Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation

Gatchell, Michael; Stockett, Mark H.; Ruette, N. de; Chen, Tao; Giacomozzi, Linda; Nascimento, R F; Wolf, Michael O.; Anderson, E. K.; Delaunay, Rudy; Vizcaino, Violaine; Rousseau, Patrick; Adoui, L.; Huber, B. A.; Schmidt, H. T.; Zettergren, Henning; Cederquist, H.

doi:10.1103/physreva.92.050702

Cited by 44 publications

(69 citation statements)

References 40 publications

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“…Our experimental results are consistent with density functional theory calculations of the dissociation energies of these ions which have been reported by Gatchell et al (2015). Based on the laser pulse energy dependence, the total energy deposited in photo-absorption events leading to backbone fragmentation of C 16 H + 10+m is 8.17 eV (3 × 2.73 eV), 5.77 eV (2 × 2.88 eV), and 2.95 eV for m = 0, 6, and 16, respectively.…”

Section: Laser Pulse Energy Dependencesupporting

confidence: 92%

PHOTO-STABILITY OF SUPER-HYDROGENATED PAHs DETERMINED BY ACTION SPECTROSCOPY EXPERIMENTS

Wolf

Kiefer

Langeland

et al. 2016

ApJ

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We have investigated the photo-stability of pristine and super-hydrogenated pyrene cations (C 16 H + 10+m , m = 0, 6, or 16) by means of gas-phase action spectroscopy. Optical absorption spectra and photo-induced dissociation mass spectra are presented. By measuring the yield of mass-selected photo-fragment ions as a function of laser pulse intensity, the number of photons (and hence the energy) needed for fragmentation of the carbon backbone was determined. Backbone fragmentation of pristine pyrene ions (C 16 H + 10 ) requires absorption of three photons of energy just below 3 eV, whereas super-hydrogenated hexahydropyrene (C 16 H + 16 ) must absorb two such photons and fully hydrogenated hexadecahydropyrene (C 16 H + 26 ) only a single photon. These results are consistent with previously reported dissociation energies for these ions. Our experiments clearly demonstrate that the increased heat capacity from the additional hydrogen atoms does not compensate for the weakening of the carbon backbone when pyrene is hydrogenated. In photodissociation regions, super-hydrogenated Polycyclic Aromatic Hydrocarbons (PAHs) have been proposed to serve as catalysts for H 2 -formation. Our results indicate that carbon backbone fragmentation may be a serious competitor to H 2 -formation at least for small hydrogenated PAHs like pyrene.

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Section: Laser Pulse Energy Dependencesupporting

confidence: 92%

PHOTO-STABILITY OF SUPER-HYDROGENATED PAHs DETERMINED BY ACTION SPECTROSCOPY EXPERIMENTS

Wolf

Kiefer

Langeland

et al. 2016

ApJ

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“…Attachment of small numbers of H atoms to coronene cations can for instance quench photoionization-induced H loss from a C 24 H + 12 precursor cation (Reitsma et al 2014(Reitsma et al , 2015. For superhydrogenation of neutral pyrene molecules, an opposite effect was observed.The C-backbone is weakened and fragmentation upon ion collisions or photoionization is increased (Wolf et al 2016;Gatchell et al 2015). Attachment of a single H atom to a PAH radical cations has a dramatic influence on its IR spectrum (Knorke et al 2009) and substantially decreases the HOMO-LUMO gap (Pathak & Sarre 2008).…”

Section: Introductionmentioning

confidence: 99%

Molecular hydrogen formation on interstellar PAHs through Eley-Rideal abstraction reactions

Foley

Cazaux

Egorov

et al. 2018

Monthly Notices of the Royal Astronomical Society

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We present experimental data on H 2 formation processes on gas-phase polycyclic aromatic hydrocarbon (PAH) cations. This process was studied by exposing coronene radical cations, confined in a radio-frequency ion trap, to gas phase H atoms. Sequential attachment of up to 23 hydrogen atoms has been observed. Exposure to atomic D instead of H allows one to distinguish attachment from competing abstraction reactions, as the latter now leave a unique fingerprint in the measured mass spectra. Modeling of the experimental results using realistic cross sections and barriers for attachment and abstraction yield a 1:2 ratio of abstraction to attachment cross sections. The strong contribution of abstraction indicates that H 2 formation on interstellar PAH cations is an order of magnitude more relevant than previously thought.

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“…For instance, for superhydrogenated neutral pyrene molecules, a weakening effect was observed. Here, fragmentation involving C-backbone rupture was found to be the highest decay channel, both in collision and in photoionization studies (Gatchell et al 2015;Wolf et al 2016). For coronene, Reitsma et al (2014) have shown that the attachment of multiple hydrogen atoms to the cation strongly influences the molecular response upon core ionization by soft X-rays, whereas the coronene radical cation [C 24 H 12 ] + undergoes predominantly H-and H 2 -loss, leading to underhydrogenated species, for superhydrogenated species C H n…”

Section: Resultsmentioning

confidence: 85%

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

Cazaux

Arribard

Egorov

et al. 2019

ApJ

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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.

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Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation

Cited by 44 publications

References 40 publications

PHOTO-STABILITY OF SUPER-HYDROGENATED PAHs DETERMINED BY ACTION SPECTROSCOPY EXPERIMENTS

PHOTO-STABILITY OF SUPER-HYDROGENATED PAHs DETERMINED BY ACTION SPECTROSCOPY EXPERIMENTS

Molecular hydrogen formation on interstellar PAHs through Eley-Rideal abstraction reactions

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

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