Infrared spectra of isomers of protonated and hydrogenated phenanthridine isolated insolid para-hydrogen
Yi-Shan Chung1, Man-Lin Yang1, Jun-Ying Feng1 and Yuan-Pern Lee1,21Department of Applied Chemistry and Institute of Molecular Science, National Yang Ming
Chiao Tung University, 1001, Ta-Hsueh Rood, Hsinchu 300093, Taiwan
2

Center for Emergent Functional Matter Science, National Yang Ming Chiao TungUniversity, 1001, Ta-Hsueh Rood, Hsinchu 300093, Taiwan
e-mail: cindy40232@gmail.com and yplee@nycu.edu.tw

Abstract
Large polycyclic aromatic hydrocarbons (PAH) and polycyclic aromatic nitrogen
heterocycles (PANH) have been proposed to be potential carriers responsible for theunidentified infrared (UIR) emission bands in the interstellar medium.1 The nitrogen atom inPANH induces a blue shift of the CC-stretching mode of the PAH, occurring around 6.3 μm. In
addition to PANH, several protonated polycyclic aromatic hydrocarbons (H+PAH) and thecorresponding nitrogen heterocycles (H+PANH) also exhibit blue shifts.2 Therefore, the CCstretching band of the H+PANH might agree better with the UIB band at 6.2 μm and H+PANH
might be potential candidates that contribute to the UIR bands.
We report the infrared (IR) spectra of two isomers of protonated phenanthridine(H+C13H9N) and seven isomers of mono-hydrogenated phenanthridine (HC13H9N), produced
upon electron-bombardment of a mixture of phenanthridine (C13H9N) and para-hydrogen (pH2) during matrix deposition at 3.2 K. IR spectra of protonated species C13H9NH+ and 4H+C13H9N were observed. In addition to the hydrogenation on the N atom of C13H9N to form
C13H9NH, mono-hydrogenation at six carbon sites to form 1-, 2-, 3-, 7-, 9-, and 10-HC13H9N,
respectively, were also observed. To confirm the spectral identification of hydrogenatedphenanthridine, additional experiments on a C13H9N/Cl2/p-H2 matrix were performed; thematrix was irradiated at 365 nm to generate Cl atoms, followed by IR irradiation to generate Hatoms via Cl + H2 (v = 1) → HCl + H. The H atoms thus generated propagated throughout thematrix and reacted with C13H9N via quantum tunneling. In these experiments, six hydrogenatedphenanthridine species, except the one hydrogenated at the carbon site 3, were clearly observed.
Spectral groupings were achieved according to the behaviors of lines upon secondary photolysis
at various wavelengths, chosen according to the vertical electronic transitions of various
isomers of HC13H9N predicted with the TD-B3LYP/6-311++G(d,p) method. The spectral
assignments were supported via comparison of the experimental results with the vibrationalwavenumbers and IR intensities of possible isomers predicted with the B3LYP/6-311++G(d,p)
method. All spectra of H+C13H9N and HC13H9N are new.
For H+C13H9N, major observed lines are at 2.95, 6.22 and 12.53 μm; the CC-stretching
band of C13H9NH+ was observed at 6.22 μm, blue shifted from that of the corresponding PAH,
10-C14H11+. For HC13H9N, major lines were observed at 13.43, 13.16, and 13.55 μm for
C13H9NH, 9-HC13H9N, and 2-HC13H9N, respectively. Based on the overall spectral pattern,these protonated and hydrogenated species are unlikely to be the carriers of the UIR bands,
characterized by emission bands near 3.3, 6.2, 7.7, 8.6, 11.2, and 12.0 μm.
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