Gas-phase hydrogenation of large, astronomically relevant PAH cations

TL;DR

This study investigates the gas-phase hydrogenation of large PAH cations relevant to interstellar chemistry, combining experimental collision reactions with quantum calculations to understand their hydrogenation behavior and structural influences.

Contribution

It provides new experimental data and theoretical insights into the hydrogenation mechanisms of large PAH cations in space environments, highlighting factors affecting their reactivity.

Findings

Hydrogenated PAH cations are efficiently formed in collisions.

No even-odd hydrogenation mass pattern observed.

Bonding ability influences hydrogenation reactivity.

Abstract

To investigate the gas-phase hydrogenation processes of large, astronomically relevant cationic polycyclic aromatic hydrocarbon (PAH) molecules under the interstellar environments, the ion-molecule collision reaction between six PAH cations and H-atoms is studied. The experimental results show that the hydrogenated PAH cations are efficiently formed, and no even-odd hydrogenated mass patterns are observed in the hydrogenation processes. The structure of newly formed hydrogenated PAH cations and the bonding energy for the hydrogenation reaction pathways are investigated with quantum theoretical calculations. The exothermic energy for each reaction pathway is relatively high, and the competition between hydrogenation and dehydrogenation is confirmed. From the theoretical calculation, the bonding ability plays an important role in the gas-phase hydrogenation processes. The factors that affect the hydrogenation chemical reactivity are discussed, including the effect of carbon skeleton structure, the side-edged structure, the molecular size, the five- and six-membered C-ring structure, the bay region structure, and the neighboring hydrogenation. The IR spectra of hydrogenated PAH cations are also calculated. These results we obtain once again validate the complexity of hydrogenated PAH molecules, and provide the direction for the simulations and observations under the coevolution interstellar chemistry network. We infer that if we do not consider other chemical evolution processes (e.g., photo-evolution), then the hydrogenation states and forms of PAH compounds are intricate and complex in the interstellar medium (ISM).