Isomers of Hydrogenated Polycyclic Aromatic Hydrocarbons Explain the Presence of Infrared Bands in the 3μm Region

A large number of hydrogenated coronene, circumcoronene, and circumcircumcoronene isomers have been investigated by means of density functional theory calculations. The computation of their IR spectra puts forward significant differences between the different isomers in the 3 mu m region and shows that hydrogenated polycyclic aromatic hydrocarbons can account for the aromatic infrared bands resulting from the absorption of light by the interstellar medium. In particular, the intense 3.3 mu m band, as well as the weak features observed at 3.40, 3.46, 3.51, and 3.56 mu m are reasonably reproduced by the present calculations. The latter two are only observed when hydrogenation takes place in tertiary carbon atoms, showing that the frequencies at which these IR bands appear are a consequence of their position and chemical environment within the molecule.