Structures and Energies of Disilicon Dicarbide, C₂Si₂

The structures and energies of the binary disilicon dicarbide C₂Si₂ in the lowest singlet and triplet states have been investigated by ab initio MO theory. Full fourth-order Møller-Plesset (MP4) perturbation theory is employed on HF/6-31G*-optimized geometries. Rhombic dicarbide 5s is the global C₂Si₂ minimum and 8.5 kcal/mol more stable than the rhomboidal structure 6s, which contains an inverted tricoordinate carbon as well as an inverted tricoordinate silicon. Both cyclic structures are energetically favored over a linear triplet isomer. Whereas the relative energies are very sensitive to electron correlation effects, addition of diffuse functions at the Hartree-Fock level (HF/6-31+G*) has little influence.