Polycyclic aromatic hydrocarbons (PAHs) are abundant in many regions of the Universe, representing a major reservoir for cosmic carbon. However, their formation pathways in cold regions of space remain elusive. Recent astronomical detections show that current astrochemical models drastically underestimate the abundance of aromatic molecules and suggest that additional formation pathways such as ion–molecule reactions need to be considered. Here we reveal efficient low-temperature formation pathways towards nitrogen-containing PAHs via exothermic pyridine+ and acetylene ion–molecule reactions. The experimental approach combines kinetics with spectroscopic probing and unambiguously identifies key reaction intermediates and the final nitrogen-containing PAH product quinolizinium+, a structure that is thought to contribute to the 6.2 μm interstellar emission feature. This study not only provides competing formation pathways relevant in the chemistry of the interstellar medium and Titan’s atmosphere, but also delivers information to verify in-silico potential energy surfaces, astrochemical models and infrared observations.
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The code used for data reduction in this work can be found here: https://github.com/aravindhnivas/FELion_GUI3.
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We gratefully acknowledge the support of Radboud University and of the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), for providing the required beam time at the FELIX laboratory and the skilful assistance of the FELIX staff. This work was sponsored by NWO Exact and Natural Sciences as part of the research programme ROSAA (NWO START-UP grant 740.018.010; S.B., A.N.M.) and through the use of supercomputer facilities at SURFsara in Amsterdam (NWO Rekentijd grant 2021.055). We thank the Cologne Laboratory Astrophysics group for providing the FELion ion trap instrument for the current experiments and the Cologne Center for Terahertz Spectroscopy funded by the Deutsche Forschungsgemeinschaft (grant SCHL 341/15-1) for supporting its operation. We thank J. Oomens for helpful discussion.
The authors declare no competing interests.
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Extended Data Fig. 1 First order reaction rate constants obtained from the kinetic profiles of (a) m/z 104, (b) m/z 105, (c) m/z 106 and (d) m/z 130 plotted against the acetylene number density.
Second order reaction rate coefficients are obtained using a linear fit to obtain (a) k2, (b) kRA, (c) kproto and (d) k2,C7. When using large acetylene number densities, required for the measurements on m/z 106 and 130, the reaction towards m/z 104 and 105 has almost proceeded completely during the helium pulse phase (which is not included in the fit) and yields a larger fit error for the latter. The error bars indicate the 1σ errors and the grey shaded bands define the 2σ uncertainty regions of the fits.
Extended Data Fig. 2 Exemplary kinetic profile of the reaction of pyridine+ with acetylene in a continuous high-pressure regime at 150 K.
An ion pulse length of 5 ms and acetylene number densities of 1.8 x 109 to 3.1 x 109 have been used. Measurements are shown as points with 1σ error bars from multiple iterations on each datapoint. The solid lines display the results of the fitted reaction equation model. The grey area is not included in the fit, as the ions are still being thermalized after entering the trap. The value for k2 that describes the bimolecular reaction to m/z 104 is fixed to 4.9 x 10−10 cm3 molecule−1 s−1 as determined from low pressure regime scans.
Extended Data Fig. 3 Experimental IRMPD infrared spectrum of m/z 105 (grey) and comparison with scaled harmonic infrared spectra of different C7H7N•+ isomers (coloured sticks).
The vibrational frequencies have been calculated for (a) two conformers of 2-vinyl-β-distonic-H-pyridine+ (Ca-3, blue sticks), (b) two conformers of 2-acetylene-H-pyridine+ (Ca-2 and Cb-2, pink sticks), (c) pyridine-C2H2+ complex (C-1, blue-green sticks) and 2-vinyl-pyridine+ (Cb-3, green sticks) and (d) N-acetylene-pyridine+ (N-1, purple sticks) and N-vinyl-α-distonic-pyridine+ (N-2, dark-purple sticks) at the B3LYP/N07D-GD3 level of theory. The assigned structures for m/z 105 are shown in panel (a).
Extended Data Fig. 4 Comparison between the experimental IRMPD spectrum of m/z 130 produced in the reaction in the ion trap (grey) and the IRPD spectrum of m/z 130 formed inside the ion storage source (red).
To evaluate the B3LYP/N07D level of theory, the calculated anharmonic infrared spectra of quinolizinium+ (N-5) and H-quinoline+ (Ca-6) are plotted as orange and green sticks, respectively.
Extended Data Fig. 5 Calculated potential energy surface of the first part of the C formation pathway (black) as shown in Fig. 5 and an alternative route via N-acetylene-pyridine+ (N-1).
The energies are zero-point energy corrected and calculated with respect to the entrance energy of pyridine+ and acetylene at the B3LYP/6-311++G(d,p) level of theory. Energies obtained from additional calculations at the DSD-PBEP86-GD3BJ/aug-cc-pVQZ//M06-2X/6-31++G(2df,p) level of theory for the transition states TS-NC-2 and TS-Cb-1 and the product Cb-2 are shown next to it.
Extended Data Fig. 6 Experimental IRMPD infrared spectrum of m/z 104 (grey) and comparison with scaled harmonic infrared spectra of different C7H6N+ isomers (coloured sticks).
The vibrational frequencies have been calculated for (a) 2-ethynyl-H-pyridine+ (Ca-7, red sticks), (b) N-ethynyl-pyridine+ (N-6, blue sticks) and (c) 3 and 4-ethynyl-H-pyridine+ (green and orange sticks, respectively) at the B3LYP/N07D-GD3 level of theory. The assigned structure for m/z 104 is shown in panel (a).
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Rap, D.B., Schrauwen, J.G.M., Marimuthu, A.N. et al. Low-temperature nitrogen-bearing polycyclic aromatic hydrocarbon formation routes validated by infrared spectroscopy. Nat Astron 6, 1059–1067 (2022). https://doi.org/10.1038/s41550-022-01713-z