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.
