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

TL;DR

This study uses DFT calculations to analyze the IR spectra of large irregular PAHs, revealing how their geometry, size, charge, and nitrogenation influence astronomical IR features and the PAH cation-anion ratio.

Contribution

It provides new insights into how irregular PAHs and nitrogenation affect IR spectral features relevant to astronomical observations.

Findings

Irregular PAHs lack strong 6.2 um CC stretch in pure C-H forms.

Nitrogenated PAH anions shift the CC stretch to 6.3 um, similar to class B spectra.

The 7.7 um feature arises from a mix of small and large PAHs, while 8.6 um is from large compact PAHs.

Abstract

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.