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The mid-IR spectra of six large, irregular PAHs with formulae (C84H24 -C120H36) have been computed using Density Functional Theory (DFT). Trends inthe dominant band positions and intensities are compared to those of large,compact PAHs as a function of geometry, size and charge. Irregular edgemoieties that are common in terrestrial PAHs, such as bay regions and ringswith quartet hydrogens, are shown to be uncommon in astronomical PAHs. As forall PAHs comprised solely of C and H reported to date, mid-IR emission fromirregular PAHs fails to produce a strong CCstr band at 6.2 um, the positioncharacteristic of the important, class A astronomical PAH spectra. Earlierstudies showed inclusion of nitrogen within a PAH shifts this to 6.2 um for PAHcations. 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 ofthe CC stretch near 6.2 um appears to directly reflect the PAH cation to anionratio. Large irregular PAHs exhibit features at 7.8 um but lack them near 8.6um. Hence, the 7.7 um astronomical feature is produced by a mixture of smalland large PAHs while the 8.6 um band can only be produced by large compactPAHs. As with the CCstr, the position and profile of these bands reflect thePAH cation to anion ratio.

astrophysical journal/the astrophysical journal

The Infrared Spectra of Very Large Irregular Polycyclic Aromatic  Hydrocarbons (PAHs): Observational Probes of Astronomical PAH Geometry, Size  and Charge