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Unimolecular Reaction Energies for Polycyclic Aromatic Hydrocarbon Ions

Abstract

Imaging photoelectron photoion coincidence spectroscopy was employed to explore the unimolecular dissociation of the ionized polycyclic aromatic hydrocarbons (PAHs) acenaphthylene, fluorene, cyclopenta[d,e,f]phenanthrene, pyrene, perylene, fluoranthene, dibenzo[a,e]pyrene, dibenzo[a,l]pyrene, coronene and corannulene. The primary reactions for all of these species is hydrogen atom loss, with the smaller species also exhibiting loss of C2H2 to varying extents. Combined with previous work on smaller PAH ions, trends in the reaction energies for loss of H from sp2-C and sp3-C centres, along with hydrocarbon molecule loss were found as a function of carbon number for ionized PAHs ranging in size from naphthalene to coronene. In the case of molecules which possessed at least one sp3-C centre, the activation energy for the loss of an H atom from this site was 2.34 eV, with the exception of cyclopenta[d,e,f]phenanthrene (CPP) ions, for which the E0 was 3.55 ± 0.86 eV due to steric constraints. Hydrogen loss from completely unsaturated PAHs has two trends, based on whether there is an odd or even number of hydrogen “holes”. For the loss of the first hydrogen atom, the energy is consistently ~ 4.40 eV while that for the second hydrogen atom is much lower at ~ 3.16 eV. The only exception was for dibenzo[a,l]pyrene which has a unique structure due to steric constraints, resulting in a low H loss reaction energy of 2.85 eV. If C2H2 is lost directly from the precursor structure, the energy required for this dissociation is 4.16 eV. No other fragmentation channels were observed over a large enough sample set for trends to be extrapolated, though data on CH3 and C4H2 loss obtained in previous studies is included for completeness. The dissociation reactions were also studied by collision induced dissociation after ionization by atmospheric pressure chemical ionization. When modeled with a simple temperature-based theory for the post-collision internal energy distribution, there was reasonable agreement between the two sets of data.

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Publication details

The article was received on 31 Oct 2017, accepted on 09 Feb 2018 and first published on 09 Feb 2018


Article type: Paper
DOI: 10.1039/C7CP07369K
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Unimolecular Reaction Energies for Polycyclic Aromatic Hydrocarbon Ions

    B. West, S. Rodriguez, A. Sit, S. M. Mohamed, B. Lowe, C. Joblin, A. Bodi and P. M. Mayer, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C7CP07369K

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