Infrared spectra of potential interstellar molecules: protonated PANH and OCS
M. Tsuge,1 C.-Y. Tseng,1 and Y.-P. Lee1,2
1Department of Applied Chemistry, National Chiao Tung University, Taiwan
2
Instritute of Molecular Sciences, Academia Sinica, Taiwan

The infrared emission from many astronomical objects is dominated by the unidentified
infrared (UIR) emission bands, which are characterized by main features at 3.3, 6.2, 7.7, 8.6,
and 11.2 μm. Because these features are characteristic of the CH stretching, CC stretching,
and CH bending modes of aromatic compounds, polycyclic aromatic hydrocarbons (PAHs)
were proposed to be possible emitters. The nitrogen containing PAHs, known as polycyclicaromatic nitrogen heterocycles (PANHs), might be responsible for the 6.2-μm feature of UIRbands because the N atom induces a blue shift to the CC stretching modes of pure PAHs near
6.3 μm [1]. The existence of protonated PANHs is expected because of the large proton
affinity at the nitrogen site of PANHs.
We have developed a new method to investigate the infrared spectra of protonated PAHs
using electron bombardment during deposition of a PAH/para-H2 mixture at 3.2 K [2]. The
electron bombardment of para-H2 produces H3+, which subsequently react with PAH to form
protonated PAH (H+PAH). We have applied this method to PAHs up to ovalene (C32H14) toobtain infrared spectra of protonated species with narrow lines and excellent signal-to-noise
ratio [3]. In this work, we extended our investigation to quinoline and isoquinoline (C9H7N)
as a first step toward the study of protonated PANHs. The IR spectra of protonated quinolineand isoquinoline (C9H7NH+) are characterized by features originating from NH stretching, CCstretching, and CH(NH) bending modes. Agreement between observed spectra of C9H7NH+
and the UIR emission spectrum is unsatisfactory, most probably because the size of molecule
is too small; in the case of H+PAH, features close to the UIR bands were observed for only
large H+PAH such as protonated ovalene (C32H15+). However, the blue shifts of the CC
stretching bands of protonated quinolone and isoquinoline from the corresponding H+PAH,protonated naphthalene (C10H9+), are clearly observed. IR spectra of mono-hydrogenatedquinoline and isoquinoline (C9H7NH∙) radicals, with hydrogen atom attached on the N atom,
were also identified for the first time.
We also applied this method to small molecules such as OCS, which has been postulated toexist in the ISM. The OCS molecule has two possible protonation sites, leading to isomers
HOCS+ and HSCO+. Both HOCS+ and HSCO+ have been observed with microwave
spectroscopy [5], but only the OH stretching mode of HOCS+ has been identified with gasphase IR spectroscopy [6]. We identified three IR absorption lines of HOCS+ at 2945.9 (ν1),
1875.3 (ν2), and 1041.9 (ν3) cm−1 and two of HSCO+ at 2506.9 (ν1) and 2074.2 (ν2) cm−1.
Absorption lines of t-HOCS at 3563.4, 1394.8, and 1199.0 cm−1 were also identified for thefirst time. Assignments were achieved according to expected chemistry, comparison between
experimental spectra and theoretical predictions, and isotopic substitutions. The large red shiftof the ν1 of HOCS+ is attributed to partial proton sharing between OCS and H2.
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