The 3.2-3.5 μm region revisited - I. A theoretical study of the effects of aliphatic substitution on some model PAHs

Three attractive hypotheses have been proposed to interpret the origin of the satellites of the 3.3-mu m band, and especially the most prominent feature at 3.4 mu m, in connection with polycyclic aromatic hydrocarbons (PAHs): first, the hot bands hypothesis; secondly, the side groups hypothesis; thirdly, the perhydrogenation hypothesis. We considered the first hypothesis in a previous paper. Here we focus on the second one and report ab initio quantum mechanical studies of the 3.2-3.5 mu m region using adapted model compounds with -CH_3, -C_2H_5 and = CH_2 lateral substituents. Three features at 3.2 mu m (vinylic hydrogens), 3.4 mu m (aliphatic hydrogens) and 3.5 mu m (overtone) are predicted by theory. The side group hypothesis is consistent with two of the observed features at 3.4 and 3.5 mu m; the non-observation of a minor feature at 3.2 mu m shows that = CH_2 side groups should be rare in the interstellar medium, in agreement with previous photochemical modelling. The present calculations also suggest that another type of carrier, different from those considered in the present study, seems necessary to account for the observed intensity of the 3.5-mu m feature. Within the actual accuracy of the calculations, ionization does not make a significant difference for the existence of the satellite bands, but might contribute to the variations observed in the shape of the bands.