六-正-六苯并冠烯阳离子HBC +和HBC2 +的红外光谱
Infrared Spectra of Hexa-peri-hexabenzocoronene Cations:HBC+ and HBC2+
摘要：
本文首次报道了一种与天文学有关的大分子PAH阳离子（C $_{42}$ H $_{18}$ $^+$，HBC $^+$）及其双阳离子（C $_{42}$ H $_{18}$ $^{2+}$，HBC $^{2+}$）的红外气相光谱。使用自由电子激光器在530 $-$ 1800 cm $^{-1}$（5.6 $-$ 18.9 $μ$ m）范围内的强红外辐射，通过存储在四极离子阱中的离子的红外多光子解离（IRMPD）光谱来记录光谱。HBC $^{+}$在762（13.12）、1060（9.43）、1192（8.39）、1280（7.81）、1379（7.25）和1530（6.54）cm $^{-1}$（$μ$ m）处显示出主要的强吸收峰，与考虑非谐效应后的DFT计算结果非常一致。HBC $^{2+}$的主要吸收峰位于660（15.15）、766（13.05）、1054（9.49）、1176（8.50）、1290（7.75）、1370（7.30）和1530（6.54）cm $^{-1}$（$μ$ m）。鉴于在阳离子和双阳离子光谱的相似性，我们还没有确定一个明显的诊断签名，在空间中的多电荷多环芳烃的存在。虽然与IRMPD技术相关的实验问题排除了与星际光谱的详细比较，但我们确实注意到，HBC $^+$和HBC $^{2+}$在$\sim$ 6.5、7.7、8.4和13.1 $μ$ m处的强带与突出的芳香族红外带（AIB）一致。HBC只有三个CH组和两个HBC阳离子的面外CH弯曲模式测量在13.1 $μ$ m，正方形的范围内预测的理论和以前发现的小（取代）多环芳烃的研究。因此，这项研究支持使用AIBs观察到的11 $-$ 14 $μ$ m范围作为诊断工具的边缘拓扑结构的大型多环芳烃在空间。
英文摘要：
We present the first infrared (IR) gas phase spectrum of a large and astronomically relevant PAH cation (C$_{42}$H$_{18}$$^+$, HBC$^+$) and its di-cation (C$_{42}$H$_{18}$$^{2+}$, HBC$^{2+}$). The spectra are recorded via infrared multi-photon dissociation (IRMPD) spectroscopy of ions stored in a quadrupole ion trap, using the intense infrared radiation of a free electron laser in the 530$-$1800 cm$^{-1}$ (5.6$-$18.9 $μ$m) range. HBC$^{+}$ shows main intense absorption peaks at 762 (13.12), 1060 (9.43), 1192 (8.39), 1280 (7.81), 1379 (7.25) and 1530 (6.54) cm$^{-1}$($μ$m), in good agreement with DFT calculations that after scaling to take the anharmonicities effect into account. HBC$^{2+}$ has its main absorption peaks at 660 (15.15), 766 (13.05), 1054 (9.49), 1176 (8.50), 1290 (7.75), 1370 (7.30) and 1530 (6.54) cm$^{-1}$($μ$m). Given the similarity in the cationic and di-cationic spectra, we have not identified an obvious diagnostic signature to the presence of multiply charged PAHs in space. While experimental issues associated with the IRMPD technique preclude a detailed comparison with interstellar spectra, we do note that the strong bands of HBC$^+$ and HBC$^{2+}$ at $\sim$ 6.5, 7.7, 8.4 and 13.1 $μ$m coincide with prominent aromatic infrared bands (AIBs). HBC has only trio CH groups and the out-of-plane CH bending mode of both HBC cations is measured at 13.1 $μ$m, squarely in the range predicted by theory and previously found in studies of small (substituted) PAHs. This study therefore supports the use of AIBs observed in the 11$-$14 $μ$m range as a diagnostic tool for the edge topology of large PAHs in space.
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