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

Abstract

A unified low‐temperature reaction mechanism on the formation of acenes, phenacenes, and helicenes—polycyclic aromatic hydrocarbons (PAHs) that are distinct via the linear, zigzag, and ortho‐condensed arrangements of fused benzene rings—is revealed. This mechanism is mediated through a barrierless, vinylacetylene mediated gas‐phase chemistry utilizing tetracene, [4]phenacene, and [4]helicene as benchmarks contesting established ideas that molecular mass growth processes to PAHs transpire at elevated temperatures. This mechanism opens up an isomer‐selective route to aromatic structures involving submerged reaction barriers, resonantly stabilized free‐radical intermediates, and systematic ring annulation potentially yielding molecular wires along with racemic mixtures of helicenes in deep space. Connecting helicene templates to the Origins of Life ultimately changes our hypothesis on interstellar carbon chemistry.

A Free‐Radical Prompted Barrierless Gas‐Phase Synthesis of Pentacene

“…It is important to highlight that the barrier connecting the van‐der‐Waals complex [ i0 a ] to the resonantly stabilized free radical (RSFR) intermediate [ i1 ] is below the energy of the separated reactants. In this sense, a barrier to addition exists, but the latter resides lower in energy than the reactants and hence is called a submerged barrier . Intermediate [ i1 ] is central to the formation of the pentacene ( P1 ) and benzo[ a ]tetracene ( P2 ) products; these isomers are synthesized via the reaction sequences [ i1 ] → [ i3 ] → [ i4 ] → [ i5 ] → P1 and [ i1 ] → [ i6 ] → [ i7 ] → [ i8 ] → P2 involving an hydrogen atom migration from an ortho carbon in the attacked ring of tetracenyl to β‐carbon in the side chain, cyclization, another hydrogen atom migration in the newly formed six‐member ring (from the methylene moiety to the neighboring bare carbon atom), and atomic hydrogen elimination accompanied by aromatization and ring annulation.…”

An Aromatic Universe–A Physical Chemistry Perspective

“…Initially unraveled via the crossed molecular beam reaction of the phenyl radical (C 6 H 5 ·; 1 ) with vinylacetylene (HCCC 2 H 3 ) leading to naphthalene (C 10 H 8 ; 8 ), ,, HAVA is initiated via the barrierless formation of a van der Waals complex ( 23 ) in the entrance channel of the reaction. Generally speaking these van der Waals complexes are weakly stabilized by 8–19 kJ mol –1 with respect to the reactants and isomerize via an addition of the radical center of the aromatic radical reactant to the H 2 C moiety of the vinylacetylene molecule leading to a resonantly stabilized free radical (RSFR) intermediate ( 24 ) residing in deep potential energy wells of typically 179–195 kJ mol –1 with respect to the separated reactants. ,− A small barrier to addition of 4–9 kJ mol –1 does exist, but it is located below the energy of the separated reactants and hence is called a submerged barrier . Therefore, the reaction of the aromatic radical with vinylacetylene leading to the resonantly stabilized free doublet radical intermediate ( 24 ) is essentially barrierless and hence can operate even in low-temperature environments of, for example, 10 K as in cold molecular clouds.…”

“…Therefore, this complexity requires a systematic elucidation of the fundamental, elementary reactions involved in the formation of PAHs through well-defined molecular beam studies. These experiments are conducted either under single-collision conditions as provided in crossed molecular beam experiments , or in pyrolytic microreactors under very dilute conditions . Furthermore, flame-sampling molecular-beam studies can provide immediate insights into PAH formation kinetics.…”

“…We bring together emerging concepts of low- and high-temperature aromatization reactions via neutral–neutral collisions from monocyclic systems to PAHs. It compiles developing trends on the formation and molecular growth processes of PAHs via five elementary mechanisms made available by new molecular beam studies combined with high-level electronic structure calculations (for details of these calculations, we refer the reader to the original literature): H ydrogen A bstraction– C 2 H 2 (acetylene) A ddition (HACA), − H ydrogen A bstraction– V inylacetylene A ddition (HAVA), ,,,,,− P henyl A ddition–Dehydro C yclization (PAC), R adical- R adical R eactions (RRR), and M ethylidyne A ddition- C yclization- A romatization (MACA) . The experimental and computational verification of these reaction networks provides a constrained framework of PAH formation chemistry in extreme environments from low temperatures in molecular clouds (10 K) and hydrocarbon-rich atmospheres of planets and their moons (35–150 K) to high-temperature environments like circumstellar envelopes of carbon-rich AGB stars and combustion systems at a few 1000 K.…”

Formation of phenanthrene via H‐assisted isomerization of 2‐ethynylbiphenyl produced in the reaction of phenyl with phenylacetylene

“…Molecules and radicals other than acetylene, including aromatics themselves, [8][9][10]20 methyl, 11,12,[21][22][23] vinyl 24 and resonantly-stabilized propargyl 25,26 and allyl 27,28 radicals, and vinylacetylene, [29][30][31][32][33][34][35][36][37] have been proposed to complement acetylene-based HACA, especially in the production of purely benzenoid PAHs. In particular, Shukla et al 21,38 and Xiong et al 39 proposed a mechanism termed PAC (phenyl addition/dehydrocyclization) in which phenyl radical adds to a bay area (armchair edge) of a PAH molecule thus resulting in its extension by two six-member rings, see, for example, the reaction of phenanthrene with C 6 H 5 producing benzo[e]pyrene.…”