Computational study of neutral and cationic catacondensed polycyclic aromatic hydrocarbons Amit Pathak, Shantanu Rastogi * Department of Physics, D.D.U. Gorakhpur University, Gorakhpur 273 009, India Received 31 July 2004; accepted 7 January 2005 Available online 26 January 2005 Abstract 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 system- atic 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 Astrophys- ical Unidentified Infrared bands. Ó 2005 Elsevier B.V. All rights reserved. Keywords: PAH; Interstellar molecules; DFT; Charge distribution; IR spectra; Aromatic infrared bands 1. Introduction The mid infrared spectra of many Astronomical sources like HII regions, Planetary Nebulae, Reflection Nebulae, Post AGB stars, star forming regions, Inter- stellar medium (ISM) of our galaxy and even of outer galaxies show distinct emission features at 3.3, 6.2, 7.7, 8.6 and 11.2 lm (3030, 1610, 1300, 1160, 890 cm À1 ) [1–6]. These ubiquitous bands, earlier known as the unidentified infrared (UIR) bands, arise from vibrations within molecules having Aromatic moieties. Due to their Aromatic origin these bands are now also known as aro- matic infrared bands (AIBs). Polycyclic aromatic hydro- carbon (PAH) molecules are proposed as the most viable carriers of these bands [7–10], though identifica- tion of a specific PAH has not yet been possible. PAHs seem to be partially responsible for the UV extinction bump at 217 nm [11] and are also considered to be strong candidates for the carriers of diffuse interstellar bands (DIBs) [12–15], which are emission features superimposed on the interstellar extinction curve. The PAH molecules consist of several benzenoid rings forming a planar structure. They have a highly sta- ble configuration due to the entire de-localization of p electrons over the whole molecule. The large cross-sec- tion area of PAHs makes them efficient absorber of the background radiation. Depending on the strength of the background UV field the molecules may dissoci- ate, get ionized or dehydrogenated. The molecule may also experience structural reformation to a more com- pact and stable configuration. A PAH molecule may get vibrationally heated upto a temperature of 1000 K by the absorption of a single UV photon. It then under- goes Ôinternal vibrational redistributionÕ to distribute the 0301-0104/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.01.007 * Corresponding author. Tel.: +915512204517; fax: +915512330767. E-mail address: shantanu_r@hotmail.com (S. Rastogi). www.elsevier.com/locate/chemphys Chemical Physics 313 (2005) 133–150