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Theoretical Investigations of Mechanisms of Thermal Cleavage of E=E Bonds in Heavy Butadiene Systems (E = C, Si, Ge, Sn, and Pb)



The potential energy surfaces for the intramolecular reactions of heavy 1,3-butadiene have been explored using DFT. All the stationary points, which include the heavy 1,3-butadienes (R2E=ER–ER=ER2, E = group 14 element), intramolecular fragments, transition states, and the products, were completely optimized at the B3LYP/LANL2DZ level of theory. Five 1,3-butadiene species, which include carbon, silicon, germanium, tin, and lead, were chosen as model reactants. Our theoretical findings suggest the following: (1) both sterically bulky substituents attached to the heavy butadiene and the weakness of the E=E double bond lead to the easy cleavage of one E=E double bond in heavy 1,3-butadiene and (2) for two intramolecular reactions (cycloaddition and 1,2-migration) of the heavy 1,3-butadienes with sterically overcrowded substituents, the lighter the E atoms involved in the 1,3-butadiene molecule, the smaller the intramolecular barrier, the lower the reaction enthalpy, and the more facile its intramolecular reaction at room temperature. Furthermore, a configuration-mixing model has been used to rationalize the computational results, and the results obtained allow a number of predictions to be made.