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Fullerenes

Although fullerenes have long been hypothesized to occur in interstellar environments, their actual unambiguous spectroscopic identification is of more recent date. FELIX researchers have recorded the first laboratory infrared spectrum of gaseous C60H+ and C70H+.

C60, C70 and C60+ now constitute the largest molecular species individually identified in the interstellar medium (ISM). Fullerenes have significant proton affinities and it was Sir Harold Kroto who suggested that C60H+ is likely the most abundant interstellar analogue of C60.

FELIX researchers have recorded the first laboratory infrared (IR) spectrum of gaseous C60H+. The added proton breaks the high symmetry of C60 producing a much richer IR spectrum, forming a textbook example of the influence of molecular symmetry on IR spectroscopy. The experimental spectrum is used to benchmark theoretical spectra indicating that the B3LYP density functional with the 6-311+G(d,p) basis set reproduces the spectrum most closely. Moreover, and perhaps most interesting, comparison with astronomical spectra from two planetary nebulae that have been associated with high abundances of neutral C60 indicate that C60H+ is a plausible contributor to their IR emission, possibly confirming Kroto’s hypothesis.

C60H+

The laboratory IR spectrum of C60H+. The background shows the Orion Nebula. © Daniël Rap.

As compared to C60, where all C-atoms are equivalent due to the icosahedral symmetry, C70 belongs to the D5h point group and has five nonequivalent C-atoms, which are available as protonation sites. Combined analysis of the experimental spectrum and spectra computed at the density functional theory level enables us to evaluate the protonation isomers being formed

References

The infrared spectrum of protonated buckminsterfullerene C60H+ Julianna Palotás, Jonathan Martens, Giel Berden, and Jos Oomens, Nature Astronomy 4, 240-245 (2020).  PDF free download on arXiv.

The infrared spectrum of protonated buckminsterfullerene C70H+, Julianna Palotás, Jonathan Martens, Giel Berden, and Jos Oomens, Astrophyiscal Journal Letters 909, L17 (2021). PDF free download (open access).