Chemical Bonding in Three-Membered Ring Systems

The formation of the four 3-ring systems c-(CH ₂) _3−k(SiH ₂) _k ((Formula presented.) : cyclopropane, (Formula presented.) : silirane, (Formula presented.) : disilirane, (Formula presented.) : cyclotrisilane) by addition of methylene and silylene to the double bond in ethene, disilene, and silaethene, as well as the elimination of the carbene analogs from the 3-rings, was studied with CAS(4,4) wave functions in both (Formula presented.) and (Formula presented.) symmetry. To reveal the charge and spin redistribution during these reactions the CAS(4,4) wave functions were analyzed using the orthogonal valence bond method (OVB). The potential energy curves, different internal coordinates, and the results of the OVB analysis show that, frequently, the addition and elimination reactions follow different minimum energy paths, because they are indeed diabatic reactions. In these cases, there are no energy barriers corresponding to saddle points on the potential energy surfaces but the energy increases during one diabatic reaction until, at a certain point, the system jumps to the other diabatic state and, in the following, the energy decreases. This happens for reactions in (Formula presented.) symmetry; as soon as the system can change to the lower symmetry, the diabatic states combine to an adiabatic one and the reaction follows a single minimum energy path.