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.

doe_1 — 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.

doe_4 — 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.

mission36 — 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.

doe_5 — 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; ³P_j) 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)

34. L. Zhao, R.I. Kaiser, B. Xu, U. Ablikim, W. Lu, M. Ahmed, M.M. Evseev, E.K. Bashkirov, V.N. Azyazov, A.H. Howlader, S.F. Wnuk, A.M. Mebel, Gas-Phase Synthesis of Triphenylene (C₁₈H₁₂), ChemPhysChem 20, 791-797 (2019). (PDF)

35. L. Zhao, R.I. Kaiser, B. Xu, U. Ablikim, W. Lu, M. Ahmed, M.M. Evseev, E.K. Bashkirov, V.N. Azyazov, M.V. Zagidullin, A.N. Morozov, A.H. Howlader, S.F. Wnuk, A.M. Mebel, D. Joshi, G. Veber, F.R. Fischer, Gas phase synthesis of [4]-helicene, Nat. Commun. 10, 1510-1517 (2019). (PDF)

36. A. M. Thomas, C. He, L. Zhao, G. R. Galimova, A. M. Mebel, R. I. Kaiser, Combined Experimental and Computational Study on the Reaction Dynamics of the 1-Propynyl (CH₃CC)-1,3-Butadiene (CH₂CHCHCH₂) System and the Formation of Toluene under Single Collision Conditions, J. Phys. Chem. A, 123, 4104-4118, (2019). (PDF)

37. L. Zhao, M.B. Prendergast, R.I. Kaiser, B. Xu, W. Lu, U. Ablikim, M. Ahmed, A.D. Oleinikov, V.N. Azyazov, A.M. Mebel, A.H. Howlader, S.F. Wnuk, Reactivity of the Indenyl Radical (C₉H₇) with Acetylene (C₂H₂) and Vinylacetylene (C₄H₄), ChemPhysChem, 20, 1437-1447 (2019). (PDF)

38. C. He, L. Zhao, A. M. Thomas, A. N. Morozov, A. M. Mebel, R. I. Kaiser, Elucidating the Chemical Dynamics of the Elementary Reactions of the 1-Propynyl Radical (CH₃CC; X²A₁) with Methylacetylene (H₃CCCH; X¹A₁) and Allene (H₂CCCH₂; X¹A₁), J. Phys. Chem. A, 123, 5446-5462 (2019). (PDF)

39. L. Zhao, M. Prendergast, R.I. Kaiser, B. Xu, U. Ablikim, W. Lu, M. Ahmed, A.D. Oleinikov, V.N. Azyazov, A.H. Howlader, S.F. Wnuk, A.M. Mebel, How to add a five-membered ring to polycyclic aromatic hydrocarbons (PAHs) – molecular mass growth of the 2-naphthyl radical (C₁₀H₇) to benzindenes (C₁₃H₁₀) as a case study, Phys. Chem. Chem. Phys. 21, 16737-16750 (2019). (PDF)

40. C. He, A. M. Thomas, G. R. Galimova, A.M. Mebel, R.I. Kaiser, Gas Phase Formation of the Interstellar Molecule Methyltriacetylene, ChemPhysChem, 20, 1912-1917 (2019). (PDF)

41. L. Zhao, R.I. Kaiser, W. Lu, B. Xu, M. Ahmed, A.N. Morozov, A.M. Mebel, A.H. Howlader, S.F. Wnuk, Molecular mass growth through ring expansion in polycyclic aromatic hydrocarbons via radical–radical reactions, Nat. Commun. 10, 3689 (2019). (PDF)

42. A. M. Thomas, L. Zhao, C. He, G. R. Galimova, A. M. Mebel, R. I. Kaiser, Directed Gas-Phase Synthesis of Triafulvene under Single-Collision Conditions, Angew. Chem., Int. Ed., 58, 15488-15495 (2019). (PDF)

43. C. He, L. Zhao, A. M. Thomas, G. R. Galimova, A. M. Mebel, R. I. Kaiser, A Combined Experimental and Computational Study on the Reaction Dynamics of the 1-Propynyl Radical (CH₃CC; X²A₁) with Ethylene (H₂CCH₂; X¹A_1g) and the Formation of 1-Penten-3-yne (CH₂CHCCCH₃; X¹A'), Phys. Chem. Chem. Phys, 21, 22308-22319 (2019). (PDF)

44. L. Zhao, M. B. Prendergast, R. I. Kaiser, B. Xu, U. Ablikim, M. Ahmed, B-J Sun, Y-L Chen, A. H. H. Chang, R. K. Mohamed, F. R. Fischer, Synthesis of Polycyclic Aromatic Hydrocarbons by Phenyl Addition–Dehydrocyclization: The Third Way, Angew. Chem. Int. Ed. 58, 17442-17450 (2019). (PDF)

45. A. M. Thomas, S. Doddipatla, R. I. Kaiser, G. R. Galimova, A. M. Mebel A Barrierless Pathway Accessing the C₉H₉ and C₉H₈ Potential Energy Surfaces via the Elementary Reaction of Benzene with 1-Propynyl, Sci. Rep. 9, 17595 (2019). (PDF)

46. C. He, A. M. Thomas, G. R. Galimova, A. M. Mebel, R. I. Kaiser, Gas-Phase Formation of 1-Methylcyclopropene and 3-Methylcyclopropene via the Reaction of the Methylidyne Radical (CH; X²Π) with Propylene (CH₃CHCH₂; X¹A'), J. Phys. Chem. A 123, 10543-10555 (2019). (PDF)

47. C. He, A. M. Thomas, G. R. Galimova, A. N. Morozov, A. M. Mebel, R. I. Kaiser, Gas-Phase Formation of Fulvenallene (C₇H₆) via the Jahn-Teller Distorted Tropyl (C₇H₇) Radical Intermediate under Single-Collision Conditions, JACS 142, 3205-3213 (2020). (PDF)

48. L. Zhao, R. I. Kaiser, B. Xu, U. Ablikim, M. Ahmed, M. M. Evseev, E. K. Bashkirov, V. N. Azyazov, A. M. Mebel, A Unified Mechanism on the Formation of Acenes, Helicenes, and Phenacenes in the Gas Phase, Angew. Chem. Int. Ed., 59, 4051-4058 (2020). (PDF)

49. C. He, L. Zhao, S. Doddipatla, A. M. Thomas, A. A. Nikolayev, G. R. Galimova, V. N. Azyazov, A. M. Mebel, R. I. Kaiser, Gas-Phase Synthesis of 3-Vinylcyclopropene via the Crossed Beam Reaction of the Methylidyne Radical (CH; X²Π) with 1,3-Butadiene (CH₂CHCHCH₂; X¹A_g), ChemPhysChem, 21, 1295-1309 (2020). (PDF)

50. L. Zhao, R. I. Kaiser, W. Lu, M. Ahmed, M. M. Evseev, E. K. Bashkirov, V. N. Azyazov, C. Tönshoff, F. Reicherter, H. F. Bettinger, A. M. Mebel, A Free Radical Prompted Barrierless Gas-Phase Synthesis of Pentacene, Angew. Chem. Int. Ed. 59, 11334–11338 (2020). (PDF)

51. L. Zhao, R. I. Kaiser, W. Lu, M. Ahmed, A. D. Oleinikov, V. N. Azyazov, A. M. Mebel, A. H. Howlader, S. F. Wnuk, Gas phase formation of phenalene via 10π-aromatic, resonantly stabilized free radical intermediates, Phys. Chem. Chem. Phys. 22, 15381-15388 (2020). (PDF)

52. L. Zhao, R. I. Kaiser, W. Lu, O. Kostko, M. Ahmed, M. M. Evseev, E. K. Bashikirov, A. D. Oleinikov, V. N. Azyazov, A. M. Mebel, A. H. Howlader, S. F. Wnuk, Gas phase formation of cyclopentanaphthalene (benzindene) isomers via reactions of 5- and 6-indenyl radicals with vinylacetylene, Phys. Chem. Chem. Phys. 22, 22493-22500 (2020). (PDF)

53. A. H. Howlader, K. Diaz, A. M. Mebel, R. I. Kaiser, S. F. Wnuk Iodoindenes: Synthesis and application to cross-coupling, Tetrahedron Lett., 61, 152427 (2020). (PDF)

54. C. He, G. R. Galimova, Y. Luo, L. Zhao, A. K. Eckhardt, R. Sun, A. M. Mebel, R. I. Kaiser, A Chemical Dynamics Study on the Gas-phase Formation of Triplet and Singlet C₅H₂ Carbenes, Proc. Natl. Acad. Sci. U.S.A. 117, 30142-30150 (2020). (PDF)

55. S. Doddipatla, G. R. Galimova, H. Wei, A. M. Thomas, C. He, Z. Yang, A. N. Morozov, C. N. Shingledecker, A. M. Mebel, R. I. Kaiser, Low-temperature Gas-phase Formation of Indene in the Interstellar Medium, Sci. Adv., 7, eabd4044 (2021). (PDF)

56. C. He, A. A. Nikolayev, L. Zhao, A. M. Thomas, S. Doddipatla, G. R. Galimova, V. N. Azyazov, A. M. Mebel, R. I. Kaiser, Gas-Phase Formation of C₅H₆ Isomers via the Crossed Molecular Beam Reaction of the Methylidyne Radical (CH; X²Π) with 1,2-Butadiene (CH₃CHCCH₂; X¹A'), J. Phys. Chem. A 125, 126-138 (2021). (PDF)

57. L. Zhao, S. Doddipatla, R. I. Kaiser, W. Lu, O. Kostko, M. Ahmed, L. B. Tuli, A. N. Morozov, A. H. Howlader, S. F. Wnuk, A. M. Mebel, V. N. Azyazov, R. K. Mohamed, F. R. Fischer, Gas-phase synthesis of corannulene - a molecular building block of fullerenes, Phys. Chem. Chem. Phys., 23, 5740-5749 (2021). (PDF)

58. R. I. Kaiser, N. Hansen, An Aromatic Universe−A Physical Chemistry Perspective, J. Phys. Chem. A., (2021). doi: 10.1021/acs.jpca.1c00606 (PDF)

59. L. Zhao, W. Lu, M. Ahmed, M. V. Zagidullin, V. N. Azyazov, A. N. Morozov, A. M. Mebel, R. I. Kaiser, Gas-phase synthesis of benzene via the propargyl radical self-reaction, Science Advances, 7, eabf0360 (2021). (PDF)

60. L. Zhao, M. Prendergast, R. I. Kaiser, B. Xu, W. Lu, M. Ahmed, A. H. Howlader, S. F. Wnuk, A. S. Korotchenko, M. M. Evseev, E. K. Bashkirov, V. N. Azyazovde, A. M. Mebel, A molecular beam and computational study on the barrierless gas phase formation of (iso)quinoline in low temperature extraterrestrial environments, Phys. Chem. Chem. Phys., 23, 18495 (2021). (PDF)

61. A. A. Nikolayev, V. N. Azyazov, R. I. Kaiser, A. M. Mebel, Theoretical Study of the Reaction of the Methylidyne Radical (CH; X²Π) with 1-Butyne (CH₃CH₂CCH; X¹A’). J. Phys. Chem A 125, 9536-9547 (2021). (PDF)

62. C. He, K. Fujioka, A. A. Nikolayev, L. Zhao, S. Doddipatla, V. N. Azyazov, A. M. Mebel, R. Sun, R. I. Kaiser, A Chemical Dynamics Study of the Reaction of the Methylidyne Radical (CH, X²Π) with Dimethylacetylene (CH₃CCCH₃, X¹A_1g), Phys. Chem. Chem. Phys., 24, 578 (2022). (PDF)

63. R. I. Kaiser, L. Zhao, W. Lu, M. Ahmed, M. V. Zagidullin, V.N. Azyazov, A.M. Mebel, Gas-Phase Formation of Benzene (C₆H₆) and Naphthalene (C₁₀H₈) through Resonantly Stabilized Cyclopentadienyl-Mediated Radical-Radical Reactions. J. Phys. Chem Lett. 13, 208-213 (2022). (PDF)

64. R. I. Kaiser, L. Zhao, W. Lu, M. Ahmed, V. S. Krasnoukhov, V. N. Azyazov, A. M. Mebel, Unconventional Excited-State Dynamics in the Concerted Benzyl (C₇H₇) Radical Self-Reaction to Anthracene (C₁₄H₁₀). Nat. Commun. 13, 786 (2022). (PDF)

65. S. J. Goettl, C. He, D. Paul, A. A. Nikolayev, V. N. Azyazov, A. M. Mebel, R. I. Kaiser, Gas-Phase Study of the Elementary Reaction of the D1-Ethynyl Radical (C₂D; X²Σ⁺) with Propylene (C₃H₆; X¹A^') under Single-Collision Conditions. J. Phys. Chem. A., 126, 1889-1898 (2022). (PDF)

66. D. Paul, Z. Yang, S. J. Goettl, A. M. Thomas, C. He, A. G. Suits, D. H. Parker, R. I. Kaiser, Photodissociation Dynamics of Astrophysically Relevent Propyl Derivatives (C₃H₇X; X = CN, OH, HCO) at 157 nm Exploiting an Ultracompact Velocity Map Imaging Spectrometer: The (Iso)Propyl Channel, J. Phys. Chem. A, 126, 5768-5775 (2022). (PDF)

67. G. Galimova, A. Mebel, S. J. Goettl, Z. Yang, R. I. Kaiser, A Crossed Molecular Beams and Computational Study on the Unusual Reactivity of Banana Bonds of Cyclopropane (c-C₃H₆; X¹A₁') Through Insertion by Ground State Carbon Atoms (C(³P_j)), Phys. Chem. Chem. Phys., 24, 22453-22463 (2022). (PDF)

68. D. Paul, Z. Yang, A. G. Suits, D. H. Parker, R. I. Kaiser, Photodissociation Dynamics of Xylene Isomers C₆H₄(CH₃)₂ at 157 nm using an Ultracompact Velocity Map Imaging Spectrometer – The C₇H₇ Channel., Chem. Phys. Lett., 807, 140064 (2022). (PDF)

69. R. I. Kaiser, L. Zhao, W. Lu, M. Ahmed, M. M. Evseev, V. N. Azyazov, A. M. Mebel, R. K. Mohamed, F. R. Fischer, X. Li Gas-phase synthesis of racemic helicenes and their potential role in the enantiomeric enrichment of sugars and amino acids in meteorites, Phys. Chem. Chem. Phys., 24, 25077-25087 (2022). (PDF)

70. C. He, Z. Yang, S. Doddipatla, A. M. Thomas, R. I. Kaiser, G. R. Galimova, A. M. Mebel, K. Fujioka, R. Sun, Directed Gas Phase Preparation of Ethynylallene (H₂CCCHCCH; X¹A') via the Crossed Molecular Beam Reaction of the Methylidyne Radical (CH; X²Π) with Vinylacetylene (H₂CCHCCH; X¹A'), Phys. Chem. Chem. Phys., 24, 26499-26510 (2022). (PDF)

71. Z. Yang, G. R. Galimova, C. He, S. Doddipatla, A. M. Mebel, R. I. Kaiser, Gas Phase Formation of 1,3,5,7-Cyclooctatetraene (C₈H₈) through Ring Expansion via the Aromatic 1,3,5-Cyclooctatrien-7-yl Radical (C₈H₉^•) Transient , JACS, 144, 22470-22478 (2022). (PDF)

72. W. Li, L. Zhao, R. I. Kaiser, A Unified Reaction Network on the Formation of Five-Membered Ringed Polycyclic Aromatic Hydrocarbons (PAHs) and Their Role in Ring Expansion Processes Through Radical-Radical Reactions, Phys. Chem. Chem. Phys., 25, 4141-4150 (2023). (PDF)

73. C. He, R. I. Kaiser, W. Lu, M. Ahmed, P. S. Pivovarov, O. V. Kuznetsov, M. V. Zagidullin, A. M. Mebel, Unconventional Pathway in the Gas-Phase Synthesis of 9H-Fluorene (C₁₃H₁₀) via the Radical-Radical Reaction of Benzyl (C₇H₇) with Phenyl (C₆H₅), Angew. Chem. Int. Ed. 62, e202216972 (2023). (PDF)

74. C. He, R. I. Kaiser, W. Lu, M. Ahmed, Y. Reyes, S. F. Wnuk, A. M. Mebel, Exotic Reaction Dynamics in the Gas-Phase Preparation of Anthracene (C₁₄H₁₀) via Spiroaromatic Radical Transients in the Indenyl-cyclopentadienyl Radical-Radical Reaction, JACS, 145, 3084-3091 (2023). (PDF)

75. L. B. Tuli, S. J. Goettl, A. M. Turner, A. H. Howlader, P. Hemberger, S. F. Wnuk, T. Guo, A. M. Mebel, R. I. Kaiser, Gas Phase Synthesis of the C40 Nano Bowl C₄₀H₁₀ Nat. Commun., 14, 1527 (2023). (PDF)

76. A. M. Mebel, M. Agúndez, J. Cernicharo, R. I. Kaiser, Elucidating the Formation of Ethynylbutatrienylidene (HCCCHCCC; X¹A') in the Taurus Molecular Cloud (TMC-1) via the Gas-Phase Reaction of Tricarbon (C₃) with the Propargyl Radical (C₃H₃) Astrophys. J. Lett., 945, L40 (2023). (PDF)

77. C. He, R. I. Kaiser, W. Lu, M. Ahmed, V. S. Krasnoukhov, P. S. Pivovarov, M. V. Zagidullin, V. N. Azyazov, A. N. Morozov, A. M. Mebel, Unconventional Gas-Phase Preparation of the Prototype Polycyclic Aromatic Hydrocarbon Naphthalene (C₁₀H₈) via the Reaction of Benzyl (C₇H₇) and Propargyl (C₃H₃) Radicals Coupled with Hydrogen-Atom Assisted Isomerization, Chem. Sci., 14, 5369-5378 (2023). (PDF)

78. S. J. Goettl, L. B. Tuli, A. M. Turner, Y. Reyes, A. H. Howlader, S. F. Wnuk, P. Hemberger, A. M. Mebel, R. I. Kaiser, Gas-Phase Synthesis of Coronene through Stepwise Directed Ring Annulation, J. Am. Chem. Soc., 145, 15443-15455 (2023). (PDF)

79. S. J. Goettl, Z. Yang, S. Kollotzek, D. Paul, R. I. Kaiser, A. Somani, A. Portela-Gonzalez, W. Sander, A. A. Nikolayev, V. N. Azyazov, A. M. Mebel, Exploring the Chemical Dynamics of Phenylethynyl Radical (C₆H₅CC; X²A₁) Reactions with Allene (H₂CCCH₂; X¹A₁) and Methylacetylene (CH₃CCH; X¹A₁), J. Phys. Chem. A, 127, 5723-5733 (2023). (PDF)

80. I. A. Medvedkov, A. A. Nikolayev, C. He, Z. Yang, A. M. Mebel, R. I. Kaiser, A combined experimental and computational study on the reaction dynamics of the 1-propynyl (CH₃CC, X²A₁) – propylene (CH₃CHCH₂, X²A′) system: formation of 1,3-dimethylvinylacetylene (CH₃CCCHCHCH₃, X¹A′) under single collision conditions, Molecular Physics, e2234509 (2023). (PDF)

81. Z. Yang, G. R. Galimova, C. He, S. J. Goettl, D. Paul, W. Lu, M. Ahmed, A. M. Mebel, X. Li, R. I. Kaiser, Gas-Phase Formation of the Resonantly Stablized 1-Indenyl (C₉H₇^•) Radical in the Interstellar Medium, Science Advances, 9, eadi5060 (2023). (PDF)

82. W. Li, J. Yang, L. Zhao, D. Couch, M. S. Marchi, N. Hansen, A. N. Morozov, A. M. Mebel, R. I. Kaiser, Gas-Phase Preparation of Azulene (C₁₀H₈) and Naphthalene (C₁₀H₈) via the Reaction of the Resonantly Stabilized Fulvenallenyl (C₇H₅^•) and Propargyl (C₃H₃^•) Radicals, Chem. Sci., 14, 9795-9805 (2023). (PDF)

83. Z. Yang, I. A. Medvedkov, S. J. Goettl, R. I. Kaiser, Low-Temperature Gas-Phase Formation of Methanimine (CH₂NH; X¹A') – the Simplest Imine - under Single-Collision Conditions, J. Phys. Chem. Lett., 14, 8500-8506 (2023). (PDF)

84. I. A. Medvedkov, A. A. Nikolayev, C. He, Z. Yang, A. M. Mebel, R. I. Kaiser, One Collision—Two Substituents: Gas-Phase Preparation of Xylenes under Single-Collision Conditions, Angew. Chem. Int. Ed., 63, e202315147 (2024). (PDF)

85. S. J. Goettl, A. M. Turner, B.-J. Sun, A. H. H. Chang, P. Hemberger, R. I. Kaiser, Gas-Phase Preparation of the Dibenzo[e,l]pyrene (C₂₄H₁₄) Butterfly Molecule via a Phenyl Radical-Mediated Ring Annulation, Chem. Commun., 60, 1404-1407 (2024). (PDF)

86. I. A. Medvedkov, A. A. Nikolayev, Z. Yang, S. J. Goettl, A. M. Mebel, R. I. Kaiser, Elucidating the Chemical Dynamics of the Elementary Reactions of the 1-Propynyl Radical (CH₃CC; X²A₁) with 2-Methylpropene ((CH₃)₂CCH₂; X¹A₁), Phys. Chem. Chem. Phys., 26, 6448–6457 (2024). (PDF)

87. S. J. Goettl, C. He, Z. Yang, R. I. Kaiser, A. Somani, A. Portela-Gonzales, W. Sander, B.-J. Sun, S. Fatimah, K. P. Kadam, A. H. H. Chang, Unconventional Gas-Phase Synthesis of Biphenyl and its Atropisomeric Methyl-Substituted Derivatives, Phys. Chem. Chem. Phys., 26, 18321-18332 (2024). (PDF)

88. C. Y. Wang, L. Zhao, R. I. Kaiser, Gas-Phase Preparation of the 14π Hückel Polycyclic Aromatic Anthracene and Phenanthrene Isomers (C₁₄H₁₀) via the Propargyl Addition-BenzAnnulation (PABA) Mechanism, ChemPhysChem, 25, e202400151 (2024). (PDF)

89. A. M. Mebel, W. Li, L. P. Maffei, C. Cavallotti, A. N. Morozov, C.-Y. Wang, J.-Z. Yang, L. Zhao, R. I. Kaiser, Fulvenallenyl Radical (C₇H₅)-Mediated Gas-Phase Synthesis of Bicyclic Aromatic C₁₀H₈ Isomers: Can Fulvenallenyl Efficiently React with Closed-Shell Hydrocarbons?, J. Phys. Chem. A, 128, 5707-5720 (2024). (PDF)

90. Z. Yang, C. He, S. J. Goettl, A. M. Mebel, P. F. G. Velloso, M. O. Alves, B. R. L. Galvão, J.-C. Loison, K. M. Hickson, M. Dobrijevic, X. Li, R. I. Kaiser, Low-temperature formation of pyridine and (iso)quinoline via neutral–neutral reactions, Nat. Astron., 8, 856-864 (2024). (PDF)