Closed-shell polycyclic aromatic hydrocarbon cations: a new category of interstellar polycyclic aromatic hydrocarbons

Density functional theory has been employed to calculate the harmonic frequ encies and intensities of a range of polycyclic aromatic hydrocarbon (PAH) cations that explore both size and electronic structure effects on the infr ared spectroscopic properties of these species. The sample extends the size range of PAH species considered to more than 50 carbon atoms and includes several representatives from each of two heretofore unexplored categories o f PAH cations: (1) fully benzenoid PAH cations whose carbon skeleton is com posed of an odd number of carbon atoms (C-odd PAHs); and (2) protonated PAH cations (HPAH(+)). Unlike the radical electronic structures of the PAH cat ions that have been the subject of previous theoretical and experimental wo rk, the species in these two classes have a 'closed'-shell electronic confi guration. The calculated spectra of circumcoronene, C54H18, in both neutral and (radical) cationic form are also reported and compared with those of t he other species. Overall, the C-odd PAHs spectra are dominated by strong C C stretching modes near 1600 cm(-1) and display spectra that are remarkably insensitive to molecular size. The HPAH+ species evince a more complex spe ctrum consistent with the added contributions of aliphatic modes and their generally lower symmetry. Finally, for both classes of closed-shell cations , the intensity of the aromatic CH stretching modes is found to increase wi th molecular size far out of proportion with the number of CH groups, appro aching a value more typical of neutral PAHs for the largest species studied . (C) 2001 Published by Elsevier Science B.V.