Computational study of neutral and cationic pericondensed polycyclic aromatic hydrocarbons

Triangulenes and theirs ions: reaching the limits of Clar's rule

The triangulenes and their closed-shell ions are a family of polycyclic aromatic hydrocarbons (PAH) that have possible applications in fields as different as spintronics or catalysis. However, the electron delocalization in such systems is not well understood because there are several differences to classical PAHs. We found that the triangulene cations are always more delocalized than the radicals or the anions, independently of the π e^- count. Contrary to any other PAHs, the π e^- of triangulenes and their ions are delocalized throughout the whole molecule and are even more delocalized than acenes. The π sextet aromaticity does not play a central role in the stabilization of triangulenes like for other PAHs. Interestingly, neither the radicals nor the ions follow Clar's rule, which makes them a unique type of PAH.

Vibrational spectroscopic study of vinyl substituted polycyclic aromatic hydrocarbons

The mid infrared emission features observed in various astrophysical sources are attributed to polycyclic aromatic hydrocarbon (PAH) molecules. The models of emission spectra from a collection of PAHs show uncertainty in matching the 6.2 μm feature. This indicates the need to consider a larger variety of PAHs and PAH derivatives. Chemical pathways towards formation of PAHs in the astrophysical environments involve vinyl substituted PAHs as intermediate products. Vibrational spectroscopic study of vinyl-PAHs is reported in the present work. The vinyl group is substituted at similar positions in eight different PAHs. The obtained optimized structures show that vinyl substitution at 2 position in acenes gives planar geometry, while all other vinyl-PAHs are non-planar. Infrared spectra is simulated for neutrals as well as for cations. The results are compared with the spectra of corresponding plain PAHs and analyzed for possible match with astrophysical observations. New features, due to vinyl group in the composite spectra, identified at 6.64, 6.92, 7.27, 8.77 and 10.35 μm fall close to some sub features of the observed emission spectra. The paper provides data that may be used in the emission models particularly along proto planetary nebulae type cool objects.

Effect of ionization on infrared and electronic absorption spectra of methyl and ethyl formate in the gas phase and in astrophysical H2O ice: a computational study

This work reports infrared and electronic absorption spectra of trans and gauche conformers of neutral ethyl formate, trans and cis conformers of neutral methyl formate, their ions in the gas phase, and neutral ethyl and methyl formate in astrophysical H(2)O ice. The second-order Møller-Plesset perturbation (MP2) method with TZVP basis set has been used to obtain ground-state geometries. An influence of ice on vibrational frequencies of neutral ethyl and methyl formate was obtained using integral equation formalism polarizable continnum model (IEFPCM). Significant shift in vibrational frequencies for neutral methyl and ethyl formate when studied in H(2)O ice and upon ionization is observed. Rotational and distortion constants for neutral ethyl and methyl formate from this work are in excellent agreement with the available experimental values. Electronic absorption spectra of conformers of ethyl and methyl formate and their ions are obtained using time-dependent density functional method (TDDFT). The nature of electronic transitions is also identified. We suggested lines especially good to detect these molecules in interstellar medium. Using these lines, we can identify the conformers of ethyl and methyl formate in gas phase and H(2)O ice in interstellar medium. This comparative study should provide useful guidelines to detect conformers of ethyl and methyl formate and their ions in gas phase and neutral molecules in H(2)O ice in different astronomical environment.

Molecular dynamics simulations on [FePAH]+ π-complexes of astrophysical interest: anharmonic infrared spectroscopy

In this article, classical Born-Oppenheimer molecular dynamics (MD) simulations in the microcanonical ensemble are performed on neutral and cationic polycyclic aromatic hydrocarbon (PAH) species, focusing on [FePAH](+)π-complexes. Their anharmonic mid-infrared (mid-IR) spectra in the classical approximation are derived. This approach allows us to describe the influence of the energy of a system on its IR spectrum in terms of band-shifts and broadenings. The MD simulations are performed on a potential energy surface (PES) described at the self-consistent-charge density functional tight-binding level of theory. The PES is benchmarked on DFT calculations, showing the validity of the approach for complexes of Fe(+) with PAHs larger than coronene (C(24)H(12)) that are of astrophysical interest. MD simulations at high temperature show the occurrence of the diffusion of the Fe cation on the surface of the PAH. It proceeds through the edge of the carbon skeleton which is the lowest energy pathway presenting barriers smaller than 1 eV. Although only qualitative information on the band broadenings can be obtained, we show that the dependence of the computed positions of the main bands of [C(24)H(12)](0/+)and [FeC(24)H(12)](+)π-complexes on temperature can be fit by linear laws. The spectral trends determined for [FeC(24)H(12)](+) are compared to those of N-substituted [C(23)NH(12)](+)and [SiC(24)H(12)](+)π-complexes of astrophysical interest.

Theoretical infrared spectra of large polycyclic aromatic hydrocarbons

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 microm band. Presence of two sub-components of the broad 7.7 microm 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 profile variations accompanying the mid-IR bands in various interstellar environments.