Structural investigation of doubly-dehydrogenated pyrene cations

The vibrationally resolved spectra of the pyrene cation and doubly-dehydrogenated pyrene cation (C16H10●+; Py+ and C16H8●+; ddPy+) are presented. Infrared predissociation spectroscopy is employed to measure the vibrational spectrum of both species using a cryogenically cooled 22-pole ion trap. The spectrum of Py+ allows a detailed comparison with harmonic and anharmonic density functional theory (DFT) calculated normal mode frequencies. The spectrum of ddPy+ consists of absorption features from at least two isomers. DFT calculations suggest that the observed spectrum is dominated by the two most thermodynamically stable isomers (4,5-ddPy+ and 1,2-ddPy+). These findings can be extended to explore the release of hydrogen from interstellar PAH species. Our results show that this process is favoured by the loss of adjacent hydrogen atoms.