Standard enthalpies Δf H 298, entropies S 298, and Gibbs functions of formation Δf G 298 for a large number of silicon hydrides containing one to five Si atoms have been calculated. The choice of compounds includes cyclic and acyclic silanes, silyl radicals, silylenes, disilenes, and cyclic diradicals. The thermodynamic functions were calculated using an empirically corrected ab initio scheme. The electronic energies were obtained with multiconfiguration reference averaged coupled-pair functional (ACPF) wave functions. Two different basis sets were employed to demonstrate the validity of the correction scheme. The computed data are compared with experimental and theoretical data from other laboratories. Increment rules and substituent effects are presented and various reaction enthalpies are tabulated and discussed. The performance of the correction scheme and potential pitfalls in its general applicability are discussed.

Torsion potentials about the X−ẌH bond in homosubstituted primary carbenes (X = C) and silylenes (X = Si) have been investigated at the multireference averaged coupled pair functional (MRACPF) level of theory. For the triplet species, the potentials are quite flat, but large barriers of torsion have been observed for the singlet states of all carbenes and silylenes whose carbon or silicon atom adjacent to the divalent atom formes small bond angles with two of its further substitutents; other geometry parameters, even the bond angle at the divalent atom, proved to be of little or no importance. The said kind of deformation encourages the formation of a weak dative π-like bond between the two X atoms, which by its 2-fold symmetry with respect to torsion about the bond axis is responsible for the observed two-minima torsion potential.