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

Aug 30, 2019
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Polycyclic aromatic hydrocarbons (PAHs) – organic molecules carrying fused benzene rings - represent key molecular building blocks leading to carbonaceous nanoparticles identified in combustion processes (soot particles) and in extraterrestrial environments (interstellar grains). Complex astrochemical and combustion reaction networks along with astronomical surveys and flame sampling studies propose that PAHs represent the critical link between resonantly stabilized free radicals like propargyl (C3H3), allyl (C3H5), and cyclopentadienyl (C5H5) and carbonaceous nanoparticles. Whereas on Earth, PAHs represent toxic, mainly carcinogenic byproducts released in incomplete combustion, in deep space, PAHs may carry up to 20% of the galactic carbon budget and are implicated as critical precursors to molecular building blocks of life such as amino acids. The detection of PAHs in carbonaceous chondrites such as Allende along with carbon isotopic analyses reveals their likely formation in circumstellar envelopes of carbon stars through extensive molecular mass growth processes. However, an understanding of their formation and growth in these extreme environments have remained elusive. 

The underlying models rely predominantly on the Hydrogen-Abstraction/aCetylene-Addition (HACA) mechanism suggesting molecular mass growth processes via stepwise ring addition to PAHs. However, recent models centered on HACA suggest that stepwise additions of acetylene to PAHs are too slow to reproduce the quantified mass fractions of complex PAHs. In deep space, PAHs are rapidly destroyed (via photolysis, galactic cosmic rays, shock waves) resulting in lifetimes of only a few 108 years. These time scales are much shorter than those for injection of PAHs into the interstellar medium by carbon rich Asymptotic Giant Branch (AGB) stars of 2 × 10years. Hence, all PAHs should be destroyed in the interstellar medium; but the presence of such species indicates a hitherto elusive route to rapid growth of PAHs in circumstellar envelopes of carbon rich stars.

Here, we report on a prototype system of a previously poorly investigated class of reaction between two radicals leading to PAHs. The reaction of the aliphatic methyl radical (CH3) with the aromatic 1-indenyl radical (C9H7) leading eventually to the formation of naphthalene (C10H8) is used as a benchmark for the creation of the simplest PAH carrying two fused benzene rings. Indeed, there is no experiment to date under high temperature conditions that has followed the kinetics and mechanism between two hydrocarbon free radicals, and hence the reaction route through methylation of the indenyl radical provides a prototype for the conversion of a five-membered ring to a six-membered ring in PAHs. This provides a detailed view of high temperature mass growth processes that may eventually lead to graphene-type PAHs, two-dimensional nanostructures, and even graphite as detected in the L3 chondrite Khohar and in Murchison. Hence we believe our study can provide a radical new view about the transformations of carbon in our universe.

Alexander Mebel

Professor, Florida International University

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