The Sequence of Coronene Hydrogenation Revealed by Gas-phase IR Spectroscopy

Abstract

Gas-phase coronene cations (C H 24 12 + ) can be sequentially hydrogenated with up to 24 additional H atoms, inducing a gradual transition from a planar, aromatic molecule toward a corrugated, aliphatic species. The mass spectra of hydrogenated coronene cations C H n 24 12 H + + [ ] show that molecules with odd numbers of additional hydrogen atoms (n H ) are dominant, with particularly high relative intensity for "magic numbers" n H =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 C H n 24 12 H + + [ ] with n H =3-11, obtained using an infrared free electron laser coupled to a Fourier transform ion cyclotron mass spectrometer. For weakly hydrogenated systems (n H = 3, 5) multiple-photon absorption mainly leads to loss of H atoms (and/or H 2 ). With increasing n H , C 2 H 2 loss becomes more relevant. For n H =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 to each n H . 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.

On the correlation of the abundances of HNCO and NH2CHO: Advantages of solid para-H2 to study astrochemical H-atom addition and abstraction reactions

“…The observations also show that the quasi-equilibrium is shifted towards HNCO because of a small barrier for the reaction H + HNCO → H 2 NCO. Similar dual-cycle mechanism might link also other unsaturated/saturated pairs of interstellar molecules; e.g., recent studies indicated the existence of similar cycles among CO, H 2 CO and CH 3 OH (Chuang et al (2016) as between polyaromatic hydrocarbons (PAHs) and their partially hydrogenated forms (Raus & Hornekaer (2008); Menella et al 2012; Cazaux et al (2019)). Therefore, this mechanism can contribute to the formation of H 2 molecules on the surface of interstellar grains.…”

Atomic hydrogen interactions with small polycyclic aromatic hydrocarbons cations

“…Each of these reactions increases the mass M by 1 Da. In a combined experimental and theoretical study we could show that C 24 H + 12 hydrogenation at T = 300 K is following a well-defined sequence of hydrogenation sites [10] that has later been confirmed by means of infrared spectroscopy [11]. The attachment barriers almost vanish for radicals with even numbers of attached H atoms and as a consequence, cations with odd s are observed at much higher intensities than cations with even s. Hydrogenation states with s = 5, 11 and 17 are particularly prominent due to their relatively high binding energies combined with a high barrier for the next hydrogenation to occur.…”

Competitive Dehydrogenation and Backbone Fragmentation of Superhydrogenated PAHs: A Laboratory Study

“…For the smaller catacondensed PAHs anthracene and phenanthrene, the crossover occurs already between N = 2 and 4 additional H atoms (Diedhiou et al 2020). An infrared multi-photon dissociation study of super-hydrogenated coronene found the branching fractions to be comparable at N = 7 (Cazaux et al 2019). For coronene and the larger PAHs expected to be predominant in PDRs, it may be expected -though this has not been demonstrated -that hydrogenated species with the minimal amount of superhydrogenation to achieve catalytic H 2 formation could approach 100% stability against loss of native H atoms and backbone fragmentation.…”

“…This was demonstrated in experiments where super-hydrogenated coronene (C 24 H 12+N , N = 1, 3, 5) cations were found to lose fewer of their native H atoms with increasing levels of super-hydrogenation following soft x-ray excitation at 285 eV (Reitsma et al 2014). More recent studies, however, showed that higher degrees of super-hydrogenation (N 6) led to increased carbon backbone fragmentation (Rapacioli et al 2018;Cazaux et al 2019). Taken together, these studies suggest the existence of a window of hydrogenation levels consistent with truly catalytic H 2 -formation without destruction of the native PAH.…”