 2+ (n = 6-8) upon double ionization.…”

“…These reactions are denoted as ''dication dissociation'' in Table 1, and they lead to dehydrogenations (both losses of atomic as well as molecular hydrogen in several cases) and expulsions of acetylene, thereby affording the corresponding fragment ions which still carry a two-fold charge. 22 (ii) Electron transfer from neutral benzene to the C m H n 2+ projectile dication affords two sets of monocations which can either be due to non-dissociative or dissociative electron transfer (denoted as ET and DET in Table 1) and are reported separately for the projectile dication and the monocation products formed from the neutral target gas benzene. (iii) In the case of hydrogen-containing dications, proton transfer from the dication to the neutral reagent can often be of considerable importance 9 and is therefore reported in a separate column (denoted as PT in Table 1).…”

“…Finally, (iv) the bond-forming reactions under maintenance of the two-fold charge lead to the growth of the C m H n 2+ projectiles in that stabilization of the initially formed C m+6 H n+6 2+* intermediate is achieved by expulsion of either atomic and/or molecular hydrogen or loss of neutral acetylene. 1,22,25 As these processes lead to the growth of the C m H n 2+ precursors by C 6 H n and C 4 H n -units, respectively, they are accordingly denoted as bond formation in the last column of Table 1. In the evaluation of the data, the available knowledge on the dissociation of the corresponding monocations was taken into account in order to identify the channels which can be attributed to DET.…”

From physical chemistry to mass spectrometry to government lab manager in half a century

“…One line of work was an extension of our “half‐assed” attempt to exploit the abundant doubly‐charged molecular ions of PAHs for selective analyses in complex mixtures. In a series of investigations (Sim, Jamieson, & Boyd, ; Perreault et al, ; Perreault et al, ) of the CID reactions of M 2+ ions of PAHs and their heterocyclic (nitrogenous) analogs we found a qualitative difference between smaller species (<100 Da) and larger ones. The fragmentations of the smaller systems are dominated by charge‐separation and charge‐exchange processes, but the larger ones exhibit a preference for expulsion of small neutral species (H‐atoms and molecules of acetylene and/or HCN in the case of the heterocycles).…”