Previous experimental work by Necula and Scott has characterized the isomerization of polycyclic aromatic hydrocarbons (PAHs) under flash vacuum pyrolysis (FVP) conditions.1 Four mechanisms were proposed for the FVP of phenanthrene, but no one mechanism was deemed favorable. One mechanism describes the simultaneous removal of two bay-region hydrogens, forming a diradical intermediate, and its rearrangements to 1-ethynyl, 2-ethynyl, and 4-ethynylacenaphthylene and pyracylene. This computational study investigates the structures and energies of these species and the proposed intermediates. Key mechanisms include the collapse of the diradical, hydrogen shift/benzene ring contraction rearrangements, 1,2-hydrogen shifts between vinylidene and ethynyl groups, and C-H vinylidene insertions. Relative Gibbs free energies of M06-2X/6-31G* and UM06-2X/6-31G* optimized geometries are compared in order to examine the barrier heights of the mechanistic pathway. The hydrogen shift/benzene ring contraction rearrangements are shown to be degenerate pathways. Additionally, computational evidence reveals the requirement for flexibility of the system in order to undergo these rearrangements, suggesting an increase in barrier heights for larger bay-region PAHs. Results on the diradical and vinylidene mechanisms will be presented. Elucidation of the mechanisms and barrier heights by which bay-region PAHs undergo rearrangements may supplement the synthesis of novel species. 1Necula, A.; Scott, L.T., Polycyclic Aromat. Compd. 2010, 30, 260-273

Computational study on the high temperature isomerization of phenanthrene

249th National Meeting (2015)

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