Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC⁺ and HBC²⁺

Abstract

We present the first infrared (IR) gas-phase spectrum of a large and astronomically relevant polycyclic aromatic hydrocarbon (PAH) cation (C 42 H 18 + , HBC + ) and its dication (C 42 H 18 + 2 , HBC 2+ ). The spectra are recorded via infrared multiphoton dissociation (IRMPD) spectroscopy of ions stored in a quadrupole ion trap, using the intense IR radiation of a free electron laser in the 530-1800 cm −1 (5.6-18.9 μm) 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 (μm), in good agreement with density functional theory calculations 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 (μm). Given the similarity in the cationic and dicationic 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 ∼6.5, 7.7, 8.4, and 13.1 μm 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 μm, squarely in the range predicted by theory and as previously found in studies of small (substituted) PAHs. This study therefore supports the use of AIBs observed in the 11-14 μm range as a diagnostic tool for the edge topology of large PAHs in space.

Laboratory Gas-phase Infrared Spectra of Two Astronomically Relevant PAH Cations: Diindenoperylene, and Dicoronylene,

“…Thus, if this is confirmed in other PAHs, these bands can offer a tool to identify PAHs with pentagon structures. Another pronounced difference is found around 1600 cm −1 ; whereas HBC + (Zhen et al 2017) only exhibits a relatively weak feature in this range, both DIP + an DC + have much stronger bands here. Further studies are required to assess whether this reflects the less symmetric structure of the species studied here, which could "activate" more CC modes than for the more highly symmetric HBC + .…”

“…A comparison between Figures 2(A) and (B) shows clear differences. DIP + exhibits multiple CH in-plane bending vibration bands (1001, 1070, 1115, and 1152 cm −1 ) in a range (1000-1200 cm −1 ) where DC + , and also HBC + show only two resolved bands (Zhen et al 2017). The origin of these features can be traced back to a combination of CH in plane and CC stretches modes involving also, but not exclusively, the two pentagons in the DIP geometry.…”

“…In Figure 3, the obtained experimental IRMPD spectra for DIP + and DC + are replotted together with the previously reported spectrum of HBC + (Zhen et al 2017). Also included in the figure (top row) is a typical AIB emission spectrum as observed with Spitzer toward NGC 7023 (Houck et al 2004;Werner et al 2004).…”

Destruction and multiple ionization of PAHs by X-rays in circumnuclear regions of AGNs

“…Neutral and ionized PAHs present large differences in the relative intensities of their resulting IR spectra (Allamandola et al 1999;Tielens 2008). However, there is still no evidence for a unambiguous identification between singly and doubly charged PAHs in the ISM (Zhen et al 2017). The precise determination of the charge state of PAHs is a relevant astrophysical feature, which is also corroborated by our results.…”

Effect of molecular structure on the infrared signatures of astronomically relevant PAHs

“…Infrared spectra of PAHs and PAH cations in the gas phase have also been reported (e.g., Joblin et al 1994;Oomens et al 2001Oomens et al , 2006. In earlier works we demonstrated that with our instrument for Photoprocessing of PAHs (i-PoP) connected to the free electron laser for infrared experiments (FELIX) we can record infrared multiphoton dissociation (IRMPD) spectra of large aromatic cations up to C 48 H •+ 20 (Zhen et al 2017(Zhen et al , 2018. Large sets of density functional theory (DFT) computed normal modes of PAHs have also been reported (e.g., Bauschlicher et al 2008Bauschlicher et al , 2009Ricca et al 2012).…”