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Computational study of polycyclic aromatic hydrocarbons (PAHs) with phenylside group substituted at different positions is reported. The infraredspectral variations due to the position of phenyl substitution, ionizationstate and the size of the molecules are discussed and possible contribution ofphenyl-PAHs to the mid-infrared emission features from astrophysical objects isanalyzed. Structurally phenyl group substitution at 2nd position gives morestable species compared to substitution at other positions. Phenyl-PAHs exhibitnew aromatic bands near 695 and 741 cm$^{-1}$ (14.4 and 13.5 $\mu$m), due tocontribution from quintet C-H wag, that compare well with minor features at14.2 and 13.5 $\mu$m observed in several astrophysical objects. Just as inplain PAHs, the C-C stretch vibrational modes ($\sim$1600 cm$^{-1}$) havenegligible intensity in neutrals, but the cations of all phenyl-PAHs exhibitsignificantly strong phenyl group C-C stretch peak close to class B type 6.2$\mu$m astrophysical band. In 2-phenylpyrene, it is the neutral molecule thatexhibits this strong feature in the 6.2 $\mu$m range along with other featuresthat match with sub-features at 6.66 and 6.9 $\mu$m, observed in astronomicalspectra of some late type objects. The substitution of phenyl side group atsolo position shifts the C-C stretch mode of parent PAH close to the region of6.2 $\mu$m astrophysical band. The results indicate possibility of phenyl-PAHsin space and the bottom-up formation of medium sized compact PAHs with phenylside group in carbon rich cool circumstellar shells. Phenyl-PAHs need to beconsidered in modelling mid-infrared emission spectra of various astrophysicalobjects.

Study of vibrational spectra of polycyclic aromatic hydrocarbons with phenyl side group