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.

💡 Research Summary

This study investigates how the geometry, size, and charge state of polycyclic aromatic hydrocarbons (PAHs) influence their mid‑infrared (mid‑IR) spectra, with a focus on the astronomical PAH emission bands at 6.2, 7.7, and 8.6 µm. Six large, irregular PAHs ranging from C₈₄H₂₄ to C₁₂₀H₃₆ were modeled using density functional theory (DFT) at the B3LYP/4‑31G* level. For each molecule, neutral, cationic (+1), and anionic (–1) charge states were optimized, and harmonic vibrational frequencies were scaled by 0.967 to compare with laboratory data. In addition, nitrogen‑substituted analogues (N‑PAHs) were calculated to assess the effect of heteroatom incorporation.

The structural analysis shows that the irregular PAHs lack the “bay” regions and quartet‑hydrogen edge sites that are common in many terrestrial PAHs. Consequently, the characteristic 12–13 µm out‑of‑plane bending modes associated with such edge structures are weak or absent, suggesting that astronomical PAHs are dominated by more compact edge configurations.

Spectral results reveal systematic trends. Pure carbon‑hydrogen PAHs do not generate a strong C–C stretching band near 6.2 µm; instead, the dominant CC stretch appears around 6.3 µm and its intensity is modest. Charge state strongly modulates this band: cations shift the CC stretch toward shorter wavelength (≈6.2 µm), while anions shift it to longer wavelength (≈6.3 µm). Introducing a single nitrogen atom produces the same charge‑dependent shift, but the band intensities become comparable to the observed astronomical features. Thus, the distinction between class A (6.2 µm) and class B (6.3 µm) PAH spectra can be interpreted as a diagnostic of the PAH cation‑to‑anion ratio in a given environment.

The 7.7 µm complex, composed of sub‑features near 7.6 µm and 7.8 µm, behaves differently. Irregular PAHs display a pronounced 7.8 µm component but lack a significant 8.6 µm band (≈1160 cm⁻¹). Large, compact PAHs, by contrast, exhibit a strong 8.6 µm band and a more balanced 7.6/7.8 µm contribution. This indicates that the 8.6 µm astronomical feature originates almost exclusively from large, compact PAHs, whereas the 7.7 µm feature requires a mixture of small (≈30–50 carbon atoms) and large (>100 carbon atoms) PAHs.

Charge effects on band positions are quantitatively consistent with electronic structure considerations: cations have reduced electron density, leading to slightly shorter C–C bonds and higher vibrational frequencies; anions have excess electron density, lengthening bonds and lowering frequencies. Consequently, the precise peak positions and profiles of the 6.2/6.3, 7.7, and 8.6 µm bands serve as direct probes of the PAH ionisation balance.

Comparisons with astronomical observations across diverse environments (photodissociation regions, planetary nebulae, reflection nebulae, and the diffuse interstellar medium) support these conclusions. Regions with intense UV radiation and high electron densities show dominant class A 6.2 µm bands, consistent with a PAH population skewed toward cations. Conversely, environments with softer radiation fields exhibit class B 6.3 µm bands, indicating a larger fraction of anions. The breadth of the 7.7 µm feature correlates with the presence of both small and large PAHs, while a sharp 8.6 µm peak signals a substantial population of large, compact PAHs.

In summary, the paper establishes that (1) irregular PAHs are a minor component of the interstellar PAH inventory; (2) nitrogen incorporation is essential for reproducing the strong 6.2 µm CC stretch, and the exact wavelength depends on charge state; (3) the 7.7 µm emission arises from a size‑diverse PAH mixture, whereas the 8.6 µm band is a signature of large, compact PAHs; and (4) the relative abundances of PAH cations and anions can be inferred directly from the positions and shapes of the mid‑IR bands. These insights provide a robust framework for interpreting PAH infrared emission and for constraining the physical conditions of the astrophysical environments in which they reside.