Infrared Spectrum of Protonated Corannulene H⁺C₂₀H₁₀ in Solid para-Hydrogen and its Potential Contribution to Interstellar Unidentified Infrared Bands

Polycyclic aromatic hydrocarbons (PAH) and their derivatives, including protonated and cationic species, are suspected to be carriers of the unidentified infrared (UIR) emission bands observed from the galactic and extragalactic sources. We extended our investigations of infrared (IR) spectra of protonated planar PAH to a nonplanar PAH, corannulene (C₂₀H₁₀), which is regarded as a fragment of a fullerene, C₆₀. The protonated corannulene H⁺C₂₀H₁₀ was produced on bombarding a mixture of corannulene and para-hydrogen (p-H₂) with electrons during deposition at 3.2 K. During maintenance of the electron-bombarded matrix in darkness the intensities of IR lines of protonated corannulene decreased because of neutralization by electrons that were slowly released from the trapped sites. The observed lines were classified into two groups according to their responses to secondary irradiation at 365 nm. Eighteen lines in one group are assigned to the lowest-energy species among five possible isomers, hub-H⁺C₂₀H₁₀, and 17 in another group to rim-H⁺C₂₀H₁₀, the species of second lowest energy. Spectral assignments were derived based on a comparison of the observed spectra with those predicted with the B3PW91/6-311++G(2d,2p) method. The observed IR spectrum of hub-H⁺C₂₀H₁₀ resembles several bands of the Class-A UIR bands.