  By applying quantum chemical calculation, interstellar infrared spectrum was
categorized to three classes. Type-A show unusual feature of strong peaks at
11.3,12.9, and 14.0 micrometer. Usually observed 6.2, 7.7, and 8.6 micrometer
bands are weak or not recognized. Typical examples are NGC1316 and NGC4589.
Such spectrum could be identified for the first time by pure carbon molecule
(C23)2+ (dication) having two carbon pentagons combined with five hexagons.
Calculated spectrum coincided well at 11.3, 13.0, and 14.0 micrometer. Also we
could find more coincidence at 5.2, 5.6, 7.6, 8.8,10.6,15.7 and 17.2
micrometer. Type-B is ubiquitously observed IR, but show medium strength at
11.3 micrometer. Examples are NGC6946 and the red triangle nebula. Coronene
modified PAH (C23H12)2+ show best coincidence for both wavelength and strength.
Type-C is usualy observed one featuring very strong peak at 11.3 micrometer.
Examples are NGC7023, NGC2023 and M17SW. One capable expanation of large 11.3
micrometer band is a mixture of Type-A and Type-B. Combination of polycyclic
pure-carbon-molecule and hydrocarbon-molecule may give a variety of IR
spectrum.
