Unusual isomers of disilacyclopropenylidene (Si2CH2)

Nine electronic singlet state structures of Si(2)CH(2) have been systematically investigated by high level theoretical methods. This research employed coupled cluster (CC) methods with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] using the correlation-consistent polarized valence cc-pVXZ/cc-pV(X+d)Z (X = D, T, and Q) basis sets. Full valence complete active space self-consistent-field (CASSCF) wave functions were used for the interpretation of geometries and physical properties. Among the nine singlet stationary points, six structures (1S-6S) are found to be minima, two structures (7S and 8S) are transition states, and one structure (9S) is a second-order saddle point. The existence of the two peculiar hydrogen bridged isomers, 1S (Si...H...Si) and 4S (agostic CH...Si) is established. Extensive focal point analyses are used to obtain complete basis set (CBS) limit energies. For the six lowest-lying singlet minima, after focal point analyses, the energy ordering and energy differences (in kcal mol(-1), with the zero-point vibrational energy corrected values in parentheses) are predicted to be 1S [0.0 (0.0)] < 3S [14.7 (14.5)] < 4S [25.1 (25.3)] < 5S [28.2 (26.0)] < 6S [45.0 (45.4)] < 2S [73.8 (72.0)]. Their relative energies are strikingly different from those for the isovalent parent C(3)H(2) molecule. Geometries, dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities are reported for all equilibrium structures.