The Infrared Spectra of Very Large Irregular Polycyclic Aromatic
  Hydrocarbons (PAHs): Observational Probes of Astronomical PAH Geometry, Size
  and Charge
  The mid-IR spectra of six large, irregular PAHs with formulae (C84H24 -
C120H36) have been computed using Density Functional Theory (DFT). Trends in
the dominant band positions and intensities are compared to those of large,
compact PAHs as a function of geometry, size and charge. Irregular edge
moieties that are common in terrestrial PAHs, such as bay regions and rings
with quartet hydrogens, are shown to be uncommon in astronomical PAHs. As for
all PAHs comprised solely of C and H reported to date, mid-IR emission from
irregular PAHs fails to produce a strong CCstr band at 6.2 um, the position
characteristic of the important, class A astronomical PAH spectra. Earlier
studies showed inclusion of nitrogen within a PAH shifts this to 6.2 um for PAH
cations. Here we show this band shifts to 6.3 um in nitrogenated PAH anions,
close to the position of the CC stretch in class B astronomical PAH spectra.
Thus nitrogenated PAHs may be important in all sources and the peak position of
the CC stretch near 6.2 um appears to directly reflect the PAH cation to anion
ratio. Large irregular PAHs exhibit features at 7.8 um but lack them near 8.6
um. Hence, the 7.7 um astronomical feature is produced by a mixture of small
and large PAHs while the 8.6 um band can only be produced by large compact
PAHs. As with the CCstr, the position and profile of these bands reflect the
PAH cation to anion ratio.
