The averaged theoretical harmonic IR spectra for PAHs containing 1–10 hexagons in neutral (N), cationic (C), and anionic (A) charge states in cata- and peri-condensed forms in the gas phase. Three spectral regions are shown: (a) C–H stretching (3.0–3.4 μm), (b) C–C stretching and C–H in-plane bending (6.0–9.0 μm), and (c) C–H out-of-plane bending (11.0–13.0 μm). Observational data are obtained from ISO-SWS and are processed using baseline correction, emission-line suppression, and normalization. Dotted vertical lines indicate the observed positions of AIBs.