The sequence of coronene hydrogenation revealed by gas-phase IR spectroscopy

Stéphanie Cazaux, Dmitrii Egorov, Julianna Palotás, Romke Hoekstra, Giel Berden, Jos Oomens, Thomas Schlathölter

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Abstract

Gas-phase coronene cations (C24H12+) can be sequentially hydrogenated with up to 24 additional H atoms, inducing a gradual transition from a planar, aromatic molecule towards a corrugated, aliphatic species. The mass spectra of hydrogenated coronene cations [C24H12+nH ]+ show that molecules with odd numbers of additional hydrogen atoms (nH) are dominant with particularly high relative intensity for ”magic numbers” nH = 5, 11, and 17, for which hydrogen atoms have the highest binding energies. Reaction barriers and binding energies strongly affect the hydrogenation sequence and its site specificity. In this contribution, we monitor this sequence experimentally by the evolution of infrared multiple-photon dissociation (IRMPD) spectra of gaseous [C24H12+nH]with nH = 3 − 11, obtained using an infrared free electron laser coupled to a Fourier transform ion cyclotron mass spectrometer.
For weakly hydrogenated systems (nH = 3, 5) multiple-photon absorption mainly leads to loss of Hatoms (and/or H2). With increasing nH, C2H2 loss becomes more relevant. For nH = 9, 11, the carbon skeleton is substantially weakened and fragmentation is distributed over a large number of channels. A comparison of our IRMPD spectra with density functional theory calculations clearly shows that only one or two hydrogenation isomers contribute for each nH. This confirms the concept of hydrogenation
occurring along very specific sequences. Moreover, the atomic sites participating in the first 11 steps of this hydrogenation sequence are clearly identified.
Original languageEnglish
Article number27
Number of pages8
JournalThe Astrophysical Journal
Volume875
Issue number1
DOIs
Publication statusPublished - 10-Apr-2019

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