IUCAA Preprints
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Item Theoretical infrared spectra of large polycyclic aromatic hydrocarbons(2007-11-18) Pathak, Amit; Rastogi, ShantanuTheoretical and experimental spectroscopic studies have underlined the contribution of large PAHs towards the astrophysical mid-infrared emission bands. Quantum chemical study of eight large PAHs using density functional theory approach is reported along with their infrared spectra. Systematic variation of bands with PAH size is noted and a better agreement with the observed astrophysical bands is obtained. Compared to small and medium sized PAHs there is substantial C ¡ H stretch intensity in the cation spectra. This is attributed to smaller change in charge on the hydrogens upon ionization. For the C ¡ H out-of-plane mode large PAHs correlate well with observed features on the shorter wavelength side of the 11.2 ¹m band. Presence of two sub-components of the broad 7.7 ¹m band in large PAHs compares very well with the corresponding astrophysical band and point to the abundance of large PAH cations in interstellar environments. The data presented here may be used for a more detailed study on the pro¯le variations accompanying the mid-IR bands in various interstellar environments.Item Computational study of neutral and catoinic catacondensed polycyclic aromatic hydrocarbons(2005-06-01) Pathak, Amit; Rastogi, ShantanuTheoretical calculations have been done for neutral and cationic catacondensed polycyclic aromatic hydrocarbons (PAH) using density functional theory approach. Optimized geometries and charge istributions 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.Item Computational study for neutral and cationic pericondensed polycyclic aromatic hydrocarbons(2006-02-10) Pathak, Amit; Rastogi, ShantanuQuantum chemical calculations using density functional theory are presented for small to medium sized pericondensed PAHs includ- ing some being reported for the first time. Bond lengths and charge distribution have been computed for these PAHs in both neutral and cationic forms. Upon ionization, significant change in fractional charge on atoms is present particularly for the outer carbon atoms. The charge on the internal carbon atoms tends towards zero in cations. Vibrational frequencies and infrared intensities have been calculated for the optimized structures of PAH neutrals and cations. The drastic intensity alterations occurring upon ionization are discussed and related to specific changes occurring in the charge distribution. The C-H stretch intensity depends on the partial charge on peripheral hydrogen atoms and reduces in cations as hydrogen atoms become more positive. Pericondensed PAHs show better matching with the observed interstellar infrared bands. The co-added model spectra show profiles similar to the observed astrophysical bands.