THEORETICAL INFRARED-SPECTRA OF SOME MODEL POLYCYCLIC AROMATIC-HYDROCARBONS - EFFECT OF IONIZATION

In order to test the hypothesis of ionized polycyclic aromatic hydroca rbons (PAHs) as possible carriers of the UIR bands, we realized a comp utational exploration on selected PAHs of small dimension in order to identify which changes ionization would induce on their IR spectra. In this study we performed ab initio calculations of the spectra of neut ral and positively ionized naphthalene, anthracene, and pyrene. The re sults are significantly important. The frequencies in the cations are slightly shifted with respect to the neutral species, but no general c onclusion can be reached from the three molecules considered. By contr ast, the relative intensities of most vibrations are strongly affected by ionization, leading to a much better agreement between the calcula ted CH/CC vibration intensity ratios and those deduced from observatio ns.