defects in intensities and positions of the lines can be found.

Thus we notice in all areas of the spectra more significant deviations as in previous works. Yu & Nyman ([@yu]) has raised the suspicion that there is a size dependence for the effects of the (5, 7)-member ring defect on the IR spectra of PAHs. The impact of such defects could increase with decreasing size of the PAHs. Also Ricca et al. ([@ricca]) found that the differences appear to become smaller as the size of the PAH increases. Boersma et al. ([@boersma2]) and Ricca et al. ([@ricca2]) discuss the difficulties of drawing any definitive conclusions about the specific PAH species that might be responsible for the UIR bands attributed to the PAHs. Our investigations represent the limiting cases with respect to the size of the PAHs. The investigation of these small systems is assumed to be an important step to a significant understanding of the role of the (5,7)-member ring defects for IR spectra of PAHs.

![Theoretical IR spectra of coronene and pyrene and the defected forms in the region of 20 - 100$\mu$m. The calculations were performed using the B3LYP density functional. Additionally there is the cm$^{-1}$ axis 