Computational study of neutral and cationic catacondensed polycyclic aromatic hydrocarbons

Theoretical calculations have been done for neutral and cationic catacondensed polycyclic aromatic hydrocarbons (PAH) using density functional theory approach. Optimized geometries and charge distributions have been calculated and the change in structure and charge distribution upon ionization of PAHs is studied and discussed. The calculated infrared vibrational modes show systematic variations with size in the linear polyacenes while no regular variation is apparent in non-linear catacondensed PAHs. The prominent features in the spectra of neutral PAHs are due to C-H stretch and C-H wag motions. In the spectra of PAH cations C-C stretch and C-H in plane modes are the most intense. The changes in charge distributions of cations causing these intensity changes have been identified. The C-H stretch intensity depends on the partial charge on peripheral Hydrogen atoms and reduces in cations as Hydrogen atoms become more positive. The prospect of catacondensed PAHs is discussed in the context of Astrophysical Unidentified Infrared bands.