Modelling the signatures of interstellar polycyclic aromatic hydrocarbons with quantum chemistry

Abstract

A posteriori calculations that reproduce already well-documented experiments are shown to be a prerequisite to a priori calculations of unknown spectra. Anticipating the ISO observations to come, we applied this strategy to the prediction of the IR signatures of molecular ions derived from polycyclic aromatic hydrocarbons (PAHs). After calibration on naphthalene and verification on pyrene, two points of important astrophysical interest are addressed: (1) the infrared signature of doubly ionized species (PAH++) which might exist in regions submitted to strong radiation fields, using pyrene dication as a model compound, and (2) the infrared signature of organometallic compounds [PAH-M]+ formed by the sticking of transition metal ions on aromatic grains, taking the anthracene-Fe+ complex as an example. It is anticipated that the spectra of doubly ionized PAHs should show only weak features around 3000 cm-1, but that a set of high intensity bands can be expected in the 12001500 cm-1 region as in singly ionized PAHs. The same should be true of [PAH-M]+ when the formation of the complex results in a transfer of the positive charge of the ion to the organic moiety. An interesting point found in this study is that the vibrations with the strongest intensities in the complex are closer in position to the PAH neutral bands than to the PAH cation bands.