Cyclic SiC3 was observed toward the carbon star IRC +10216 by
Apponi et al. and reported in 1999; the
experimental data upon which the astronomical detection was based were published separately in
papers. The detection has not been
confirmed to date; a recent search for c-SiC3 in Orion KL by
Tercero et al. placed an upper
limit on its column density of 1.3 x 1013 cm-2 in that source.
The SiC3 system is intriguing, with two rhomboidal, bicyclic minima.
In addition to the observed interstellar species (isomer 1), where the three carbon atoms are arranged
in a nearly equilateral triangle, there is a second
form (isomer 2) in which the three carbon atoms are roughtly linear. Isomer 2 is only slightly less stable
than isomer 1 [by about 6 kcal/mol at the CCSD(T)/cc-pCVQZ
level]. The charge densities suggest that there is a great deal of Si+C3-
character in both isomers, which is consistent with their substantial dipole moments [4.1 and 2.1 D for
isomers 1 and 2, respectively, at the CCSD(T)/aug-cc-pV(T+d)z level].