Structural investigation of doubly-dehydrogenated pyrene cations

TL;DR

This study uses infrared spectroscopy and DFT calculations to analyze the vibrational spectra of pyrene cations and their doubly-dehydrogenated forms, revealing isomer contributions and implications for hydrogen loss in interstellar PAHs.

Contribution

It provides the first detailed vibrational spectra comparison of doubly-dehydrogenated pyrene cations with theoretical models, identifying dominant isomers and hydrogen loss mechanisms.

Findings

Spectra of ddPy+ dominated by two isomers

Infrared spectra match DFT calculations for Py+

Hydrogen loss favors adjacent hydrogen atoms

Abstract

The vibrationally resolved spectra of the pyrene cation and doubly-dehydrogenated pyrene cation (C$_{16}$H$_{10}$$^{.+}$; Py$^+$ and C$_{16}$H$_{8}$$^{.+}$; ddPy$^+$) are presented. Infrared predissociation spectroscopy is employed to measure the vibrational spectrum of both species using a cryogenically cooled 22-pole ion trap. The spectrum of Py$^+$ allows a detailed comparison with harmonic and anharmonic density functional theory (DFT) calculated normal mode frequencies. The spectrum of ddPy$^+$ is dominated by absorption features from two isomers (4,5-ddPy$^+$ and 1,2-ddPy$^+$) with, at most, minor contributions from other isomers. These findings can be extended to explore the release of hydrogen from interstellar PAH species. Our results suggest that this process favours the loss of adjacent hydrogen atoms.