Peculiarities in the infrared emission of polycyclic aromatic hydrocarbon-C60 adducts

Abstract

[Abstract] The coexistence of polycyclic aromatic hydrocarbons (PAHs) and the C60 fullerene in different astrophysical environments can give rise to the formation of new complex species denoted as PAH-C60 adducts, which may contribute to the infrared (IR) emission observed. These PAH-C60 adducts have previously been reported experimentally due to the high reactivity between PAHs and C60. From an astrophysical point of view, however, they have not been considered in detail yet. Here, we performed a combined experimental and theoretical study in order to characterize the IR spectra of PAH-C60 adducts, including multiple adducts. Using new advanced experimental techniques, we were able to synthesize some specific PAH-C60 adduct isomers and measure their IR spectra. These experimental data were used to correct their harmonic scaled spectra, as obtained from quantum-chemistry calculations performed at the density functional theory (DFT) level under the B3LYP-GD3/6-31+G(d) approach. This way, we simulated the IR (∼3−25 µm) spectra of multiple PAH-C60 adducts, composed by a different number of PAH units: mostly one or two units. In addition, the chemical kinetics data available in the literature were used to tentatively estimate the possible order of magnitude of the abundances of these PAH-C60 adducts using the available observational data. Essentially, our results revealed a possible strong modification of the IR spectra when astronomically estimated abundances are considered. Several spectral peculiarities are observed, such as a broad ∼3.4–3.6 µm feature, and important modifications in the 6–10 and 12–16 µm spectral regions together with contributions to the C60 features at 7.0 and 18.9 µm. Interestingly, these PAH-C60 adducts lack aliphatic CH bonds, but they display IR features around 3.4 µm, challenging previous interpretations of this astronomical feature.