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