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 H-2 by acting as a surface catalyst for the adsorption of hydrogen atoms and desorption of H-2 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(+center dot)) and 1,2,3,4,5,6,7,8-octahydroanthracene (OHA(+center dot)), having opposite patterns of hydrogenation, and 1,2,3,4-tetrahydrophenanthrene (THP+center dot) and 1,2,3,4,9,10-hexahydrophenanthrene (HHP+center dot). DHA(+center dot) exhibits the same reactions previously observed for 1,2-dihydronaphthalene and 9,10-dihydrophenanthrene, namely competing loss of H-center dot and CH3 center dot. However, the energy required for H-center dot-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(+center dot) behaves similarly to ionized tetralin, displaying losses of H-center dot, CH3 center dot, C2H4 and C3H5 center dot in its collision induced dissociation (CID) mass spectra, but under iPEPICO conditions CH3 center dot-loss is not observed. THP+center dot and HHP+center dot have aspects of both DHA(+center dot) and OHA(+center dot) chemistries, displaying losses of H center dot, CH3 center dot, C2H4 and C3H5 center dot. 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 H-2 in the ISM, but rather sources of small hydrocarbons.