Structures and Energies of Singlet Silacyclopropenylidene and 14 Higher Lying C₂SiH₂Isomers

The closed-shell molecular structures and relative energies of 15 different isomers of the formula C₂SiH₂have been investigated via nonempirical molecular electronic structure theory. Eight structures were found to be minima on the self-consistent-field potential energy hypersurface by using a double-ζ (DZ) basis set. The three lowest lying isomers have further been optimized with the DZ basis set augmented by polarization functions (DZ+P), and vibrational frequencies and IR intensities were obtained at this level of theory by using analytical gradients and second derivatives. Correlation energies have been predicted at the DZ+P level via configuration interaction including all single and double excitations of the Valence orbitals (DZ+P CI). 3-Silacyclopropenylidene (1) has clearly been found to be the global minimum on the singlet C₂SiH₂potential energy hypersurface. Vinylidenesilene (2) and silylenylacetylene (3) are 17 and 22 kcal/mol higher in energy, respectively (DZ+P CI). While 1, 2, and 3 might experimentally be observed, the other isomers are higher in energy, and it is less likely that they will soon be identified.