How does successive hydrogen addition to PAH ions impact their unimolecular chemistry?

Hydrogenated polycyclic aromatic hydrocarbons (PAHs) have been proposed to contribute to the formation of interstellar H₂ by acting as a surface catalyst for the adsorption of hydrogen atoms and desorption of H₂ molecules. In the present study, imaging photoelectron photoion coincidence (iPEPICO) spectroscopy and tandem mass spectrometry were employed to examine the unimolecular chemistry of four hydrogenated polycyclic aromatic hydrocarbon ions: 9,10-dihydroanthracene (DHA^+•) and 1,2,3,4,5,6,7,8-octahydroanthracene (OHA^+•), having opposite patterns of hydrogenation, and 1,2,3,4-tetrahydrophenanthrene (THP^+•) and 1,2,3,4,9,10-hexahydrophenanthrene (HHP^+•). DHA^+• exhibits the same reactions previously observed for 1,2-dihydronaphthalene and 9,10-dihydrophenanthrene, namely competing loss of H^• and CH₃^•. However, the energy required for H^•-loss, as predicted by RRKM modeling of the iPEPICO results, was lower than the latter ions, presumably due to charge delocalization across the central ring upon dehydrogenation. OHA^+• behaves similarly to ionized tetralin, displaying losses of H^•, CH₃^•, C₂H₄ and C₃H₅^• in its collision induced dissociation (CID) mass spectra, but under iPEPICO conditions CH₃^•-loss is not observed. THP^+• and HHP^+• have aspects of both DHA^+• and OHA^+• chemistries, displaying losses of H^•, CH₃^•, C₂H₄ and C₃H₅^•. RRKM modeling produced minimum energies for all observed reaction channels, which were also computationally explored at the B3LYP/6–31+G(d,p) level of theory. The results indicate that small PAH ions may not be effective surfaces for the catalytic formation of H₂ in the ISM, but rather sources of small hydrocarbons.