The infrared spectrum of protonated buckminsterfullerene C₆₀H⁺

Although fullerenes have long been hypothesized to occur in interstellar environments, they have only recently been unambiguously identified through spectroscopy. C₆₀, C₇₀ and C₆₀⁺ now constitute the largest molecular species individually identified in the interstellar medium. Fullerenes have substantial proton affinities and it has been suggested that C₆₀H⁺ is likely the most abundant interstellar analogue of C_60. We present here a laboratory infrared (IR) spectrum of gaseous C₆₀H⁺. Symmetry breaking in C₆₀H⁺ produces an IR spectrum that is much richer than that of C₆₀. The experimental spectrum is used to benchmark theoretical spectra indicating that the B3LYP density functional with the 6-311+G(d,p) basis set accurately reproduces the spectrum. Comparison with IR emission spectra from two planetary nebulae, SMP LMC56 and SMC16, which have been associated with high C₆₀ abundances, indicates that C₆₀H⁺ is a plausible contributor to their IR emission.