Infrared spectra of protonated pyrene and its neutral counterpart in solid para-hydrogen

Protonated polycyclic aromatic hydrocarbons (H⁺PAHs) have been reported to have infrared (IR) bands at wavenumbers near those of unidentified infrared (UIR) emission bands from interstellar objects. We produced 1-C ₁₆H₁₁⁺ and 1-C₁₆H₁₁ upon electron bombardment during matrix deposition of p-H₂ containing pyrene (C₁₆H₁₀) in a small proportion. Intensities of absorption features of 1-C₁₆H₁₁⁺ decreased after the matrix was maintained in darkness or irradiated with light at 365 nm, whereas those of 1-C₁₆H₁₁ increased. The observed line wavenumbers and relative intensities of 1-C₁₆H₁₁ ⁺ and 1-C₁₆H₁₁ agree satisfactorily with the scaled vibrational wavenumbers and IR intensities predicted with the B3PW91/6-311++G(2d,2p) method. Our method, being relatively clean with negligible fragmentation, is applicable to larger H⁺PAH; it has the advantages of producing excellent IR spectra covering a broad spectral range with narrow lines and accurate intensities, so that structural identification among various isomers is feasible.