Infrared Spectra of 1-Quinolinium (C9H7NH+) Cation and Quinolinyl Radicals (C9H7NH and 3-, 4-, 7-, and 8-HC9H7N) Isolated in Solid para-Hydrogen.

Abstract

Large protonated polycyclic aromatic hydrocarbons (H+PAH) and the corresponding nitrogen heterocycles (H+PANH) have been proposed as possible carriers of unidentified infrared (UIR) emission bands from galactic objects. The nitrogen atom in H+PANH is expected to induce a blue shift of the band associated with the CC-stretching mode of H+PAH near 6.3 μm so that their emission bands might agree better with the UIR band near 6.2 μm. We report the IR spectrum of protonated quinoline (1-quinolinium cation, C9H7NH+) and its neutral species (1-quinolinyl radical, C9H7NH) measured upon electron bombardment during the deposition of a mixture of quinoline (C9H7N) and para-hydrogen (p-H2) at 3.2 K, indicating that the protonation and hydrogenation occur mainly at the N atom site. Additional experiments on the irradiation of C9H7N/Cl2/p-H2 matrices at 365 nm to generate Cl atoms, followed by irradiation with IR light to generate H atoms via Cl + H2 (v = 1), were performed to induce the reaction H + C9H7N. This method proved to be efficient for hydrogenation reactions in solid p-H2; we identified, in addition to C9H7NH observed in electron-bombardment experiments, four radicals with hydrogenation at the C-atom site─3-, 4-, 7-, and 8-HC9H7N. Spectral assignments were achieved according to the behavior upon secondary photolysis and a comparison of experimental results with vibrational wavenumbers and IR intensities predicted with the B3LYP/6-311++G(d,p) method. The observed lines at 1641.4, 1598.4, and 1562.0 cm-1 associated with the CC-stretching mode of C9H7NH+ are blue-shifted from those at 1618.7, 1580.8, 1556.7, and 1510.0 cm-1 of the corresponding protonated naphthalene (C10H9+).