Coronene and pyrene (5, 7)-member ring defects

Infrared spectra, energetics, and alternative formation pathways

¹ Institute for Astro- and Particle Physics, Leopold Franzens Universität Innsbruck, Technikerstrasse 25, 6020 Innsbruck, Austria
e-mail: Silvia.Oettl@uibk.ac.at
² Institute of Ion Physics and Applied Physics, Leopold Franzens Universität Innsbruck, Technikerstrasse 25, 6020 Innsbruck, Austria
e-mail: S.Huber@uibk.ac.at; Michael.Probst@uibk.ac.at
³ Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile
e-mail: Stefan.Kimeswenger@gmail.com

Received: 13 June 2014
Accepted: 15 July 2014

Abstract

Context. Polycyclic aromatic hydrocarbons (PAHs) are known today to be one of the carriers of the ubiquitous aromatic infrared (IR) bands. The IR spectra of many astrophysical objects show IR emission features derived from PAH molecules of different size. Space-based observations have shown that these IR emission features are omnipresent and can be found in most objects. However, some of the characteristics of the emitting population remain unclear. The emission bands show details that cannot be explained so far. These unidentified IR features require more laboratory and observational investigations.

Aims. We present a theoretical study of the IR spectra of PAHs containing (5, 7)-member ring defects, focusing on pyrene (C₁₆H₁₀) and coronene (C₂₄H₁₂).

Methods. Using density functional theory, we investigated the effects of these defects on the IR spectra of pyrene and coronene and their cations and anions. In addition, we explored parts of the potential energy surface of the neutral species and discuss alternative formation pathways.

Results. The addition of (5, 7)-membered ring defects in pyrene and coronene results in a change of the IR spectra: both molecules lose their typical spectroscopic signature. We find shifts in the positions of the band as well as different intensities and an increase in the number of features. The boundaries in terms of the size of the PAHs exhibiting a (5, 7)-membered ring defect are studied and shown. Investigating the minimum energy pathway leads to a result of 8.21 eV for pyrene and 8.41 eV for coronene as the minimum activation barriers for the transformation from ground to defected state. Whereas pyrene retains some of its symmetry because of the symmetry exhibited by the Stone-Wales defect itself, coronene loses much more of its symmetry. The formation of these (5, 7)-ring defects in PAHs may be well supported in asymptotic giant branch stars or planetary nebulae. These environments strongly enable the transition from the ground to the defected state. Therefore the knowledge of the IR spectra of these molecules will support future investigations aiming for a thorough understanding of the unidentified IR emission bands.