Infrared Spectroscopy of Neutral and Cationic Sumanene (C21H12 & C21H12+) in the Gas Phase: Implications for Interstellar Aromatic Infrared Bands (AIBs)

Polycyclic aromatic hydrocarbons (PAHs) are known to be omnipresent in various astronomical sources. Ever since the discovery of C₆₀ and C₇₀ fullerenes in a young planetary nebula in 2010, uncovering the reaction pathways between PAHs and fullerenes has been one of the primary goals in astrochemistry. Several laboratory studies have attempted to elucidate these pathways through experiments simulating top-down and bottom-up chemistry. Recently, indene (c-C₉H₈, a fused pentagon and hexagonal ring) has been detected in the TMC-1 molecular cloud. This is a significant finding since pentagon-bearing PAHs could be key intermediates in the formation of fullerenes in space. Spectroscopic studies of pentagon-bearing PAHs are thus essential for their detection in molecular clouds, which would eventually lead to unraveling the intermediate steps in PAH’s chemistry. This work reports the infrared (IR) spectra of both neutral and cationic sumanene (C₂₁H₁₂ and C₂₁H₁₂⁺): a bowl-shaped PAH containing three pentagon rings. Apart from its relevance for furthering our understanding of the chemistry of PAHs in an astronomical context, the presence of three sp³ hybridized carbons makes the vibrational spectroscopy of this molecule highly interesting also from a spectroscopic point of view, especially in the CH stretching region. The experimental IR spectra of both species are compared with quantum chemically calculated IR spectra as well as with the aromatic infrared bands (AIBs) of the photodissociation regions of the Orion Bar obtained using the James Webb Space Telescope (JWST).