Reactivity of the Indenyl Radical (C9H7) with Acetylene (C2H2) and Vinylacetylene (C4H4). Issue 11 (18th April 2019) - Zhao, Long
Abstract: The reactions of the indenyl radicals with acetylene (C2 H2 ) and vinylacetylene (C4 H4 ) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1‐indenyl radical (C9 H7 . ) is always formed in the pyrolysis of 1‐, 2‐, 6‐, and 7‐bromoindenes at 1500 K. The 1‐indenyl radical reacts with acetylene yielding 1‐ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C6 H5 . ) and acetylene forming phenylacetylene (C6 H5 CCH), the 1‐indenyl+acetylene→1‐ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol −1 ) and slow, contrary to the exoergic (−38 kJ mol −1 ) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1‐indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces. Abstract : Radical formation : The 1‐indenyl radical can be formed from the pyrolytic decomposition of 1‐, 2‐, 6‐, and 7‐bromoindenes, and appears not to be an efficient precursor for further polycyclic aromatic hydrocarbon growth either via hydrogen abstraction‐acetylene addition or hydrogen abstraction–vinylacetylene addition mechanisms.