Exploring the Reaction Dynamics of Elementary Reactions Leading to Polycyclic Aromatic Hydrocarbons (PAHs) and their Hydrogen Deficient Precursors


The primary objectives of this project are to experimentally explore the energetics, dynamics, and potential energy surfaces (PESs) of reactions of key aromatic radicals (ARs) with unsaturated C2 to C4 hydrocarbons leading to prototype polycyclic aromatic hydrocarbons (PAHs) carrying six membered and five membered rings in the gas phase under single collision conditions at the most fundamental, microscopic level. These molecular mass growth processes represent a fundamental unsolved challenge and are of critical importance to the gas phase reaction dynamics community to ultimately untangle the formation of three-dimensional carbonaceous nanostructures and soot particles. PAHs carrying five membered rings such as fluorene and cyclopentanaphthalenes represent essential molecular building blocks of non-planar PAHs like corannulene along with fullerenes. These species require five-membered rings in the carbon backbone of the PAH to ‘bent’ PAHs like corannulene out of the plane. The intimate knowledge of the elementary mechanisms to synthesize PAHs carrying five-membered ring(s) is therefore critical to our understanding of the early stage chemistry in combustion systems how precursor PAHs to three dimensional (bowl-shaped) carbonaceous structures and ultimately soot particles form. However, the inherent elementary steps, reaction dynamics, energy flow processes, and reaction mechanisms to form these PAHs on the molecular level are still elusive as detailed synthetic routes have not been investigated experimentally under single collision conditions to date.


To achieve these goals, we conduct two sets of complementary experiments. First, reactions are initiated in a crossed molecular beams machine under single collision conditions by intersecting two supersonic reactant beams containing radicals and closed shell species under a well-defined collision energy and crossing angle (UH Manoa). By recording angular-resolved time of flight (TOF) spectra, we obtain information on the reaction products, intermediates involved, branching ratios of competing reaction channels, reaction energetics, and on the underlying reaction mechanisms to form PAHs along with their acyclic isomers. Second, in collaboration with Dr. Musahid Ahmed (Advanced Light Source, Lawrence Berkeley National Laboratory), reactions have been carried out in a chemical reactor at well characterized pressure and temperature distributions with reaction products interrogated isomer-selectively by tunable vacuum ultraviolet light (VUV) via photoionization (PI) coupled with a reflectron time of flight mass spectrometer (ReTOFMS). Both sets of experiments are merged with electronic structure calculations by Prof. Alexander M. Mebel (Florida International University), and are supported by synthetic efforts Prof. Felix Fischer (UC Berkeley); Prof. Stanislaw Wnuk (Florida International University). This endeavor helps to extract the underlying chemical dynamics, reaction mechanisms, products and intermediates, energetics, branching ratios, and enthalpies of formation of complex polycyclic aromatic hydrocarbons (PAHs). This also assists to comment on the vital role of these processes to form distinct PAHs and their isomers in combustion flames and links to fundamental molecular mass growth processes leading ultimately to soot particles. These data are very much required by the combustion chemistry community to understand the formation of polycyclic aromatic hydrocarbons (PAHs) and their hydrogen deficient precursors from the ‘bottom up’.

Bicyclic Aromatic Hydrocarbons

Considering the unique capability of our setups to untangle the energetics and dynamics of bimolecular reactions leading to prototype bicyclic polycyclic aromatic hydrocarbons (PAHs) indene (C9H8), naphthalene (C10H8), and dihydronaphthalene (C10H10) via reactions of the phenyl radical (C6H5) with unsaturated C3 (methylacetylene, allene) and C4 hydrocarbons (vinylacetylene, 1,3-butadiene) under single collision conditions, we expanded our studies to the next level and investigated the formation of (di)methyl-substituted PAHs with indene and naphthalene cores in crossed molecular beams experiments and by exploiting the chemical reactor. In the crossed beams machine, we prepared intense supersonic beams of para- and meta-tolyl (4- and 3-methylphenyl) radicals (C6H4CH3) via photodissociation of helium-seeded para- and meta-chlorotoluene (4- and 3-chlorotoluene) at 193 nm and probed the reactions with unsaturated C3 to C5 hydrocarbons. With the exception of methyl-substituted indene, these bimolecular reactions lead to the formation of (di)methyl-substituted polycyclic aromatic hydrocarbons (PA-Hs) with naphthalene and 1,4-dihydronaphthalene cores in exoergic and entrance barrier-less reactions under single collision conditions. Most importantly, the reaction mechanism involves the initial formation of a van-der-Waals complex and addition of the phenyl-type radical to the C1 position of a vinyl-type group through a submerged barrier. Our investigations suggest that in the hydrocarbon reactant, the vinyl-type group must be in conjugation with a -C≡CH or -HC=CH2 group to form a resonantly stabilized free radical (RSFR) intermediate, which eventually isomerizes to a cyclic intermediate followed by hydrogen loss and aromatization with PAH formation. The barrierless formation of (dimethyl-substituted) PAHs defies conventional wisdom that PAH synthesis necessitates elevated temperatures. Studies in the chemical reactor revealed the complimentary nature of the hydrogen abstraction – acetylene addition (HACA) mechanism leading to naphthalene (C10H8) at elevated temperatures via reactions of phenyl radicals with acetylene; on the other hand, reactions of the phenyl radical with vinylacetylene via the hydrogen abstraction – vinylacetylene addition (HAVA) mechanism synthesize naphthalene (C10H8) without entrance barrier at low temperatures as well. These systems were also expanded to explore the formation of nitrogen-bearing PAH counterparts.

Prototype polycyclic aromatic hydrocarbons (PAHs) indene, naphthalene, and dihydronaphthalene (top row) together with their (di)methylsubstituted counterparts (bottom row) formed in the reactions of phenyl-type radicals (phenyl, para- and meta-tolyl) with C3 to C5 hydrocarbons. Indene can also be synthesized via the reaction of the benzyl radical with acetylene.

Formation of Tricyclic Aromatic Hydrocarbons

We elucidated fundamental reaction mechanisms leading to the formation of three-ring PAHs carrying three six membered rings [phenanthrene (C14H10), dihydrophenanthrene (C14H12), and anthracene (C14H10)] and two six membered plus one five membered ring [acenaphthylene (C12H8)]. These studies revealed that the HACA mechanism neither yields anthracene (C14H10) nor phenanthrene (C14H10) from naphthyl radical reactions (C10H7) with acetylene, but solely acenaphthylene (C12H8) However, HACA starting from non-PAH radicals such as biphenylyl (C12H9) reacting with acetylene (C2H2) can synthesize via bay-closure phenanthrene (C14H10), but not the anthracene isomer (C14H10). On the other hand, HAVA commencing from 1- and 2- naphthyl radicals (C10H7) reacting with vinylacetylene (C4H4) can form via ring annulation phenanthrene (C14H10) and anthracene (C14H10) via barrierless reactions of aryl-type aromatic radicals with vinylacetylene. These studies highlight the complementary nature of HACA and HAVA to form two dimensional (planar) PAHs via bay-closure and barrierless ring annulation with HAVA operating even at ultralow temperatures.

Complementary pathways to bi- and tricyclic PAHs via the hydrogen abstraction – acetylene addition (HACA) (black) and the barrier-less hydrogen abstraction – vinylacetylene addition (HAVA) pathways (blue). Pathways involving barrierless reactions with 1,3-butadiene are color coded in green.
Barrierless formation of naphthalene via the hydrogen abstraction - vinylacetylene addition (HAVA) reaction mechanism.

Formation of Tetracyclic Aromatic Hydrocarbons

We expanded our studies on the growth of PAHs from bi- and tricyclic systems to aromatic systems carrying four six membered rings utilizing pyrene (C16H10) as a prototype. By exploring the reaction of the 4-phenanthrenyl radical ([C14H9]) with acetylene (C16H10) under conditions prevalent in high temperature combustion systems, we provide testimony on a facile, isomer-selective formation of pyrene (C16H10). Along with the Hydrogen Abstraction – Vinylacetylene Addition (HAVA) mechanism, molecular mass growth processes from pyrene may lead through systematic ring annulation not only to more complex PAHs, but ultimately to two-dimensional graphene-type nanostructures thus facilitating an understanding toward soot growth in combustion systems.

The atomic structure of pyrene molecules are represented in an artist’s rendering of an asteroid belt, with carbon atoms shown in black and hydrogen atoms in white. A new study shows chemical steps for how pyrene, a type of hydrocarbon found in some meteorite samples, could form in space.
Recent Selected Publications

1.   A. M. Mebel, V.V. Kislov, R.I. Kaiser, Photoinduced Mechanism of Formation and Growth of Polycyclic Aromatic Hydrocarbons in Low-Temperature Environments via Successive Ethynyl Radical Additions, JACS 130, 13618-13629  (2008).   (PDF)

2.   F. Zhang, B. Jones, P. Maksyutenko, R.I. Kaiser, C. Chin, V.V. Kislov, A.M. Mebel, Formation of the Phenyl Radical [C6H5(X2A1)] under Single Collision Conditions: A Crossed Molecular Beam and Ab Initio Study, JACS 132, 2672-2683  (2010).   (PDF)

3.   B.M. Jones, F. Zhang, R.I. Kaiser, A. Jamal, A.M. Mebel, M.A. Cordiner, S.B. Charnley, Formation of Benzene in the Interstellar Medium, Proceedings of the National Acadamy of Sciences USA 108, 452-457  (2011).   (PDF)

4.   D.S.N. Parker, F. Zhang, Y.S. Kim, R.I. Kaiser, A. Landera, V.V. Kislov, A.M. Mebel, A.G.G.M. Tielens, Low Temperature Formation of Naphthalene and its Role in the Synthesis of PAHs (Polycyclic Aromatic Hydrocarbons) in the Interstellar Medium, PNAS, 109, 53-58  (2012).   (PDF)

5.   B.B. Dangi, D.S.N. Parker, R.I. Kaiser, A. Jamal, A.M. Mebel, A Combined Experimental and Theoretical Study on the Gas-Phase Synthesis of Toluene under Single Collision Conditions, Angew. Chem. Int. Ed., 52, 7186-7189  (2013).   (PDF)

6.   B.B. Dangi, D.S.N. Parker, T. Yang, R.I. Kaiser, A.M. Mebel, Gas Phase Synthesis of the Benzyl Radical (C6H5CH2), Angew. Chem. Int. Ed., 53, 4608-4613  (2014).   (PDF)

7.   D.S.N. Parker, R.I. Kaiser, T.P. Troy, M. Ahmed, Hydrogen Abstraction/Acetylene Addition Revealed, Angew. Chem. Int. Ed., 53, 7740-7744  (2014).   (PDF)

8.   R.I. Kaiser, D. S. N. Parker, A. M. Mebel, Reaction Dynamics in Astrochemistry: Low-Temperature Pathways to Polycyclic Aromatic Hydrocarbons in the Interstellar Medium, Annual Reviews of Physical Chemistry. 66, 43-47  (2015).   (PDF)

9.   T. Yang, L. Muzangwa, D.S.N. Parker, R.I. Kaiser, A.M. Mebel, Formation of 2- and 1-Methyl-1,4-Dihydronaphthalene Isomers via the Crossed Beam Reactions of Phenyl Radicals (C6H5) with Isoprene (CH2C(CH3)CHCH2) and 1,3-Pentadiene (CH2CHCHCHCH3), Phys. Chem. Chem. Phys., 17, 530-540  (2015).   (PDF)

10.   L. Muzangwa, T. Yang, D.S.N. Parker, R.I. Kaiser, A.M. Mebel, A. Jamal, M. Ryazantsev, K. Morokuma, A Crossed Molecular Beam and Ab Initio Study on the Formation of 5- and 6-Methyl-1,4-Dihydronaphthalene (C11H12) via the Reaction of Meta-Tolyl (C7H7) with 1,3-Butadiene (C4H6), Phys. Chem. Chem. Phys., 17, 7699-7706  (2015).   (PDF)

11.   T. Yang, D.S.N. Parker, B.B. Dangi, R.I. Kaiser and A.M. Mebel, Formation of 5- and 6-methyl-1H-indene (C10H10) via the Reactions of the Para-tolyl Radical (C6H4CH3) with Allene (H2CCCH2) and Methylacetylene (HCCCH3) under Single Collision Conditions, Phys. Chem. Chem. Phys., 17, 10510-10519 (2015).   (PDF)

12.   D.S.N. Parker, R.I. Kaiser, O. Kostko, T.P. Troy, M. Ahmed, A.M. Mebel, A.G.G.M. Tielens, Gas Phase Synthesis of (Iso)Quinoline and Its Role in the Formation of Nucleobases in the Interstellar Medium, Ap J., 803:53, 1-10  (2015).   (PDF)

13.   D.S.N. Parker, R.I. Kaiser, B. Bandyopadhyay, O. Kostko, T.P. Troy, M. Ahmed, Unexpected Chemistry from the Reaction of Naphthyl and Acetylene at Combustion-Like Temperatures, Angew. Chem. Int. Ed. 54, 5421-5424  (2015).   (PDF)

14.   D.S.N. Parker, R.I. Kaiser, O. Kostko and M. Ahmed, Selective Formation of Indene through the Reaction of Benzyl Radicals with Acetylene, Chem. Phys. Chem. 16, 2091-2093  (2015).   (PDF)

15.   D.S.N. Parker, R.I. Kaiser, T.P. Troy, O. Kostko and M. Ahmed, Toward the Oxidation of the Phenyl Radical and Prevention of PAH Formation in Combustion Systems, J. Phys. Chem. A 119, 7145-7154  (2015).   (PDF)

16.   T. Yang, L. Muzangwa, R.I. Kaiser, A. Jamal, and K. Morokuma, A Combined Crossed Molecular Beam and Theoretical Investigation of the Reaction of the Meta-tolyl Radical with Vinylacetylene- toward the Formation of Methylnaphthalenes, Phys. Chem. Chem. Phys. 34, 461-514  (2015).   (PDF)

17.   A.M. Mebel, R.I. Kaiser, Formation of resonantly stabilised free radicals via the reactions of atomic carbon, dicarbon, and tricarbon with unsaturated hydrocarbons: theory and crossed molecular beams experiments, International Reviews in Physical Chemistry. 17, 21564-21575  (2015).   (PDF)

18.   D.S.N. Parker, R.I. Kaiser, O. Kostko, T.P. Troy, M. Ahmed, B-J Sun, S-H Chen, A.H.H. Chang, On the formation of pyridine in the interstellar medium, Phys. Chem. Chem. Phys., 17, 32000-32008  (2015).   (PDF)

19.   D.S.N. Parker, T. Yang, B.B. Dangi, R.I. Kaiser, P.P. Bera, and T.J. Lee, Low Temperature Formation of Nitrogen-substituted Polycyclic Aromatic Hydrocarbons (PANHs)- Barrierless Routes to Dihydro(iso)quinolines, Ap. J., 815, 11  (2015).   (PDF)

20.   T. Yang, T.P. Troy, B. Xu, O. Kostko, M. Ahmed, A.M. Mebel, R.I. Kaiser, Hydrogen-Abstraction/Acetylene-Addition Exposed, Angew. Chem. Int. Ed., 55, 14983-14987  (2016).   (PDF)

21.   A.M. Thomas, T. Yang, B.B. Dangi, R.I. Kaiser, G-S Kim, A.M. Mebel Oxidation of the para-Tolyl Radical by Molecular Oxygen under Single-Collison Conditions: Formation of the para-Toloxy Radical, J. Phys. Chem. Lett., 7, 5121-5127  (2016).   (PDF)

22.   D.S.N. Parker, R.I. Kaiser, On the Formation of Nitrogen-Substituted Polycyclic Aromatic Hydrocarbons (NPAHs) in Circumstellar and Interstellar Environments, Chem. Soc. Rev., 46, 452-463  (2017).   (PDF)

23.   A.M. Mebel, A. Landera, R.I. Kaiser, Formation Mechanisms of Naphthalene and Indene: From the Interstellar Medium to Combustion Flames, J. Phys. Chem. A, 121, 901-926  (2017).   (PDF)

24.   T. Yang, R.I. Kaiser, T.P. Troy, B. Xu, O. Kostko, M. Ahmed, A.M. Mebel, M.V. Zagidullin, V.N. Azyazov, HACA's Heritage: A Free-Radical Pathway to Phenanthrene in Circumstellar Envelopes of Asymptotic Giant Branch Stars, Angew. Chem. Int. Ed., 56, 4515-4519  (2017).   (PDF)

25.   A.M. Thomas, M. Lucas, T. Yang, R.I. Kaiser, L. Fuentes, D. Belisario-Lara, A.M. Mebel, A Free-Radical Pathway to Hydrogenated Phenanthrene in Molecular Clouds - Low Temperature Growth of Polycyclic Aromatic Hydrocarbons, Chem. Phys. Chem., 18, 1971-1976  (2017).   (PDF)

26.   M. Lucas, A.M. Thomas, L. Zhao, R.I. Kaiser, G-S Kim, A.M. Mebel, Gas-Phase Synthesis of the Elusive Cyclooctatetraenyl Radical (C8H7) via Triplet Aromatic Cyclooctatetraene (C8H8) and Non-Aromatic Cyclooctatriene (C8H8) Intermediates, Angew. Chem. Int. Ed., 56, 13655-13660  (2017).   (PDF)

27.   M. Lucas, A.M. Thomas, R.I. Kaiser, E.K. Bashkirov, V.N. Azyazov, A.M. Mebel Combined Experimental and Computational Investigation of the Elementary Reaction of Ground State Atomic Carbon (C; 3Pj) with Pyridine (C5H5N; X1A1) via Ring Expansion and Ring Degradation Pathways, J. Phys. Chem. A 122, 3128-3139  (2018).   (PDF)

28.   A.M. Thomas, M. Lucas, L. Zhao, J. Liddiard, R.I. Kaiser, A.M. Mebel A combined crossed molecular beams and computational study on the formation of distinct resonantly stabilized C5H3 radicals via chemically activated C5H4 and C6H6 intermediates, Phys. Chem. Chem. Phys. 20, 10906-10925  (2018).   (PDF)

29.   L. Zhao; R.I. Kaiser, B. Xu, U. Ablikim, M. Ahmed, D. Joshi, G. Veber, F.R. Fischer, A.M. Mebel Pyrene synthesis in circumstellar envelopes and its role in the formation of 2D nanostructures, Nature Astronomy 2, 413-419  (2018).   (PDF)

30.   L. Zhao, R.I. Kaiser, B. Xu, U. Ablikim, M. Ahmed, M.V. Zagidullin, V.N. Azyazov, A.H. Howlader, S.F. Wnuk, A.M. Mebel VUV Photoionization Study on the Formation of the Simplest Polycyclic Aromatic Hydrocarbon: Naphthalene (C10H8), J. Phys. Chem. Lett. 9, 2620-2626  (2018).   (PDF)

31.   A.M. Thomas, L. Zhao, C. He, A.M. Mebel, R.I. Kaiser, A Combined Experimental and Computational Study on the Reaction Dynamics of the 1-Propynyl (C3H3)-Acetylene (HCCH) System and the Formation of Methyldiacetylene (CH3CCCCH), J. Phys. Chem. A 122, 6663-6672 (2018).   (PDF)

32.   L. Zhao, R.I. Kaiser, B. Xu, U. Ablikim, M. Ahmed, M.M. Evseev, E.K. Bashkirov, V.N. Azyazov, A.M. Mebel, Low-Temperature Formation of Polycyclic Aromatic Hydrocarbons in Titan's Atmosphere, Nat. Astron. 2, 973–979, (2018).   (PDF)

33.   M.V. Zagidullin, R.I. Kaiser, D.P. Porfiriev, I.P. Zavershinskiy, M. Ahmed, V.N. Azyazov, A.M. Mebel, Functional Relationships between Kinetic, Flow, and Geometrical Parameters in a High-Temperature Chemical Microreactor, J. Phys. Chem. A, 8819–8827, (2018).   (PDF)