Multiple ionization, charge separation and charge stripping reactions involving polycyclic aromatic compounds



The 70 eV electron ionization mass spectra of polycyclic aromatic compounds are characterized by the presence of relatively stable multiply charged molecular ions [M]n+ (n=2–4). When generated from the compounds benzene, napthalene, anthracene, phenanthrene, 2,3-benzanthracene, 1,2-benzanthracene, chrysene, 9,10-benzophenanthrene and pyrene, the relative abundances of the multiply charged ions increase dramatically with the number of rings. These compounds form multiply charged molecular ions (n=2, 3) which undergo unimolecular decompositions indicative of considerable ionic rearrangement. The main charge separation processes observed here [M]2+→m1++m2+, [M]3+˙→m3++m→+m42+) involve, in almost every case, one or more of the products [CH3]+, [C2H3]+˙ and [C3H3]+. This suggests the existence of preferred structures amongst the metastable parent ions. Information on the relative importance of the various fragmentation pathways is presented here along with translational energy release data. Some tentative structural information about the metastable ions has been inferred from the translational energy release on the assumption that the released energy is due primarily to coulombic repulsion within the transition state structure. For the triply charged ions these interpretations have necessitated the use of a coulombic repulsion model which takes account of an extra charge. Vertical ionization energies for the process [M]n++G→[M](n+1)+G+e (charge stripping) have also been determined where possible for n=1 and 2 and the results from these experiments allow the derivation of simple empirical equations which relate successive ionization energies for the formation of [M]2+ and [M]3+˙ to the appearance energy of [M]+˙.