Giant Infrared Resonances of Cyclocarbon Cations

Carbon clusters containing between 10 and 30 atoms predominantly exist as rings, a structure that persists for clusters containing up to at least 60 carbon atoms. Although carbon rings were once regarded as fleeting entities, only observable as charged species via mass spectrometry, their recent synthesis and atomic-scale imaging on surfaces have reignited interest in their extraordinary properties. Here, we present infrared spectra of charged cyclocarbons (C4n⁺, n = 5-11) showing that they exhibit intense infrared resonances over the 550-1800 cm⁻¹ range that are at least 50 times stronger than infrared transitions of coexisting fullerene and bi-ring isomers, and which, density functional theory calculations suggest, are associated with a localized Kekulé-type distortion oscillating back and forth across the ring. The resonances should serve as distinctive spectroscopic signatures for cyclocarbon cations in terrestrial and extraterrestrial environments, and may expedite radiative cooling of isolated charged cyclocarbons, facilitating their formation through bimolecular association reactions in low pressure environments, including in interstellar space.