Browsing by Author "Rastogi, Shantanu"

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Computational study for neutral and cationic pericondensed polycyclic aromatic hydrocarbons
(2006-02-10) Pathak, Amit; Rastogi, Shantanu
Quantum 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.
Computational study of neutral and catoinic catacondensed polycyclic aromatic hydrocarbons
(2005-06-01) Pathak, Amit; Rastogi, Shantanu
Theoretical 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.
Experimental and theoretical study on the infrared spectroscopy of astrophysically relevant PAH derivatives 2- and 9-vinylanthracene
(2012-07-03) Maurya, Anju; Rastogi, Shantanu; Rouill´, Ga¨el; et.al
We propose to evaluate the contribution of polycyclic aromatic hydrocarbon molecules that carry side groups to the mid-infrared emission spectra. Within this framework, the IR absorption spectra of 2- and 9-vinylanthracene were measured in Ar matrices at 12 K and in CsI and polyethylene pellets at room temperature. The laboratory spectra were analyzed with the support of simulations based on the density functional theory. For each PAH molecule, eight IR spectra were computed by combining the B3LYP functional with as many different basis sets, namely 4-31G, 4-31G(d), 6-31G, 6-311G, 6-31G(d), 6-31G(d,p), 6-31+G(d,p), and 6-31++G(d,p). The comparison of the theoretical spectra with the laboratory data allowed us to determine the most suitable combinations for modeling the IR spectra of neutral PAH molecules that carry a vinyl side group. It was concluded from the examples of 2- and 9-VA that the optimum basis set is 6-31G unless a steric interaction has to be taken into account, in which case the optimum basis set is 6-31G(d). Thus, in the presence of such an interaction, the use of d-type polarization functions is recommended. We discuss the possibility for neutral vinyl-substituted PAHs to contribute to the mid-infrared emission spectra and find that their specific features do not match with the mid-infrared aromatic emission band
Interstellar extinction: Amorphous carbon and nanodiamonds
(2011-11-22) Rai, Rakesh K.; Rastogi, Shantanu
IR spectral variations in PAHs substituted with multi-vinyl and acetylenic side groups
(2011-11-22) Maurya, Anju; Rastogi, Shantanu
Modeling the interstellar aromatic infrared bands with co-added spectra of PAHs
(2008-04) Pathak, Amit; Rastogi, Shantanu
Aims. The observed variations in pro les of the interstellar aromatic infrared bands correlate with the object type and are indicative of PAH populations existing in di erent sources. Spectroscopic studies of PAHs can provide tools for interpreting the variations accompanying the AIBs. As the observed spectra results from a mix of possible species in the region, we try to model this composite spectra by co-adding emissions from PAHs in di erent size groups. Methods. Theoretical IR data of PAHs having 10 to 96 carbon atoms is used to obtain emission spectra. The models are taken in size groups made up of small, medium, and large PAHs. Results. The models show a good pro le match with observations for the 7.7 m complex having sub-features at 7.6 and 7.8 m. The 7.6 m sub-feature dominates in the spectra of medium-sized PAH cations, matching observations from UV-rich interstellar environments. The 7.8 m component is more intense in the spectra of large PAH cations (model III), correlating with observations from benign astrophysical regions. A possible interpretation of the observations of C H out-of-plane bend modes and the weak outliers on the blue side of the intense 11.2 m band is proposed. The models provide pointers to possible PAH populations in di erent regions
Scattering and Extinction Properties of Nanodiamonds
(2009-11-01) Rai, Rakesh K.; Rastogi, Shantanu
The study of scattering and extinction properties of possible nanodiamond grains in the ISM are reported. Calculations using Discrete Dipole Approximation (DDA) for varying ellipsoidal shapes and sizes from 2.5 to 10 nm are considered. Nanodiamonds show negligible extinction from IR to near-UV and very sharp far-UV rise. Comparison with observations rule out possibility of independent nanodiamond dust but point towards possibility of nanodiamonds as a component in the ISM. Radiation induced transformations may lead to carbonaceous grains with diﬀerent core and mantles. So calculations are also performed for a core-mantle target model with nanodiamond core in graphite mantles. The graphite extinction features get modiﬁed with the peak at 2175 ˚ A being lowered, broadened, blue shifted and accompanied by enhanced extinction in the far-UV. Such variations in the 2175 ˚ A band and simultaneous far-UV rise are observed along some sources. A three component dust model incorporating silicate, graphite and graphite with nanodiamond core is also considered. The model extinction compares very well with the average galactic extinction in the complete range from 0.2 to 10 m−1. The best ﬁt requires small size and small number of nanodiamonds
Theoretical infrared spectra of large polycyclic aromatic hydrocarbons
(2007-11-18) Pathak, Amit; Rastogi, Shantanu
Theoretical 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.
Theoretical spectra of PAHs in modeling astrophysical IR features
(2007-11-18) Pathak, Amit; Rastogi, Shantanu
Vibrational spectroscopy of PAHs
(2011-11-22) Rastogi, Shantanu