Coronene and pyrene (5, 7)-member ring defects: Infrared spectra,
  energetics and alternative formation pathways
  PAHs are known to be one of the carriers of the ubiquitous aromatic IR bands.
The IR spectra of many objects show IR emission features derived from PAH
molecules of different size. Still some of the characteristics of the emitting
population remain unclear. The emission bands show details which cannot be
explained so far. These unidentified IR features require further laboratory and
observational investigations. We present a theoretical study of the IR spectra
of PAHs containing (5,7)-member ring defects, focusing on pyrene and coronene.
Using density functional theory, we investigate the effects of such defects on
the IR spectra of pyrene and coronene and their cations and anions. In
addition, we explore parts of the potential energy surface of the neutral
species and discuss alternative formation pathways. 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 a rise
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. Investigation of
the minimal energy pathway leads to a result of 8.21 eV for pyrene and 8.41 eV
for coronene as minimum activation barriers for the transformation from ground
state to defected state. Whereas pyrene retains some of its symmetry due to 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 AGB stars or PNe. Those environments strongly enable the
transition from the ground state to the defect 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.
