Infrared Spectra of Protonated Phenanthridine (C ₁₃ H ₉ NH ⁺ ) and Isomers of Monohydrogenated Phenanthridine (C ₁₃ H ₉ NH and 1-, 2-, 7-, 9-, and 10-HC ₁₃ H ₉ N) Isolated in Solid Para-Hydrogen

Laboratory infrared (IR) spectra of protonated or hydrogenated polycyclic aromatic nitrogen heterocycles offer valuable insights for identifying potential carriers of interstellar unidentified infrared (UIR) emission. We report the IR spectra of protonated phenanthridine (5-phenanthridinium cation, C₁₃H₉NH⁺), its neutral radical counterpart (5-phenanthridinyl radical, C₁₃H₉NH), and additional monohydrogenated phenanthridine, produced via the electron bombardment of a mixture of phenanthridine (C₁₃H₉N) in excess para-hydrogen (p-H₂) during matrix deposition at 3.2 K. Complementary experiments involving ultraviolet/IR irradiation of a C₁₃H₉N/Cl₂/p-H₂ matrix yielded improved spectra of HC₁₃H₉N, facilitating spectral identification of five monohydrogenated phenanthridine (1-, 2-, 7-, 9-, and 10-HC₁₃H₉N). Additionally, spectra of 3- and 6-HC₁₃H₉N were tentatively assigned because of the limited number of firmly identified lines. Spectral groupings were based on responses to secondary photolysis at varied wavelengths. Vibrational assignments were supported on comparison of experimental results with scaled harmonic vibrational wavenumbers and IR intensities of possible candidates computed using the B3LYP/6-311++G(d,p) method. Implications of the observed IR bands in relation to the UIR emission features are discussed.