  Computational study of polycyclic aromatic hydrocarbons (PAHs) with phenyl
side group substituted at different positions is reported. The infrared
spectral variations due to the position of phenyl substitution, ionization
state and the size of the molecules are discussed and possible contribution of
phenyl-PAHs to the mid-infrared emission features from astrophysical objects is
analyzed. Structurally phenyl group substitution at 2nd position gives more
stable species compared to substitution at other positions. Phenyl-PAHs exhibit
new aromatic bands near 695 and 741 cm$^{-1}$ (14.4 and 13.5 $\mu$m), due to
contribution from quintet C-H wag, that compare well with minor features at
14.2 and 13.5 $\mu$m observed in several astrophysical objects. Just as in
plain PAHs, the C-C stretch vibrational modes ($\sim$1600 cm$^{-1}$) have
negligible intensity in neutrals, but the cations of all phenyl-PAHs exhibit
significantly 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 that
exhibits this strong feature in the 6.2 $\mu$m range along with other features
that match with sub-features at 6.66 and 6.9 $\mu$m, observed in astronomical
spectra of some late type objects. The substitution of phenyl side group at
solo position shifts the C-C stretch mode of parent PAH close to the region of
6.2 $\mu$m astrophysical band. The results indicate possibility of phenyl-PAHs
in space and the bottom-up formation of medium sized compact PAHs with phenyl
side group in carbon rich cool circumstellar shells. Phenyl-PAHs need to be
considered in modelling mid-infrared emission spectra of various astrophysical
objects.

Astrophysics of Galaxies;Solar and Stellar Astrophysics;Chemical Physics