Infrared spectroscopy of the benzylium-like (and tropylium-like) isomers formed in the –H dissociative ionization of methylated PAHs

Abstract

Cationic benzylium and tropylium are known to be two competitive isomers for the –H fragment of the methylbenzene (toluene) cation. Methylated polycyclic aromatic hydrocarbon (PAH) cations are expected to be abundant in space and their dehydrogenation could lead to the formation of both the benzylium- and tropylium-like cations, which are expected to be the two lowest-energy isomers. Here, we considered 1-methylpyrene and two less compact acene-substituted species, namely 2-methylnaphthalene and 2-methylanthracene, as precursors. The cationic –H fragments, C17H11+, C11H9+, and C15H11+, were produced by dissociative ionization, and their neon tagged complexes were formed in the 22-pole cryogenic ion trap instrument FELion that is coupled to the FELIX free electron laser. Infrared (IR) predissociation spectroscopy was performed showing that the strongest depletion band is located at about 1620cm−1, which reveals the predominance of the benzylium-like, XCH2+, isomers, where X = Pyr, Nap, or Ant. Saturation depletion measurements showed that only this isomer is present in the case of C17H11+, whereas for the acene-derived species at least two are present with a large abundance. Synthetic spectra were generated from the theoretical anharmonic IR spectra of the two lowest-energy isomers, namely XCH2+ and the tropylium-like isomers, XC7+. Spectral comparison led us to conclude that there is no evidence for PyrC7+ but clear evidence for NapC7+. No specific spectral features could be identified for AntC7+ due to a high spectral congestion. These results support the important role of PAH compactness in preventing the formation of XC7+ species. They also reveal the potential of XCH2+ species to account for the aromatic infrared band observed in emission at 6.2μm in astrophysical environments.