Low-temperature formation of polycyclic aromatic hydrocarbons in Titan’s atmosphere

Abstract

The detection of benzene in Titan’s atmosphere led to the emergence of polycyclic aromatic hydrocarbons (PAHs) as potential nucleation agents triggering the growth of Titan’s orange-brownish haze layers. However, the fundamental mechanisms leading to the formation of PAHs in Titan’s low-temperature atmosphere have remained elusive. We provide persuasive evidence through laboratory experiments and computations that prototype PAHs like anthracene and phenanthrene (C14H10) are synthesized via barrierless reactions involving naphthyl radicals (C10H7) with vinylacetylene (CH2=CH–C≡CH) in low-temperature environments. These elementary reactions are rapid, have no entrance barriers, and synthesize anthracene and phenanthrene via van der Waals complexes and submerged barriers. This facile route to anthracene and phenanthrene—potential building blocks to complex PAHs and aerosols in Titan—signifies a critical shift in the perception that PAHs can only be formed under high-temperature conditions, providing a detailed understanding of the chemistry of Titan’s atmosphere by untangling elementary reactions on the most fundamental level.

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Fig. 1: Schematic pathways involved in the synthesis of anthracene and phenanthrene.
Fig. 2: Comparison of the mass spectra recorded at a photoionization energy of 9.50 eV.
Fig. 3: PIE curves for ion counts recorded at m/z = 178 and 179.
Fig. 4: Potential energy surfaces of the reactions of 1-naphthyl and 2-naphthyl radicals with vinylacetylene leading to phenanthrene (p1) and anthracene (p2).

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work was supported by US Department of Energy, Basic Energy Sciences grants DE-FG02-03ER15411 (experimental studies) and DE-FG02-04ER15570 (computational studies) to the University of Hawaii and Florida International University, respectively. M.A., U.A., B.X. and the experiments at the chemical dynamics beamline at the ALS were supported by the Director, Office of Science, Office of Basic Energy Sciences, US Department of Energy under contract number DE-AC02-05CH11231, through the Gas Phase Chemical Physics Program, Chemical Sciences Division. Ab initio calculations of the C14H11 potential energy surface relevant to the reactions of 1- and 2-naphthyl radicals with vinylacetylene at Samara University were supported by the Ministry of Education and Science of the Russian Federation under grant number 14.Y26.31.0020. The authors thank V. Vuitton (Grenoble) and R. Yelle (Arizona) for stimulating discussions.

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R.I.K. designed the experiment. L.Z., B.X. and U.A. carried out the experimental measurements. M.A. supervised the experiment. L.Z. performed the data analyses. M.M.E., E.K.B., V.N.A. and A.M.M. carried out the theoretical analyses. R.I.K., A.M.M. and M.A. discussed the data. R.I.K. wrote the manuscript.

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Correspondence to Ralf I. Kaiser or Musahid Ahmed or Alexander M. Mebel.

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Supplementary Figures 1–6, Supplementary Table 1, Supplementary text, Supplementary references

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Zhao, L., Kaiser, R.I., Xu, B. et al. Low-temperature formation of polycyclic aromatic hydrocarbons in Titan’s atmosphere. Nat Astron 2, 973–979 (2018). https://doi.org/10.1038/s41550-018-0585-y

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