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Mechanisms of Formation of Nitrogen-Containing Polycyclic View Article Online / Journal Homepage / Table of Contents for this issue PAPER www.rsc.org/faraday_d | Faraday Discussions Mechanisms of formation of nitrogen-containing polycyclic aromatic compounds in low- temperature environments of planetary atmospheres: A theoretical study Alexander Landera and Alexander M. Mebel Received 23rd February 2010, Accepted 31st March 2010 DOI: 10.1039/c003475d Polycyclic aromatic hydrocarbons (PAHs) are believed to be responsible for the formation of organic haze layers in Titan’s atmosphere, but the nature of PAHs on Titan and their formation and growth mechanisms are not well understood. Considering the high abundance of nitrogen in Titan’s atmosphere, it is likely that the haze layers hold not only pure hydrocarbon PAHs but also their nitrogenated analogs, N-containing polycyclic aromatic compounds (N-PACs) with ‘hetero’ N atoms in aromatic rings. Laboratory studies of Titan’s tholins also support the hypothesis that, together with pure PAHs and their cations, N-PACs may be the fundamental building blocks of microphysical tholin particles. In the present work, we carried out 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. This includes mechanisms analogous to the Ethynyl Addition Mechanism (EAM) recently proposed by us for the growth of PAH by sequential C2H additions to benzene. We consider consecutive C2H and CN additions to C6H6,C6H6 +CN/ C6H5CN + H, C6H5CN + C2H / C6H4(CN)(C2H) + H, C6H5CN + CN / C6H4(CN)2 +H,C6H4(CN)(C2H) + C2H / 2-aza-4-ethynyl-1-naphthyl/2-aza- 1-ethynyl-4-naphthyl, C6H4(CN)2 +C2H / C6H4(CN)(NCCCH), and Published on 13 July 2010. Downloaded by California Institute of Technology 13/02/2017 02:31:34. C6H4(CN)(NCCCH) + C2H / 1,4-diethynylphthalazine. Although these reactions are found to be barrierless and exothermic and therefore feasible at low temperatures, the steps leading to the aza-ethynyl-naphthyl radicals, C6H4(CN)(NCCCH), and 1,4-diethynylphthalazine can give N-PACs as final products only upon their collisional or radiational stabilization. Alternatively, an N-PAC can be synthesized via the reaction of 2-methyleneaminobenzonitrile with C2H, producing 4-ethynyl-quinoline + H without an entrance barrier via a three- step sequence including C2H addition to C of CN, ring closure, and H elimination. 2-Methyleneaminobenzonitrile itself can be formed in the reaction of methyleneaminobenzene with cyano radical, C6H5(NCH2)+CN/ C6H5(NCH2)(CN) / C6H4(NCH2)(CN) + H, which also does not have any entrance barrier. Methyleneaminobenzene can be produced through recombination of phenyl and methylene-amidogen radicals followed by collisional stabilization of the product, via the barrierless C6H5 +CH3N / C6H5(NCH3) / C6H5(NCH2) + H reaction, or in the reaction of phenyl with methyleneimine, C6H5 +CH2NH / C6H5(NHCH3) / C6H5(NCH2) + H. The Florida International University, Department of Chemistry & Biochemistry, Miami, 33199 This journal is ª The Royal Society of Chemistry 2010 Faraday Discuss., 2010, 147, 479–494 | 479 View Article Online latter would be slow at low-temperature conditions owing to the barriers of 4.5 and 2.8 kcal molÀ1 relative to the initial reactants, but feasible if the reactants possess sufficient internal energy to overcome these barriers. We anticipate that the presented mechanisms are viable to form N-PACs in hydrocarbon and nitrogen rich, low temperature atmospheres of planets and their moons such as Titan. Introduction Polycyclic aromatic hydrocarbons (PAHs), present in the Earth environment in the form of atmospheric aerosols, soot, and volatile particles, are also believed to play an important role in interstellar chemistry1–6 and in the atmospheric chemistry of planets and theirs moons in the Solar System.7 PAHs have been identified in labo- ratory experiments simulating Jupiter’s atmosphere and also the atmospheric chem- istry of Saturn’s moon Titan together with its colored organic haze layers.8 The detection of the simplest aromatic hydrocarbon, benzene, which is considered as a critical precursor toward complex PAHs, in the low-temperature atmospheres of Jupiter and Saturn9 and recently on Titan10 strongly suggests that the PAH synthesis may proceed in these environments. It is widely believed that PAHs are responsible for the formation of organic haze layers in Titan’s atmosphere,7 but the nature of PAHs on Titan and their formation and growth mechanisms are not well under- stood. Considering the high abundance of nitrogen in Titan’s atmosphere, it is prob- able that the haze layers contain not only pure hydrocarbon PAHs but also their nitrogenated analogs, i.e., hetero-PAHs or PAHs including ‘hetero’ N atoms in their aromatic rings. Laboratory studies of a laboratory analogue of Titan’s haze, called tholin, also support the hypothesis that, together with pure PAHs and their cations, nitrogenated heterocyclic PAHs may be the fundamental building blocks of micro- physical particles in Titan’s hazes.11,12 For instance, nitrogen-containing heteroaro- matics and PAHs were previously detected in tholins by Khare et al.,13 Sagan et al.,14 and Ehrenfreund et al.15 In the last decade, Khare and co-workers synthesized a Titan tholin and measured the time variation of its infrared spectrum as a thin film developed.11 They found that one of the earliest features to emerge in the IR spectra was a very strong absorption feature at 1351 cmÀ1, which was ascribed to skeletal ring vibrations16 between members of C and N rings, perhaps arranged in relatively dehydrogenated, positively charged heterocyclic ring structures reminis- cent of graphitic structures. The IR spectra of the Titan tholin indicated an upper Published on 13 July 2010. Downloaded by California Institute of Technology 13/02/2017 02:31:34. limit of 6% by mass in benzenes, heterocyclics, and PAHs with more than four rings.11 Quirico et al.17 generated powdered samples of carbon–nitrogen–hydrogen tholins that mimic Titan’s atmosphere aerosols under levitation conditions in the laboratory with a dusty plasma and studied them using various spectroscopic tech- niques. The observed first-order UV Raman bands at 690 and 980 cmÀ1 suggested that C3N3 rings are present as a species inserted in the macromolecular network. A recent laboratory study of Titan’s tholin IR spectra by Carrasco et al.18 showed intense absorption bands peaking at 1560 and 1630–1650 cmÀ1, which involve the contribution of C]N groups, as imine and/or in N-bearing heteroaromatic or heterocyclic groups. Imanaka et al.12 produced tholins in an inductively coupled plasma from a nitrogen–methane gas mixture at various pressures ranging from 13 to 2300 Pa. A careful structural analysis by spectroscopic methods allowed them to conclude that tholins formed at low pressures include clusters of nitrogen-containing polycyclic aromatic compounds (N-PACs) in a matrix of carbon and nitrogen branched chain networks, which are connected tightly to each other with hydrogen bonding of N–H bonds. Nitrogen was found to be incor- porated into tholin more efficiently at lower deposition pressures. Applying the experimental results to Titan, Imanaka et al. suggested12 that the tholins formed at low pressures (13 and 26 Pa) may be better representations of Titan’s haze than 480 | Faraday Discuss., 2010, 147, 479–494 This journal is ª The Royal Society of Chemistry 2010 View Article Online those formed at high pressures and thus, the N-PACs found in tholin formed at low pressure may be present in Titan’s haze. These aromatic macromolecules might have a significant influence over the thermal structure and complex organic chemistry in Titan’s atmosphere, because they are efficient absorbers of UV radiation and effi- cient charge exchange intermediaries.12 For understanding the origin and evolution of Titan’s haze layers, it is important to find synthetic routes leading to the N-PACs from small molecules, abundant in Titan’s atmosphere, and radicals, which can be produced by their photodissociation. To our best knowledge, the only theoretical study of the mechanism of nitrogen inclusion into an aromatic ring to form N-PACs has been reported by Ricca et al.19 The authors used density functional B3LYP calculations to compute potential energy profiles for reaction sequences analogous to the Hydrogen-Abstrac- 20 tion–C2H2-Addition (HACA) mechanism of PAH growth, but with HCN or cya- noacetylene HCCCN replacing C2H2 at certain reaction steps. They found that the presence of a nitrogen atom increases the barrier heights relative to the pure hydro- carbon species to about 15 kcal molÀ1 and concluded that the calculated barriers are probably too high to allow reactions to occur in the atmosphere of Titan unless the reaction rates are enhanced by vibrational energy of the aromatic molecule, although the presence of a nitrogen atom in the aromatic ring promotes the forma- tion of an additional ring. Similar to the HACA mechanism, we anticipate that the reactions suggested by Ricca et al.19 would be more applicable to high-temperature conditions. As a practicable alternative to the HACA sequences to form PAH-like species under low temperature conditions in the interstellar medium and in hydro- carbon rich atmospheres of planets and their moons, we have recently established computationally a novel Ethynyl Addition Mechanism (EAM).21 Initiated by an addition of the ethynyl radical (C2H) to the ortho carbon atom of the phenylacety- lene (C6H5C2H) molecule, the reactive intermediate rapidly loses a hydrogen atom forming 1,2-diethynylbenzene. The latter can react with a second ethynyl molecule via addition to a carbon atom of one of the ethynyl side chains. A consecutive ring closure of the intermediate leads to an ethynyl-substituted naphthalene core.
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