used in this work. In section 3.1, we discuss the effect of (5,7)-membered ring defects on the IR absorption spectra of pyrene, coronene and their (singly charged) cations and anions. This is followed by an exploration of parts of the potential energy surface of the neutral species to investigate their thermodynamic stability in section 3.2. In section 4 we present an alternative pathway that could lead to the formation of these defected PAHs in the interstellar matter and discuss their stability, observability and detectability from an astrophysical point of view.

![The PAH molecules studied in this work. Above you can see coronene and pyrene in its normal state, below there is the defected form. In $C_{24}H_{12}$d and $C_{16}H_{10}$d the 5-member rings and 7-member rings are shown.[]{data-label="molecules.fig"}](molekul.eps){width="8.8cm"}

Models and computational methods
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Pyrene ($C_{16}H_{10}$) consists in its original condition of four 6-membered rings. In the defected state, all four rings are changed into two 5-membered and two 7-membered rings. The size of the molecule stays the same. The defected state is referred as $C_{16}H_{10}$d. The normal coronene ($C_{24}H_{12}$) consists of a 6-membered ring in the center, surrounded by a ring of six more 6-membered rings. The defect changes the ring in 