February 2011

Cyclic SiC3

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 two 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].


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