Ab initio study of cyclic siloxanes (H2SiO)n: n = 3, 4, 5

The geometry optimizations for several conformations of tri-, tetra-, and pentacyclosiloxane (H₂SiO)_n (n = 3, 4, and 5) were carried out, and the relative stabilities were compared at the Hartree-Fock (HF) and second order perturbation theory (MP2) levels of theory using the 6–31G* and 6–311G(d, p) basis sets. At the highest levels of theory, the only minimum for n = 4 (D₄) occurs at the highly symmetric D_4h structure. In contrast, several, nearly isoenergetic, minima are found on the D₅ surface. These have C₁, C₂, C_s, and D_5h symmetries. While the C₁ structure has the lowest MP2/6–311G(d, p) energy, this species is predicted to be highly fluxional, and the distribution of isomers is dependent on temperature. © 1996 by John Wiley & Sons, Inc.