Formation and infrared identification of protonated fluoranthene isomers 3-, 9-, and 10-C16H11+ in solid para-H2

Abstract

Polycyclic aromatic hydrocarbons (PAH) and their derivatives are prospective carriers of unidentified infrared (UIR) emission features observed in interstellar media. Fluoranthene (C₁₆H₁₀) is a simple planar PAH with five- and six-membered rings; it can be considered as a fragment of C₆₀, which, along with its cationic counterpart, has been identified in interstellar media. Protonated fluoranthene, C₁₆H₁₁⁺, was generated upon electron bombardment during deposition at 3.2 K of p-H₂ containing fluoranthene in a small proportion. The intensities of infrared features of C₁₆H₁₁⁺ decreased after maintaining the matrix in darkness because of its neutralization with trapped electrons. According to the correlations in intensities upon neutralization and secondary photolysis, observed lines were classified into three groups which are assigned to isomers 3-C₁₆H₁₁⁺, 9-C₁₆H₁₁⁺, and 10-C₁₆H₁₁⁺. Experimental vibrational wavenumbers and relative IR intensities of the features agree with corresponding calculated values predicted for these three isomers of C₁₆H₁₁⁺ with the B3PW91/6-311++G(2d,2p) method. 3-C₁₆H₁₁⁺ and 9-C₁₆H₁₁⁺ are predicted to have the lowest energy (within 5 kJ mol⁻¹), whereas 10- and 1-C₁₆H₁₁⁺ are lying above the global minimum 3-C₁₆H₁₁⁺ by ∼20 kJ mol⁻¹. However, definitive identification of 1-C₁₆H₁₁⁺ could not be made as only the most intense line is tentatively assigned. Although the observed spectra of these isomers match unsatisfactorily with the UIR bands, they will facilitate the potential terrestrial and extraterrestrial identification of these species.