Spectroscopy of C₁₂₀^- and larger fulleride cluster monoanions in the mid-infrared

An optical spectrum of the singly charged anionic dimer of C₆₀, the C₁₂₀^- anion, is reported for the first time. This spectrum, recorded in the mid-infrared and extending from 800 cm^-1 through to 2200 cm^-1, shows a mixture of discrete peaks and broader features. An assignment of this spectrum poses a major challenge for theory. A broad feature observed above 1600 cm^-1 can be unambiguously assigned to one or many electronic transitions. However, it is not clear which isomer is responsible for it, as many isomers show electronic transitions in this spectral range. The origin of peaks below 1600 cm^-1 in the experimental spectrum remains uncertain, as calculations predict both electronic and vibrational transitions in this spectral range for various isomers. The proximity of several electronic and vibrational transitions suggests a breakdown of the Born-Oppenheimer approximation. This suspicion is supported by the fact that the vibrational spectrum is very dependent on the computational method, much more than expected for small molecules in the gas phase. All in all, C₁₂₀^- seems to be at the brink of what is computationally feasible with current methods of quantum chemistry. We also report mid-IR spectra for the larger cluster anions, C₁₈₀^- and C₂₄₀^-, whose spectra show significant similarities to that of C₁₂₀^-.