# Chemical Bonding in Three-Membered Ring Systems

## TL;DR

This paper studies the chemical bonding and reaction pathways in three-membered ring systems using advanced computational methods.

## Contribution

The study reveals that addition and elimination reactions in these systems often follow distinct diabatic paths with no energy barriers.

## Key findings

- Addition and elimination reactions in 3-ring systems follow different minimum energy paths.
- Energy increases until the system transitions to a lower symmetry state, combining diabatic states into an adiabatic one.
- CAS(4,4) wave functions and OVB analysis show diabatic reactions lack energy barriers in C2v symmetry.

## Abstract

The formation of the four 3-ring systems c-(CH2)3−k(SiH2)k (k=0: cyclopropane, k=1: silirane, k=2: disilirane, k=3: 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 C2v and Cs 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 C2v 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.