First IR spectrum protonated C70 reported

Sir Harry Kroto, astronomer and recipient of the 1996 Nobel Prize in Chemistry for the discovery of C₆₀, predicted 30 years ago that protonated fullerenes are among the most abundant fullerene derivatives in interstellar environments. Julianna Palotás and her coworkers at the FELIX Laboratory have now recorded the first IR spectrum of C₇₀H⁺. This spectral signature may guide astronomers in the search for these species, to corroborate Kroto’s predictions.

In 2019, we published the first IR spectrum of C₆₀H⁺ in Nature Astronomy. All 60 carbon atoms in C₆₀ are equivalent, so that there exists only one isomer of C₆₀H⁺ (excluding protonation on the inside of the cage as indicated by the experimental spectrum). In contrast, the proton in C₇₀H⁺ may be located on one of five distinct carbon atoms. The present experimental IR spectrum in combination with DFT computed spectra for the five protonation isomers reveals unambiguously which of these carbon atoms carries the proton (see figure).

The Infrared Spectrum of Protonated C₇₀, Julianna Palotás, Jonathan Martens, Giel Berden, Jos Oomens, The Astrophysical Journal Letters (2021), volume 909, article L17. (open access, free download of pdf)