Per-Halogenated Polycyclic Aromatic Hydrocarbons (PAH)

Abstract

Previous investigations showed that halogen substitutions affect PAH planarity and aromaticity. In this study, a series of 21 linear and non-linear per-halogenated PAHs were considered to explore the minimum energy geometries and their aromaticity. Halogens (Cl, Br, I, and At) change naphthalene from a planar to a propeller- shaped geometry. Per-fluorinated naphthalene remains planar like unsubstituted naphthalene. The larger acene systems with more than two benzene rings have a trend of alternating structures, with the benzene rings being less twisted and halogen atoms alternating in pointing above or below the plane of the benzene rings. Non- linear PAHs structures are twisted, even the fluorinated PAHs. In terms of aromaticity, HOMA and NICS indices were employed to detect π-electron delocalization. Per-halogenation reduces aromaticity significantly, which increases from lighter to heavier halogens. Notably, the distorted structure loses aromaticity. All computed calculations were performed at the B3LYP/def2-SVP level of theory.