CH Bond Dissociation Energies of Polycyclic Aromatic Hydrocarbon Molecular Cations: Theoretical Interpretation of the (M‐1)+ Peak in the Mass Spectra

PM3 molecular orbital calculations were carried out to estimate CH bond dissociation energies of 13 polycyclic aromatic hydrocarbon (PAH) molecular cations. The dissociation energy of a CH bond turned out to be almost independent of the size of the molecular cation and the environment in the cation. Therefore, when a high-energy electron impacts a PAH molecule, all hydrogen atoms must be detached with almost equal probability from the resulting molecular cation. It follows that the [M - 1] + peak in the mass spectra of PAH molecules cannot be assigned to any particular arylium ion. The highest and the next highest occupied π-molecular orbital in some PAH molecular cations are exchanged on dehydrogenation.