Abstraction Reactions of Heavy Cyclobutenes with Carbon Tetrachloride. A Theoretical Study

The potential energy surfaces for the abstraction reactions of heavy cyclobutenes with CCl4 have been characterized in detail by using density functional theory (B3LYP/LANL2DZdp), including zero-point corrections. Seven heavy cyclobutene species including A (C-C-C=C), B (Si-Si-Si=Si), C (Ge-Ge-Ge=Ge), D (Si-Si-Ge=Ge), E (Si-Si-Ge=Sn), F (Sn-Sn-Sn=Sn), and G (Pb-Pb-Pb=Pb) have been chosen in this work as model reactants. All the interactions involve a Cl or CCl3 shift via a two-center transition state. The activation barriers and enthalpies of the reactions were compared in order to determine the relative heavy cyclobutene reactivity as well as the influence of substituents on the reaction potential energy surface. As a result, our theoretical investigations suggest that a heavy cyclobutene species that contains more massive and less electronegative atoms in the double bond should undergo radical abstraction reactions with CCl4 more readily than one containing less massive and more electronegative atoms. Moreover, we show that having undergone an initial chlorine atom abstraction, a heavy cyclobutene will then proceed to undergo a second abstraction to give a tetrachloro derivative. Furthermore, a configuration mixing model based on the work of Pross and Shaik is used to rationalize the computational results. The results obtained allow us to make several predictions.