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A Unified Mechanism on the Formation of Acenes, Helicenes

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A Unified Mechanism on the Formation of Acenes, Helicenes Angewandte Research Articles Chemie International Edition:DOI:10.1002/anie.201913037 PolycyclicAromatic Hydrocarbons German Edition:DOI:10.1002/ange.201913037 AUnified Mechanism on the Formation of Acenes,Helicenes,and Phenacenes in the Gas Phase Long Zhao,Ralf I. Kaiser,* Bo Xu, Utuq Ablikim, Musahid Ahmed,* Mikhail M. Evseev, Eugene K. Bashkirov, Valeriy N. Azyazov,and Alexander M. Mebel* Abstract: Aunified low-temperature reaction mechanismon as molecular tracers in untangling the underlying molecular the formation of acenes,phenacenes,and helicenes—polycyclic mass growth processes leading to PAHs in combustion aromatic hydrocarbons (PAHs) that are distinct via the linear, systems and in the interstellar medium (ISM) at the most zigzag,and ortho-condensed arrangements of fused benzene fundamental, microscopic level (Scheme 1). Although the rings—is revealed. This mechanism is mediated through presence of PAHs[4] along with their methylated and hetero- abarrierless,vinylacetylene mediated gas-phase chemistry utilizing tetracene,[4]phenacene,and [4]helicene as bench- marks 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 an- nulation 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. Introduction Scheme 1. Representatives of key classes of PAHs differing by the linear,zigzag, and ortho-condensed arrangements of fused benzene [1] rings:acenes (i), phenacenes (ii), and helicenes (iii). Tetracene (naphthacene;C18H12), [4]phenacene (chrys- [2] ene;C18H12), and [4]helicene (benzo[c]phenanthrene; [3] [5] C18H12) isolated nearly acentury ago are the simplest atom substituted counterparts have been firmly established representatives of three key classes of polycyclicaromatic in carbonaceous chondrites,such as Allende and Murchison hydrocarbons (PAHs)—acenes,phenacenes,and helicenes— with PAHs possibly accounting for up to 20%ofthe cosmic structural isomers of aromatic systems differentiated by carbon budget,[6] the underlying mechanisms to their forma- [7] linear, zigzag, and ortho-condensed arrangements of fused tion beyond anthracene (C14H10)and phenanthrene (C14H10) benzene rings.These species received considerable attention in deep space and in combustion systems have remained largely elusive. [*] Dr.L.Zhao, Prof. R. I. Kaiser Recent laser desorption–laser multiphoton ionization Department of Chemistry mass spectrometry (L2MS) along with D/H and 13C/12C UniversityofHawaii at Manoa isotopic analyses of meteoritic PAHs revealed that these Honolulu, HI 96822 (USA) meteoritic PAHs are likely synthesized in circumstellar E-mail:[email protected] envelopes of carbon-rich Asymptotic Giant Branch Stars Dr.B.Xu, Dr.U.Ablikim, Dr.M.Ahmed (AGB) and planetary nebulae as the descendants of AGB Chemical Sciences Division [4a,c,8] Lawrence Berkeley NationalLaboratory stars via extensive molecular mass growth processes. Berkeley,CA94720 (USA) However,contemporary astrochemical models of PAHfor- E-mail:[email protected] mation as derived from combustion chemistry reaction net- M. M. Evseev,Prof. E. K. Bashkirov, Prof. V. N. Azyazov, works[9] predict time scales for the injection of PAHs from Prof. A. M. Mebel carbon stars into the interstellar medium of some 109 years, Samara National Research University which is much longer than the predicted lifetimes of PAHs of Samara 443086 (Russia) only afew 108 years.[10] These models rely on the much Prof. A. M. Mebel discussed hydrogen-abstraction/acetylene-addition (HACA) Department of Chemistry and Biochemistry mechanism, which involves repetitive sequences of atomic Florida International University Miami, FL 33199 (USA) hydrogen abstractions from an aromatic hydrocarbon like E-mail:[email protected] benzene followed by consecutive addition of one or two Supportinginformation and the ORCID identification number(s) for acetylene molecule(s) prior to cyclization and aromatiza- [11] the author(s) of this article can be found under: tion. Interstellar PAHs can be rapidly destroyed by https://doi.org/10.1002/anie.201913037. photolysis,[12] galactic cosmic rays,[13] and interstellar shocks,[10] Angew.Chem. Int.Ed. 2020, 59,4051 –4058 2019 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 4051 Angewandte Research Articles Chemie but—along with their derivatives—they are still present as Briefly,ahigh-temperature chemical reactor was utilized evidenced from the diffuse interstellar bands (DIBs)[14]— to investigate the reaction of distinct anthracenyl and discrete absorption features overlaid on the interstellar phenanthrenyl radicals ([C14H9]C)with vinylacetylene [7,9] extinction curve from the blue part of the visible (400 nm) (C4H4). This reactor consists of aheated silicon carbide to the near-infrared (1.2 mm)—and from unidentified infra- (SiC) tube and is incorporated within the source chamber of red (UIR) emission bands[15] in the 3–14 mm wavelength amolecular beam machine equipped with aWiley–McLaren range.Thus,the spectroscopic observation of PAHs infers reflectron time-of-flight mass spectrometer (Re-TOF-MS) acritical, hitherto unexplained route to their rapid chemical (Figure 1). To generate the radical reactants,thermally labile growth in the cold interstellar medium at temperatures down brominated precursor molecules were pyrolyzed in situ via to 10 K. cleavage of aweak carbon–bromine bond. These precursors Herein, we reveal an isomer-selective,versatile reaction mechanism involving vinylacetylene mediated gas phase formation of acenes,helicenes,and phenacenes with the simplest 18-p-electron tetracene,[4]phenacene,and [4]heli- cene isomers acting as critical benchmarks.Instrong contrast to the aforementioned routes to PAHs synthesis involving HACA,our mechanistical studies of the elementary reactions of distinct anthracenyl and phenanthrenyl radicals ([C14H9]C; 177 amu) with vinylacetylene (C4H4 ;52amu) display barrier- less pathways via the initial formation of along-range van- der-Waals complexes in the entrance channels followed by isomerization through addition of the aromatic radical involving asubmerged barrier leading to resonantly stabilized free [C18H13]C radicals (RSFRs) [Equation (1)].The latter C H C C H C H C C H HC 1 ½ 14 9 þ 4 4 !½ 18 13 ! 18 12 þ ð Þ undergo hydrogen migration and ring closure followed by aromatization through atomic hydrogen loss yielding distinct C18H12 isomers through targeted, stepwise ring expansion involving free radical reaction intermediates.This pathway represents aversatile reaction mechanism to synthesize Figure 1. Schematic diagram of the high-temperature reactor along acenes,helicenes,and phenacenes in low temperature inter- with the molecularbeam sampling reflectron time-of-flightmass stellar environments down to 10 Kthrough elementary gas spectrometer.[21] phase reactions of aryl radicals with vinylacetylene.Consid- ering the low temperature,bimolecular gas phase reactions have to be exoergic and all transition states involved shall be were 2- and 3-bromophenanthrene along with 1- and 2- lower than the energy of the separated reactants.Since these bromoanthracene (C14H9Br) seeded in separate experiments requirements are fulfilled, the proposed pathway provides in vinylacetylene/helium. Thetemperature of the reactor was ahitherto ignored low temperature route to complex PAHs 1400 10 K. At this temperature,each brominated precursor Æ via ring annulation. Interstellar PAHs are rapidly destroyed in dissociates to the corresponding radical plus atomic bromine the interstellar medium by photolysis,cosmic rays,and in situ followed by reaction of the radical with vinylacetylene. interstellar shocks leading to life times of only afew Thereaction products were expanded, passed through 108 years.[10] This time scale is much shorter than the time askimmer downstream the reactor, and entered the main scale for injection of PAHs synthesized in carbon-rich out- chamber,which houses the Re-TOF-MS.Tunable vacuum flows of AGBstars of some 109 years.[10] Therefore,the ultraviolet (VUV) light from the Advanced Light Source presence of PAH-like material in the interstellar medium crossed the neutral molecular beam downstream of the suggests that acritical synthetic pathway to PAHs at low skimmer in the extraction region of the Re-TOF-MS.Amass temperatures is missing.The mechanisms elucidated here spectrum was collected by measuring the arrival time of the might fill the gap and can provide aunique route to PAHs at ions,asafunction of mass-to-charge (m/z)ratios.Finally, temperatures as low as 10 K. These mechanisms are of photoionization efficiency (PIE) curves reporting the ion interest in organic chemistry and can be linked to material counts of well-defined m/z ratios versus the VUVenergy were sciences as they provide insight into reactivity,bond-breaking recorded by integrating the ion signal at mass-to-charges of processes,and synthesis of extended p-conjugated systems interest and normalizing it to the photon flux. VUV single involving acyclic precursors (vinylacetylene) with extensive photon ionization represents afragment-free ionization applications as building blocks for molecular wires,[16] carbon technique and is dubbed as a soft ionization method nanotubes[17] and graphene[18]
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