The Infrared Spectrum of Protonated C₇₀

With the detection of C₆₀, C₇₀, and C⁺₆₀ in the interstellar medium, fullerenes are currently the largest molecules identified in space. The relatively high proton affinities of C₆₀ and C₇₀ support the hypothesis that protonated fullerenes may also be abundant in the interstellar matter. Here, we present the first experimental vibrational spectrum of C₇₀H⁺, recorded in the gas phase. The attachment of a proton to C₇₀ causes a drastic symmetry lowering, which results in a rich vibrational spectrum. As compared to C₆₀, where all C-atoms are equivalent due to the icosahedral symmetry, C₇₀ belongs to the D_5h 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. We compare the IR spectra of C₆₀H⁺ and C₇₀H⁺ to IR emission spectra from planetary nebulae, which suggests that a mixture of these fullerene analogs could contribute to their IR emission.