  It is well known since 2010 that fullerene C60 is widespread through the
interstellar space. Also, it is well known that graphene is a source material
for synthesizing fullerene. Here, we simply assume the occurrence of graphene
in space. Infrared spectra of graphene molecules are calculated to compare both
to astronomical observational spectra and to laboratory experimental one. Model
molecules for DFT calculation are selected by one astronomical assumption, that
is, single void in charge neutral graphene of C13, C24 and C54, resulting C12,
C23 and C53. They have a carbon pentagon ring within a hexagon network.
Different void positions are classified as different species. Single void is
surrounded by 3 radical carbons, holding 6 spins. Spin state affects molecular
configuration and vibrational spectrum. It was a surprise that the triplet
state is stable than the singlet. Most of charge neutral and triplet spin state
species show closely resembling spectra with observed one of carbon rich
planetary nebulae Tc1 and Lin49. We could assign major bands at 18.9
micrometer, and sub-bands at 6.6, 7.0, 7.6, 8.1, 8.5, 9.0 and 17.4 micrometer.
It is interesting that those graphene species were also assigned in the
laboratory experiments on laser-induced carbon plasma, which are analogies of
carbon cluster creation in space. The conclusion is that graphene molecules
could potentially contribute to the infrared emission bands of carbon-rich
planetary nebulae.
