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The 11.3 $\mu$m emission feature is a prominent member of the family ofunidentified infrared emission (UIE) bands and is frequently attributed toout-of-plane bending modes of polycyclic aromatic hydrocarbon (PAH) molecules.We have performed quantum mechanical calculations of 60 neutral PAH moleculesand found that it is difficult to reconcile the observed astronomical featurewith any or a mix of these PAH molecules. We have further analyzed the fittingof spectra of several astronomical objects by the NASA PAH database program andfound that reasonable fittings to the observed spectra are only possible byincluding significant contributions from oxygen and/or magnesium containingmolecules in the mix. A mixed of pure PAH molecules, even including units ofdifferent sizes, geometry and charged states, is unable to fit the astronomicalspectra. Preliminary theoretical results on the vibrational spectra of simplemolecules with mixed aromatic/aliphatic structures show that these structureshave consistent bundles of vibrational modes and could be viable carriers ofthe UIE bands.

On the origin of the 11.3 micron unidentified infrared emission feature