Vibrational dynamics of phenylphenanthrenes with phenyl group at different positions

Polycyclic aromatic hydrocarbons (PAHs) appear to be ubiquitous in terrestrial atmosphere as well as in the interstellar medium (ISM). In the astrophysical context presence of PAHs is interpreted by the observations of mid-IR emission bands at 3.3, 6.2, 7.7, 8.6, 11.2 and 12.6 micron (3030, 1610, 1300, 1160, 890 and 790~cm$^{-1}$). The profile of these emission bands exhibit definite variations from source to source, which are explained due to sources having different types of PAHs in different ionization states. The model spectra of mixtures of PAHs show good fit for most bands except for the 6.2 micron feature. Thus a wider variety of PAHs need to be considered for this feature. PAH formation in circumstellar medium of planetary nebulae and dense clouds indicate formation of phenyl radical and phenyl substituted PAHs. To understand the modifications in infrared lineaments upon phenyl substitution and also due to substitution at different positions, study is made on five possible isomers of phenylphenanthrene. The features that may account for possible phenylphenanthrenes in protoplanetary nebulae atmospheres are also discussed. Possible contribution of phenylphenanthrenes towards the 6.2 micron feature and other astrophysical emission bands is assessed.