2009ApJ...697..311B

The mid-infrared (IR) spectra of six large, irregular polycyclic aromatic hydrocarbons (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 CC_str band at 6.2 µm, the position characteristic of the important, class A astronomical PAH spectra. Earlier studies showed that inclusion of nitrogen within a PAH shifts this to 6.2 µm for PAH cations. Here we show that this band shifts to 6.3 µm 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 µm appears to directly reflect the PAH cation to anion ratio. Large irregular PAHs exhibit features at 7.8 µm but lack them near 8.6 µm. Hence, the 7.7 µm astronomical feature is produced by a mixture of small and large PAHs while the 8.6 µm band can only be produced by large compact PAHs. As with the CC_str, the position and profile of these bands reflect the PAH cation to anion ratio.