Ion Chemistry of Hydrogenated PAHs

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are a class of organic molecules suggested to constitute roughly 20% of carbon in the interstellar medium (ISM). These species exist in both neutral and ionic forms and both are potentially involved in processes such as H2 formation. Catalyzing H2 formation necessarily involves the participation of hydrogenated PAHs as reaction intermediates. Employing tandem mass spectrometry and imaging photoelectron photoion coincidence spectroscopy and theory, we have explored the unimolecular reactions of five ionized hydrogenated PAHs that vary in degree and position of hydrogenation: tetralin (1,2,3,4-tetrahydronaphthalene), 9,10-dihydroanthracene (DHA+•), 1,2,3,4-tetra- and 1,2,3,4,9,10-hexa-hydrophenanthrene (THP+• and HHP+•) and 1,2,3,4,5,6,7,8-octahydroanthracene (OHA+•). The major reactions observed were the losses of the hydrocarbons CH3•, C2H4 and C3H5• together with H• atom loss. RRKM modeling of the iPEPICO data suggested that the unimolecular chemistries were based around a two-well potential energy surface in each case. Ionized tetralin can lose all four neutrals via H-shift and ring-opening reactions, or CH3• and C2H4 after interconversion to the 1-methylindane ion, a process similar to that found for ionized 1,2-dihydronaphthalene (isomerizing to form the 1-methylindene ion structure). 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 an expansion of the electron 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+• iii and HHP+• have aspects of both DHA+• and OHA+• chemistries, displaying losses of H•, CH3•, C2H4 and C3H5•. Minimum energies for all observed reaction channels were thus obtained, together with selected mechanisms computationally explored at the B3-LYP/6-31+G(d,p) level of theory. The trend in reactivity in going from tetralin and DHA+• to THP+•, HHP+• and ultimately OHA+• sees decreasing abundance of H• and CH3•-loss and an increasing dominance of the formation of C2H4, C3H5• and higher hydrocarbons with degree of hydrogenation as isomerization to a methyl-substituted ion becomes less significant. As this isomerization decreases in significance, the ions become sources of small hydrocarbon molecules and not hydrogen atoms or molecules.