Angewandte Chemie International Edition:DOI:10.1002/anie.201411987 Gas-Phase Chemistry German Edition:DOI:10.1002/ange.201411987 Unexpected Chemistry from the Reaction of Naphthyl and Acetylene at Combustion-LikeTemperatures** Dorian S. N. Parker,Ralf.I.Kaiser,* Biswajit Bandyopadhyay, Oleg Kostko,Tyler P. Troy,and Musahid Ahmed* Abstract: The hydrogen abstraction/acetylene addition Thehydrogen abstraction/acetylene addition (HACA) (HACA)mechanism has long been viewed as akey route to mechanism has been proposed as akey route to PAH aromatic ring growth of polycyclic aromatic hydrocarbons formation in high-temperature environments up to 2500 K.[1,7] (PAHs) in combustion systems.However,doubt has been This fundamental route involves the sequential reaction of an drawn on the ubiquity of the mechanism by recent electronic aromatic radical such as the phenyl radical (C6H5C)with two structure calculations whichpredict that the HACA mechanism acetylene (C2H2)molecules leading eventually to the simplest [1,8] starting from the naphthyl radical preferentially forms ace- PAH, naphthalene (C10H8 ;Equations (1)–(3)). After for- naphthylene,therebyblocking cyclization to athird six- mation of naphthalene (C10H8), the HACA mechanism has membered ring. Here,byprobing the products formed in the been proposed to continue through hydrogen atom abstrac- reaction of 1- and 2-naphthyl radicals in excess acetylene under tion from the naphthalene ring leading to the 1- and/or 2- [9] combustion-like conditions with the help of photoionization naphthyl radical (C10H7C), upon which athird six-membered mass spectrometry,weprovideexperimental evidence that this aromatic ring is proposed to be formed through successive reaction produces 1- and 2-ethynylnaphthalenes (C12H8), reactions with two acetylene molecules leading to anthracene acenaphthylene (C12H8)and diethynylnaphthalenes (C14H8). and phenanthrene (C14H10). Reaction of 1-naphthyl radicals Importantly,neither phenanthrene nor anthracene (C14H10) with asingle acetylene molecule has been proposed to was found, which indicates that the HACA mechanism does synthesize acenaphthylene (C12H8)—a prospective precursor not lead to cyclization of the thirdaromatic ring as expected to nonplanar PAHs like corannulene and possibly to fullerene but rather undergoes ethynyl substitution reactions instead. (Scheme 1).[10] he gas-phase reactions of unsaturated hydrocarbons and C10H7C 127 u C2H2 26 u C12H8 152 u HC 1u 1 T ð Þþ ð Þ! ð Þþ ð ÞðÞ their radicals rapidly,exothermically,and sequentially form CH 152 u C H C 151 u HC 1u 2 hexagonal six-carbon ring structures leading to large poly- 12 8ð Þ! 12 7 ð Þþ ð ÞðÞ cyclic aromatic hydrocarbons (PAH) in sooting flames[1] of, [2] [3] [4] CH C 151 u C H 26 u C H 176 u HC 1u 3 for example,methane, benzene, and toluene. PAHs and 12 7 ð Þþ 2 2ð Þ! 14 8ð Þþ ð ÞðÞ their derivatives are considered to be toxic byproducts produced during the incomplete combustion of fossil fuels; However,recently,the molecular growth processes from these compounds are also linked to food poisoning,cancer,[5] naphthalene to anthracene and phenanthrene involving the and global warming.[6] An intimate understanding of their HACA mechanism have come under scrutiny.Comprehen- formation and growth mechanisms is therefore crucial to sive electronic structure calculations demonstrate that upon eventually eliminate the formation of PAHs in combustion formation of the 1-naphthyl radical (C10H7C), addition of processes. acetylene (C2H2)would form predominantly acenaphthylene [8a] (C12H8). Mebel et al. provide compelling evidence that the rapid cyclization of the C12H9 intermediate to form afive- [*] Dr.D.S.N.Parker,Prof. Dr.R.I.Kaiser membered ring is much faster than the addition of asecond Department of Chemistry,UniversityofHawaii at Manoa acetylene molecule leading to C14H11 followed by hydrogen Honolulu, HI 96822, (USA) loss to yield athird aromatic ring:anthracene and/or E-mail:[email protected] phenanthrene.Acomparison of these predictions with data Homepage:http://www.chem.hawaii.edu/Bil301/welcome.html from combustion flames shows that the HACA-type mech- Dr.B.Bandyopadhyay,Dr. O. Kostko, Dr.T.P.Troy,Dr. M. Ahmed ChemicalSciences Division, Lawrence Berkeley National Laboratory anism only accounts for up to 6% of phenanthrene and Berkeley,CA947200, (USA) E-mail:[email protected] [**] This work was supported by the US Department of Energy,Basic Energy Sciences (DE-FG02-03ER15411) to the University of Hawaii. The authors M.A.,B.B.,O.K.,and T.P.T., and the Advanced Light Source are supported by the Director,Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, through the Chemical Sciences Division. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201411987. Scheme 1. Structures of acenaphthylene, corannulene, and fullerene. Angew.Chem. Int.Ed. 2015, 54,5421 –5424 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 5421 Angewandte. Communications anthracene at combustion-relevant temperatures from 1000 to 2000 K.[11] Doubts were raised earlier still on the ability of the HACA mechanism to reach the large mass of high-order PAHs such as coronene (300 u) at the experimentally observed rates,byonly adding 24 uineach step.[12] Although certainly present in combustion environments,HACAroutes have often been labeled as the most important routes to PAH formation, simply based on the observation of abundant acetylene,the knowledge that aromatic radicals play apivotal role in PAHformation, and the neatness of the empirical formula by addition of C2H2 followed by hydrogen emission. Recent work by Shukla et al. has shown that phenyl addition cyclization (PAC)forms large-mass PAHs at amuch more convincing rate than the HACA mechanism, by addition of 77 uateach step.[13] Our investigation aims to identify the product distribution for the HACA mechanism leading from the prototypical bicyclic radical, the 1- and 2-naphthyl radicals to tricyclic PAHs phenanthrene and anthracene by sequential addition to acetylene molecules.Although the HACA mechanism operates over arange of aromatic radicals and experimental conditions,our experiment investigates acentral pathway from bi- to tricyclic PAHs that is representative of the sequential two-step acetylene addition mechanism that builds an extra aromatic ring onto aPAH radical. By studying the reactions of the 1- and 2-naphthyl radicals (C10H7C)with acetylene (C2H2)under simulated combustion conditions,weprovide experimental evidence that the HACA mechanism forms firstly 1- and 2-ethynylnaphthalene (C12H8) and acenaphthylene (C12H8)and secondly diethynyl naph- thalenes (C14H8)with surprisingly no formation of the Figure 1. Mass spectra of the products of the reaction of a) 1-naphthyl previously postulated tricyclic PAHs phenanthrene and and b) 2-naphthyl radicals with acetylene in the pyrolysis reactor at apressure of 300 Torr recorded at aphoton energy of 9.0 eV. anthracene (C14H10). Theisomer-specific products of these reactions were monitored and identified by exploiting single- photon ionization by tunable vacuum ultraviolet (VUV) C14H8 (176 u) products are formed through the reaction of the [14] light. Ionized molecules were detected by reflectron time- naphthyl radical (C10H7C;127 u) with one and two acetylene of-flight mass spectrometry (ReTOF). In separate experi- molecules (C2H2 ;26u)through aseries of acetylene addition/ ments,continuous beams of 1-naphthyl and 2-naphthyl hydrogen atom eliminations [Equations (1)–(3)] with the radicals (C10H7C)were generated in situ through quantitative molecules indicated in bold detected in the present study. pyrolysis of their 1- and 2-iodonaphthalene (C10H7I) precur- Theinterpretation of the mass spectra alone provides clear sors,respectively.Each precursor was seeded in pure acety- proof that in the reaction of naphthyl radicals with acetylene, lene (C2H2)atpressures of 300 Torr and expanded into only the hydrocarbons of the molecular formulae C12H8 and aresistively heated silicon carbide tube (“pyrolytic reactor”) C14H8 are formed and no C14H10 isomers.Therefore,we at 1200 100 K.[15] Theacetylene molecules did not only act conclude that neither phenanthrene nor anthracene are Æ as aseeding gas,but also as areactant with the pyrolytically reaction products under the present experimental conditions. generated naphthyl radicals.The neutral molecular beam was It should be noted that both, phenanthrene and anthracene, then interrogated by quasicontinuous tunable vacuum ultra- have been observed and studied under identical conditions in violet (VUV) radiation in the extraction region of aReTOF the experimental apparatus in which they are fully observable mass spectrometer.The photoionized molecules were col- and identifiable.Furthermore,inthe preparation stages of the lected by amicrochannel plate detector with mass spectra experiment, the reactions were conducted at arange of being collected at intervals of 0.05 eV between 8.00 and temperatures while monitoring the mass spectra for the signal 10.00 eV (see the Supporting Information, SI). at m/z 178 (C14H10); no signal was observed at any temper- Figure 1depicts the mass spectra for the reaction of 1- ature from no heating to 1500 K. naphthyl and 2-naphthyl radicals with acetylene recorded at We now focus on the identification of each isomer by aphotoionization energy of 9.0 eV.These data provide clear analyzing the photoionization efficiency(PIE) curves,which