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.

The thermal decomposition of C₆₀Br₂₄•2Br₂ has been studied by Differential Scanning Calorimetry (DSC) at different heating rates. Depending from the heating rate, the thermal decomposition peak occurred between 163 °C and 201 °C with an activation energy E^# = 108.6 kJ/mol. The decomposition products are C₆₀ (assessed by FT-IR) and molecular bromine. From the decomposition enthalpy measured by DSC, the enthalpy of formation ΔH_f°(C₆₀Br_24(c)) = 2114 ± 34 kJ/mol has been determined. By group increment or group additivity method ΔH_f°(C₆₀Br_{24 (g)}) = 2184 kJ/mol has been calculated, in fair agreement with the experimental value. Furthermore, through the group increment method ΔG_f°(C₆₀Br_{24 (g)}) = 3264 kJ/mol has been estimated. The enthalpy of the C-Br bond in the C₆₀Br₂₄ molecule was found comprised between 108 and 120 kJ/mol.

Small metal-containing molecules have been detected and recognized as one of the hybrid species that are efficiently formed in space, especially in the circumstellar envelopes of evolved stars. It has also been predicted that more complex hybrid species such as those formed by metals and fullerenes (metallofullerenes) could be present in these circumstellar environments. Recently, quantum-chemical simulations of metallofullerenes have shown that they are potential emitters contributing to the observed mid-IR spectra in the fullerene-rich circumstellar environments of different types of evolved stars. Here we present the individual simulated mid-IR (∼5–50 μm) spectra of 28 metallofullerene species. Both neutral and charged endo- and exohedral metallofullerenes for seven different metals (Li, Na, K, Ca, Mg, Ti, and Fe) have been considered. The changes induced by the metal–C₆₀ interaction in the intensity and position of the spectral features are highlighted using charge-density difference maps and electron-density partitioning. Our calculations identify the fundamental IR spectral regions in which, depending on the metal binding nature, there should be a major spectral contribution from each of the metallofullerenes. The IR spectra of the metallofullerenes are made publicly available to the astronomical community, especially users of the James Webb Space Telescope, for comparisons that could eventually lead to the detection of these species in space.

Several planetary nebulae (PNe) have been found to contain both polycyclic aromatic hydrocarbon (PAH-like) species and fullerenes (C₆₀) distinguished by their mid-infrared emission. Previous laboratory and astronomical studies suggest that the formation of both species could be related to the decomposition, by photochemical processing, of hydrogenated amorphous carbon (HAC) grains. Then, HACs and, seemingly, big-fullerene related species (e.g. carbon onions) have been suggested as potential carriers of the ultraviolet (UV) bump at 2175 Å and the far-UV rise common to interstellar extinction curves. Our goal is to investigate the UV bump with the possible presence of an HAC extinction component in the International Ultraviolet Explorer (IUE) spectra of C-rich PNe; both with detected and non-detected fullerenes. The considered sample includes three C₆₀-PNe (Tc 1, IC 418, and IC 2501) and the non-C₆₀-PN Hen 2-5. Independently of the presence of C₆₀ in their circumstellar envelopes, we found that the UV bump in all sample PNe is well explained by interstellar extinction, suggesting that species different from those of the foreground interstellar medium, e.g. large fullerene-related species like carbon onions, are not the carrier. Interestingly, we found that PNe Tc 1 and Hen 2-5 show an absorption in the FUV rise. Their IUE continuum spectra may be very well reproduced by including the extinction curve of HAC-like very small grains. The possible presence of both species, HAC-like grains and fullerenes (C₆₀), in Tc 1 could support the HAC photochemical processing scenario for the formation of fullerenes in the complex circumstellar envelopes of PNe.