A Theoretical Study on the Vibrational Spectra of PAH Molecules with
  Aliphatic Sidegroups
  The role of aliphatic side groups on the formation of astronomical
unidentified infrared emission (UIE) features is investigated by applying the
density functional theory (DFT) to a series of molecules with mixed
aliphatic-aromatic structures. The effects of introducing various aliphatic
groups to a fixed polycyclic aromatic hydrocarbon (PAH) core (ovalene) are
studied. Simulated spectra for each molecule are produced by applying a Drude
profile at $T$=500 K while the molecule is kept at its electronic ground state.
The vibrational normal modes are classified using a semi-quantitative method.
This allows us to separate the aromatic and aliphatic vibrations and therefore
provide clues to what types of vibrations are responsible for the emissions
bands at different wavelengths. We find that many of the UIE bands are not pure
aromatic vibrational bands but may represent coupled vibrational modes. The
effects of aliphatic groups on the formation of the 8 $\mu$m plateau are qua
ntitatively determined. The vibrational motions of methyl ($-$CH$_3$) and
methyl ene ($-$CH$_2-$) groups can cause the merging of the vibrational bands
of the pa rent PAH and the forming of broad features. These results suggest
that aliphatic structures can play an important role in th e UIE phenomenon.
