Analyzing interstellar infrared spectrum by circumcoronene (C54H18)
  related molecules
  It is very important to identify carrier molecules of astronomical
interstellar infrared spectrum (IR) to understand chemical evolution step of
polycyclic aromatic hydrocarbon (PAH) in the universe. In our previous study,
it was suggested that coronene (C24H12) related PAH could reproduce such IR by
the first principles quantum chemical calculation. In this paper, PAH
candidates were enhanced to circumcoronene (C54H18) related molecules. Well
known PAH oriented wavelength of 6.2, 7.7, 8.6, 11.3, and 12.7 micrometer were
reproduced well by a void induced mono-cation (C53H18) having hydrocarbon two
pentagons combined with 17 hexagons. Typical astronomical object are the red
rectangle nebula and NGC6946, which is categorized as Type-B sectrum. Neutral
circumcoronene show very strong IR peak at 11.1 micrometer, which is noted as
Type-A spectrum observed in NGC1316 and NGC4589. Ubiquitously well observed
spectrum was noted as Type-C, which could be explained by a suitable
combination of Type-B and Type-A. Type-C objects are NGC7023, NGC2023 and so
many. It should be noted that a dehydrogenated pure carbon mono-cation molecule
(C53) show IR peaks at 6.3, 7.8, 8.5 micrometer, but no peak at 11.3
micrometer, which newly defined as Type-E. Such a strange characteristic can
contribute to explain IR intensity ratio. Observed intensity ratio between
peaks of 6.2, 7.7, 8.6, 11.3, and 12.7 micrometer each other were compared with
above calculated IR intensities. Major intensity ratio was well reproduced by
Type-B molecules. Variation of observed intensity ratio could be explained by a
mixture degree between Type-B and Type-A, and also a mixture degree between
Type-B and Type-E.
