Photolysis-induced scrambling of PAHs as a mechanism for deuterium storage

Abstract

Aims We investigate the possible role of polycyclic aromatic hydrocarbons (PAHs) as a sink for deuterium in the interstellar medium (ISM) and study UV photolysis as a potential underlying chemical process in the variations of the deuterium fractionation in the ISM. Methods The UV photo-induced fragmentation of various isotopologs of deuterium-enriched, protonated anthracene and phenanthrene ions (both C 14 H 10 isomers) was recorded in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR MS). Infrared multiple photon dissociation (IRMPD) spectroscopy using the Free-Electron Laser for Infrared eXperiments (FELIX) was applied to provide IR spectra. Infrared spectra calculated using density functional theory (DFT) were compared to the experimental data to identify the isomers present in the experiment. Transition-state energies and reaction rates were also calculated and related to the experimentally observed fragmentation product abundances. Results The photofragmentation mass spectra for both UV and IRMPD photolysis only show the loss of atomic hydrogen from [D -C 14 H 10 ] + , whereas [H -C 14 D 10 ] + shows a strong preference for the elimination of deuterium. Transition state calculations reveal facile 1,2-H and -D shift reactions, with associated energy barriers lower than the energy supplied by the photo-excitation process. Together with confirmation of the ground-state structures via the IR spectra, we determined that the photolytic processes of the two different PAHs are largely governed by scrambling where the H and the D atoms relocate between different peripheral C atoms. The ∼0.1 eV difference in zero-point energy between C -H and C -D bonds ultimately leads to faster H scrambling than D scrambling, and increased H atom loss compared to D atom loss. Conclusion We conclude that scrambling is common in PAH cations under UV radiation. Upon photoexcitation of deuterium-enriched PAHs, the scrambling results in a higher probability for the aliphatic D atom to migrate to a strongly bound aromatic site, protecting it from elimination. We speculate that this could lead to increased deuteration as a PAH moves towards more exposed interstellar environments. Also, large, compact PAHs with an aliphatic C -HD group on solo sites might be responsible for the majority of aliphatic C -D stretching bands seen in astronomical spectra. An accurate photochemical model of PAHs that considers deuterium scrambling is needed to study this further.

Do defects in PAHs promote catalytic activity in space? Stone–Wales pyrene as a test case

“…Specifically, H-loss and H-roaming might be important reactions to be considered. 55,63,64 For SW-pyrene, H-loss can be a competitive channel when H 2 is formed on the C2-C4 junction, as the difference between the two reactions is 0.35 eV, which favours the H-loss. This is not the case for the formation of H 2 on distal carbons, where the energy difference is 0.88 eV, favouring the H 2 formation (Fig.…”

IRMPD spectroscopy of a PAH cation using FELICE: The infrared spectrum and photodissociation of dibenzo[a,l]pyrene

“…In addition to the MIR spectrum, the intracavity configuration allows us to probe the FIR spectra and the dissociation mass spectra for undecorated, cationic PAHs. The observed fragmentation behavior is similar to that observed in UV dissociation experiments due to the high excitation levels achieved in IRMPD spectroscopy, making this method a useful proxy for interstellar processes [12,13].…”

Roadmap on dynamics of molecules and clusters in the gas phase

“…PAHs can also act as source of smaller hydrocarbons, which importance has recently also been assessed [279]. Not included in astronomical modeling (yet), is the role of isomerization, which has now also been shown to play a role in the dynamics of photodissociation of astro-PAHs [280][281][282].…”