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

Abstract

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 (C 20 H 10 ), which is regarded as a fragment of a fullerene, C 60 . The protonated corannulene H + C 20 H 10 was produced on bombarding a mixture of corannulene and para-hydrogen (p-H 2 ) 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 + C 20 H 10 , and 17 in another group to rim-H + C 20 H 10 , 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 + C 20 H 10 resembles several bands of the Class-A UIR bands.

Infrared spectrum of hydrogenated corannulene rim-HC20H10 isolated in solid para-hydrogen

“…25,26 Extending our research to corannulene (C20H10), a bowl-shaped PAH that is considered to be a fragment of C60, 27 we reported recently the IR spectra of protonated corannulene isomers (hub-H + C20H10 and rim-H + C20H10). 28 The IR spectrum of hub-H + C20H10 matches well the UIR emission bands, consistent with the suggestions that H + PAH and non-planar PAH (and their derivatives) might be important contributors to the UIR bands. 4,29 Hydrogenated corannulene (HC20H10) might consequently be present also in the ISM.…”

“…corannulene as discussed in our previous work. 28 To distinguish the protonated and hydrogenated products, we maintained the matrix in darkness for 30 h during which the neutralization of the protonated corannulene, HC20H10 + + e − → HC20H10, and the H-atom addition to corannulene, H + C20H10 → HC20H10, occurred. As a consequence of these reactions, lines of HC20H10 are expected to increase in intensity with time.…”

“…Para-H2 was passed through a stainless steel tube (outer diameter 6 mm) containing corannulene (C20H10, >94 % purity, Kanto Chemical); the tube was heated to 408 K to increase the vapor pressure of the sample. 28 The gaseous mixture of C20H10/p-H2 was deposited onto the substrate at 3.2 K over a period of 10 h at a flow rate ∼12 mmol h −1 . We estimated the mixing ratio of C20H10/p-H2 in these experiments to be 3.0 ± 0.3 ppm according to the method of Ruzi et al 36 using the estimated matrix thickness and quantum-chemically predicted IR intensities of some intense lines of corannulene; the error represents only the fitting error from various lines, whereas the actual relative error might be as great as 50 %.…”

“…We employed light at 405 nm instead of the absorbance maximum of Cl2 near 365 nm to avoid unwanted photolysis of corannulene. 28 During photolysis and acquisition of spectra, a long-pass filter (LP-2500 nm, Spectrogon) blocking light above 4000 cm −1 was employed to avoid the reaction between Cl and p-H2. Several ratios of Cl2/C20H10 and varied durations of visible and IR irradiations were investigated to achieve a better yield of hydrogenated products.…”

“…31 To account for the difference in the extent of anharmonicity between the CH stretching and other vibrational modes, the harmonic vibrational wavenumbers were scaled with factors of 0.958 for values >2500 cm −1 and 0.978 for values <2500 cm −1 ; these factors are the same as those used for coronene, 23 ovalene, 26 and corannulene. 28 In calculations of relative energies, correction of zero-point vibrational energy (ZPVE) was performed with unscaled harmonic vibrational wavenumbers.…”