  The mid-infrared spectra of large PAHs ranging from C54H18 to C130H28 are
determined computationally using Density Functional Theory. Trends in the band
positions and intensities as a function of PAH size, charge and geometry are
discussed. Regarding the 3.3, 6.3 and 11.2 micron bands similar conclusions
hold as with small PAHs.
  This does not hold for the other features. The larger PAH cations and anions
produce bands at 7.8 micron and, as PAH sizes increases, a band near 8.5 micron
becomes prominent and shifts slightly to the red. In addition, the average
anion peak falls slightly to the red of the average cation peak. The similarity
in behavior of the 7.8 and 8.6 micron bands with the astronomical observations
suggests that they arise from large, cationic and anionic PAHs, with the
specific peak position and profile reflecting the PAH cation to anion
concentration ratio and relative intensities of PAH size. Hence, the broad
astronomical 7.7 micron band is produced by a mixture of small and large PAH
cations and anions, with small and large PAHs contributing more to the 7.6 and
7.8 micron component respectively.
  For the CH out-of-plane vibrations, the duo hydrogens couple with the solo
vibrations and produce bands that fall at wavelengths slightly different than
their counterparts in smaller PAHs. As a consequence, previously deduced PAH
structures are altered in favor of more compact and symmetric forms. In
addition, the overlap between the duo and trio bands may reproduce the
blue-shaded 12.8 micron profile.
