The recent detection of fullerene (C₆₀) in space and the positive assignment of five diffuse interstellar bands to reinforce the notion that fullerene-related compounds can be efficiently formed in circumstellar envelopes and be present in significant quantities in the interstellar medium. Experimental studies have shown that C₆₀ can be readily hydrogenated, raising the possibility that hydrogenated fullerenes (or fulleranes, C₆₀H _m , m = 1–60) may be abundant in space. In this paper, we present theoretical studies of the vibrational modes of isomers of C₆₀H _m . Our results show that the four mid-infrared bands from the C₆₀ skeletal vibrations remain prominent in slightly hydrogenated C₆₀, but their strengths diminish in different degrees with increasing hydrogenation. It is therefore possible that the observed infrared bands assigned to C₆₀ could be due to a mixture of fullerenes and fulleranes. This provides a potential explanation for the observed scatter of the C₆₀ band ratios. Our calculations suggest that a feature around 15 μm due to the breathing mode of heavily hydrogenated C₆₀ may be detectable astronomically. A preliminary search for this feature in 35 C₆₀ sources is reported.

The 3.3 μm unidentified infrared emission feature is commonly attributed to the C–H stretching band of aromatic molecules. Astronomical observations have shown that this feature is composed of two separate bands at 3.28 and 3.30 μm, and the origin of these two bands is unclear. In this paper, we perform vibrational analyses based on quantum mechanical calculations of 153 organic molecules, including both pure aromatic molecules and molecules with mixed aromatic/olefinic/aliphatic hydridizations. We find that many of the C–H stretching vibrational modes in polycyclic aromatic hydrocarbon (PAH) molecules are coupled. Even considering the uncoupled modes only, the correlation between the band intensity ratios and the structure of the PAH molecule is not observed, and the 3.28 and 3.30 μm features cannot be directly interpreted in the PAH model. Based on these results, the possible aromatic, olefinic, and aliphatic origins of the 3.3 μm feature are discussed. We suggest that the 3.28 μm feature is assigned to aromatic C–H stretch whereas the 3.30 μm feature is olefinic. From the ratio of these two features, the relative olefinic to aromatic content of the carrier can be determined.

An unidentified infrared emission (UIE) feature at 6.0 μm is detected in a number of astronomical sources showing the UIE bands. In contrast to the previous suggestion that this band is due to C=O vibrational modes, we suggest that the 6.0 μm feature arises from olefinic double-bond functional groups. These groups are likely to be attached to aromatic rings, which are responsible for the major UIE bands. The possibility that the formation of these functional groups is related to the hydrogenation process is discussed.

The presence of neutral C₆₀ fullerenes in circumstellar environments has been firmly established by astronomical observations as well as laboratory experiments and quantum-chemistry calculations. However, the large variations observed in the C₆₀ 17.4 μm/18.9 μm band ratios indicate that either additional emitters should contribute to the astronomical infrared (IR) spectra or unknown physical processes exist besides thermal and UV excitation. Fullerene-based molecules such as metallofullerenes and fullerene-adducts are natural candidate species as potential additional emitters, but no specific specie has been identified to date. Here we report a model based on quantum-chemistry calculations and IR spectra simulation of neutral and charged endo(exo)hedral metallofullerenes, showing that they have a significant contribution to the four strongest IR bands commonly attributed to neutral C₆₀. These simulations may explain the large range of 17.4 μm/18.9 μm band ratios observed in very different fullerene-rich circumstellar environments like those around planetary nebulae and chemically peculiar R Coronae Borealis stars. Our proposed model also reveals that the 17.4 μm/18.9 μm band ratio in the metallofullerenes simulated IR spectra mainly depends on the metal abundances, ionization level, and endo/exoconcentration in the circumstellar envelopes. We conclude that metallofullerenes are potential emitters contributing to the observed IR spectra in fullerene-rich circumstellar envelopes. Our simulated IR spectra indicate also that the James Webb Space Telescope has the potential to confirm or refute the presence of metallofullerenes (or even other fullerene-based species) in circumstellar environments.

We demonstrate that by combining two robust theoretical quantum chemistry calculation techniques, stepwise ionization of C₆₀ fullerene by UV and extreme UV photons can in principle occur up to a limit as high as C₆₀²⁶⁺ before coulomb explosion of the cage. Furthermore, these highly ionized forms exhibit a comparable structural and bonding stability as for the neutral fullerene. Certain astrophysical sources like the central stars of planetary nebulae and the hottest white dwarf stars have sufficiently hard UV radiation fields that can result in a series of highly charged C₆₀^q+ species from q = 1 up to q = 16. Harsher environments, like hot X-ray bubbles in planetary nebulae, X-ray binaries and other sources, may further push the ionization right up to the q = 26 limit. These remarkable theoretical findings add new avenues to complex ion/molecule reactions, the chemistry of fragmentation products and additional pathways for spreading carbon throughout the universe. The implications for the emerging field of astrochemistry of C₆₀ fullerene in all its possible states could be profound.