An Aromatic Universe - Low Temperature Molecular Mass Growth Processes to Polycyclic
Aromatic Hydrocarbons (PAHs)
Ralf I. Kaiser1,212

Department of Chemistry, University of Hawaii at Manoa, Honolulu HI 96822, USA
W.M. Keck Research Laboratory in Astrochemistry, University of Hawaii at Manoa,
Honolulu HI 96822, USA

Polycyclic aromatic hydrocarbons (PAHs) represent key molecular building blocks leading tocarbonaceous nanoparticles identified in combustion systems and extraterrestrial environments.
However, the understanding of their formation and growth has remained elusive. Here, we
present evidence based on laboratory data combined with electronic structure calculations on
fundamental mass growth processes of PAHs in the gas phase via ring expansion and ringannulation. Key reaction pathways operate at ultralow temperatures such as at 10 K as present incold molecular clods like TMC-1 synthesizing PAHs at least from two to five six memberedrings such as naphthalene, anthracene, phenanthrene, triphenylene, [4]-helicene, and [5]-helicene.
These elementary reactions are rapid, have no entrance barriers, and synthesize PAHs via
van-der-Waals complexes and submerged barriers. This facile route to complex PAHs signifies a
critical shift in the perception that PAHs can be only formed at high-temperature combustion andcircumstellar conditions providing a detailed understanding of the low temperature chemistry
through untangling elementary reactions on the most fundamental level. An outlook is alsopresented on the synthesis of PAHs in low temperature, hydrocarbon-dominated ices in deep
space and in the outer Solar System upon interaction with (non)-ionizing radiation; theseprocesses are driven by non-adiabatic reaction dynamics and low lying triplet states of acetylene
leading to PAHs as complex as coronene. Overall, these mechanisms eventually lead tographene-type PAHs and two dimensional nanostructures providing an exciting view about thetransformation of carbon in our universe.
References

L. Zhao et al. Pyrene Synthesis in Circumstellar Envelopes and its Role in the Formation of 2D
Nanostructures. Nature Astron. 2, 413-419 (2018).
L. Zhao et al. Low-Temperature Formation of Polycyclic Aromatic Hydrocarbons in Titan’s Atmosphere.
Nature Aston. 2, 973-979 (2018).
L. Zhao, R.I. Kaiser, B. Xu, U. Ablikim, W. Lu, M. Ahmed, M. Evseev, E. K. Bashkirov, V.N. Azyazov,M.V. Zagidullin, A.N. Morozov, A.H. Howlader, S.F. Wnuk, A.M. Mebel, D. Joshi, G. Veber, F. R.
Fischer, Gas Phase Synthesis of [4]-Helicene. Nature Commun. 10, 1510 (2019).
L. Zhao et al. Molecular Mass Growth through Ring Expansion in Polycyclic Aromatic Hydrocarbons via
Radical-Radical Reactions. Nature Commun. 10, 3689 (2019).
L. Zhao et al., A Unified Mechanism on the Formation of Acenes, Helicenes, and Phenacenes in the Gas
Phase. Angewandte Chemie – International Edition 59, 4051-4056 (2020).
S. Doddipatla et al., Low-temperature Gas-phase Formation of Indene in the Interstellar Medium, Science
Advances, 7, eabd4044 (2021).
R.I. Kaiser, N. Hansen, An Aromatic Universe – A Physical Chemistry Perspective, J. Chem. Phys. A,Invited Perspective 125, 3826-3840 (2021).
L. Zhao et al., Gas Phase Synthesis of Benzene via the Propargyl Radical Self Reaction. Science
Advances 7, eabf0360 (2021).

21).