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

  title={Low-temperature formation of polycyclic aromatic hydrocarbons in Titan’s atmosphere},
  author={Long Zhao and Ralf I. Kaiser and Bo Xu and Utuq Ablikim and Musahid Ahmed and Mikhail M. Evseev and Eugene K. Bashkirov and Valeriy N. Azyazov and Alexander Moiseevich Mebel},
  journal={Nature Astronomy},
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… 

Gas phase formation of phenalene via 10π-aromatic, resonantly stabilized free radical intermediates.

Beyond PAHs, molecular mass growth processes from 1H-phenalene via ring-annulation through C3 molecular building blocks may ultimately lead to two-dimensional structures such as graphene nano flakes and after condensation of multiple layers to graphitized carbon.

Low-temperature synthesis of polycyclic aromatic hydrocarbons in Titan’s surface ices and on airless bodies

It is revealed in laboratory simulation experiments that aromatics such as benzene, naphthalene, and phenanthrene—prospective building blocks of the organic dune material—can be efficiently synthesized via galactic cosmic ray exposure of low-temperature acetylene ices on Titan's surface, hence challenging conventional wisdom that aromatic hydrocarbons are formed solely in Titan’s atmosphere.

Molecular mass growth through ring expansion in polycyclic aromatic hydrocarbons via radical–radical reactions

A mechanism through laboratory experiments and computations is presented revealing how the prototype PAH—naphthalene—can be efficiently formed via a rapid 1-indenyl radical—methyl radical reaction, providing a radical new view about the transformations of carbon in the authors' universe.

Gas-Phase Synthesis of Triphenylene (C18 H12 ).

  • Long ZhaoBo Xu R. Kaiser
  • Chemistry
    Chemphyschem : a European journal of chemical physics and physical chemistry
  • 2019
The barrier-less, exoergic nature of the reaction reveals HAVA as a versatile reaction mechanism that may drive molecular mass growth processes to PAHs and even two-dimensional, graphene-type nanostructures in cold environments in deep space thus leading to a better understanding of the carbon chemistry in the authors' universe through the untangling of elementary reactions on the most fundamental level.

Polycyclic aromatic hydrocarbon growth in a benzene discharge explored by IR-UV action spectroscopy

Infrared signatures of polycyclic aromatic hydrocarbons (PAHs) are detected towards many phases of stellar evolution. PAHs are major players in the carbon chemistry of the interstellar medium,

Gas-phase synthesis of benzene via the propargyl radical self-reaction

This work provides compelling evidence of the formation of the very first ringed aromatic and building block of PAHs—benzene—via the self-recombination of two resonantly stabilized propargyl (C3H3) radicals in dilute environments using isomer-selective synchrotron-based mass spectrometry coupled to theoretical calculations.

A Unified Mechanism on the Formation of Acenes, Helicenes, and Phenacenes in the Gas Phase.

Any enantiomeric excess generated by preferential destruction of one enantiomers might be transferred to ice-coated carbonaceous grains and ultimately to biorelevant molecules formed on these icy grains via interaction with ionizing radiation thus providing a unique perception on the Origins of Life at the most fundamental level.

Synthesis of Polycyclic Aromatic Hydrocarbons via Phenyl Addition - Dehydrocyclization: The Third Way.

This work provides compelling evidence on a novel Phenyl-Addition/dehydroCyclization (PAC) pathway leading to prototype PAHs: triphenylene and fluoranthene.

Inception of Carbonaceous Nanostructures via Hydrogen-Abstraction Phenylacetylene-Addition Mechanism.

The HAPaA mechanism can be generalized to other α-alkynyl PCAHs and thus provides an alternative covalent bond bridge for PCAH combination via an acetylene linker and may contribute toward a comprehensive understanding of soot formation, carbonaceous nanomaterials synthesis, and the origin and evolution of carbon in the authors' galaxy.



Low temperature formation of naphthalene and its role in the synthesis of PAHs (Polycyclic Aromatic Hydrocarbons) in the interstellar medium

It is shown that naphthalene can be formed in the gas phase via a barrierless and exoergic reaction between the phenyl radical (C6H5) and vinylacetylene (CH2 = CH-C ≡ CH) involving a van-der-Waals complex and submerged barrier in the entrance channel.

Mechanisms of formation of nitrogen-containing polycyclic aromatic compounds in low-temperature environments of planetary atmospheres: a theoretical study.

Ab initio quantum chemical calculations of potential energy surfaces for various reaction mechanisms of incorporation of nitrogen atoms into aromatic rings of polycyclic aromatic compounds, which may lead to the formation of N-PACs under the low-temperature and low-pressure conditions of Titan's atmosphere are carried out.

Mechanisms for the formation of benzene in the atmosphere of Titan

[1] Polycyclic aromatic hydrocarbons (PAHs) are important interstellar species, and their precursor benzene (C6H6) has been detected in our solar system. In this study the possibility of benzene

Unexpected chemistry from the reaction of naphthyl and acetylene at combustion-like temperatures.

Experimental evidence indicates that the HACA mechanism does not lead to cyclization of the third aromatic ring as expected but rather undergoes ethynyl substitution reactions instead.

Polycyclic aromatic hydrocarbons in the atmospheres of Titan and Jupiter.

Incompletely characterized complex organic solids (tholins) produced by irradiating simulated Titan atmospheres reproduce well the observed UV/visible/IR optical constants of the Titan stratospheric haze.

Hydrogen-Abstraction/Acetylene-Addition Exposed.

This study provides the first solid experimental evidence on the facile formation of naphthalene in a simulated combustion environment validating the previously postulated HACA mechanism for these two radicals.

HACA's Heritage: A Free-Radical Pathway to Phenanthrene in Circumstellar Envelopes of Asymptotic Giant Branch Stars.

By exploring the previously unknown chemistry of the ortho-biphenylyl radical with acetylene, this work delivers compelling evidence on the efficient synthesis of phenanthrene in carbon-rich circumstellar environments and implies that HACA alone cannot be responsible for the formation of PAHs in extreme environments.


Benzene has been detected in Titan's atmosphere by Cassini instruments, with concentrations ranging from sub-ppb in the stratosphere to ppm in the ionosphere. Sustained levels of benzene in the haze