There are three SiC3 isomers which have been studied in the laboratoy, two bicyclic, rhomboidal singlet isomers and a linear triplet species. The prolate bicyclic molecule, with a CC bond creating the bicyclus, is lowest in energy and was observed in space. The other two are considerably higher in energy.
A. J. Apponi, M. C. McCarthy, C. A. Gottlieb, and P. Thaddeus
reported on the Astronomical Detection of Rhomboidal SiC3
Astrophys. J. 516, L103L106 (1999).
7 emission lines of 9 rotational transitions with J" between 6 and 8 were detected in the circumstellar envelope of CW Leo (IRC +10216) with the 12 m NRAO telescope between 80 and 103 GHz.