Computational IR Spectroscopy for PAHs: from the early years to the present status

Abstract

Abstract. In the long story of interstellar PAHs, computations have played and are still playing a fundamental role in connection with experiments and observations. From the very first calculations of the IR spectra of small PAHs in the late eighties to the more recent ones, every aspect of the research linked to the PAH hypothesis has evolved dramatically: the size and the variety of the molecules considered, the techniques used, the precision of the astronomical observations . . . The initial landscape has completely changed though the quest is still the same, that is to correlate the so-called UIR bands spectra ubiquitous in the ISM (Inter Stellar Medium) with a chemical family of molecules, the PAHs. An historical review of the 25 years of this quest is presented here, focusing on the computational part.

Anharmonic vibrational spectroscopy of polycyclic aromatic hydrocarbons (PAHs)

“…In order to perform simulations that can be compared with the AIB spectrum, temperature effects are then only simply included by adding some ad-hoc band redshift and broadening, see e. g. Ref. 16 and the discussion of synthetic PAH spectra by Boersma et al 16 and Pauzat 17…”

“…16 and the discussion of synthetic PAH spectra by Boersma et al 16 and Pauzat. 17 In order to progress in the analysis of the astrophysical spectra, one has clearly to go beyond this simplified treatment. Efforts should be dedicated to describe connected states at non-zero temperatures.…”

Search for Hydrogenated C60 (Fulleranes) in Circumstellar Envelopes

“…In order to test the possible effect of ionization on the spectral behavior of fulleranes, we have computed the spectra of two ionized species with low and high degree of hydrogenation : C 60 H + 2 and C 60 H + 36 starting from the geometries of their lowest energy neutral isomers (isomer1). It is well known that due to theoretical limitations in dealing with the electronic energy states close to the ground one that arise in fullerenes and fulleranes when they have an open-shell electronic structure, the wavelengths and intensities of the C-C vibrations are not accurate (Pauzat 2011). Consequently, we focus only on the C-H stretching bands in the 3-4 µm wavelength range.…”

DFT study of five-membered ring PAHs

“…Density Functional Theory (DFT) is commonly used to study large molecules as it is computationally efficient, viable and the results closely match those of experiments. Use of this computational methods helps to calculate the harmonic frequencies and intensities of vibrational modes of PAHs in various forms including size, composition and charge states (Langhoff, 1996;Langhoff et al, 1998;Hudgins et al, 2001Hudgins et al, , 2004Pathak and Rastogi, 2005, 2006, 2007Pathak and Sarre, 2008;Pauzat, 2011;Candian et al, 2014;Buragohain et al, 2018). The present work uses DFT in combination with a B3LYP functional, along with the 6-311G** basis set used for the purpose of optimization of the molecular structures of PAHs.…”