Infrared spectroscopy of coronene+ and C60+ resolving 13C isotopologues

Infrared signatures of carbonaceous molecules in space vary among different lines of sight and astronomical objects, providing information on the local ratio of 13C-12C. The interpretation of these observations relies on high-resolution laboratory spectra capable of resolving isotopic signatures. However, such spectroscopic data are largely unavailable for larger carbonaceous molecules, such as fullerenes and polycyclic aromatic hydrocarbons. Here, we report infrared spectra of C60+ and coronene cations (C24H12+) in the fingerprint region (760-1440 cm-1) using cryogenic helium-tagging spectroscopy. In both systems, infrared spectra of 13C-substituted isotopologues occurring at natural terrestrial abundance were measured, allowing their distinct spectral signatures to be resolved. While C60+ exhibits small isotope-induced redshifts and weak band broadening upon 13C substitution, coronene cations show additional isotope-dependent splittings in several vibrational bands. This is particularly evident in the case of a narrow, out-of-plane vibrational mode of coronene for both singly and doubly 13C substituted ions, where the contributions of different isotopomers can be clearly identified. All findings are supported by quantum chemical calculations. The results demonstrate that the presence of 13C-containing isotopologues can lead to resolvable vibrational structure in large carbonaceous ions, affecting both peak positions and line shapes, and may therefore be relevant for the interpretation of astronomical infrared spectra.