Reduction pathways of organohalogen compounds: Part III. A molecular orbital (CNDO/2) study of the chlorinated benzenes, DDT, and lindane

Abstract

Molecular orbital calculations, using the CNDO/2 method which treats both the σ and π systems of molecules, were carried out for all the chlorinated benzenes, DDT (1,1,1-trichloro-2,2-bis-( p-chlorophenyl)ethane), lindane ( γ-1,2,3,4,5,6-hexachlorocyclohexane) and γ-BTC ( γ-3,4,5,6-tetrachlorocyclohexene). The energy level and electron distribution for the lowest unoccupied molecular orbital (LUMO), both LUMO- σ and LUMO- π when each system is present in the molecule, were compared with the electrochemical behavior of these compounds. The energies of both the LUMO- σ and LUMO- π are about equal and each give a linear correlation with reduction potentials for all the chlorinated benzenes. However, the CNDO/2 calculations for their respective radical anions show that the electron has entered the σ orbital. The LUMO- σ is antibonding with the electron density localized in the C−Cl bonds which undergo cleavage to the reduction products. The relative electron density for different C−Cl bonds correlates with the product ratios when several products are found. The evidence indicates that upon electrochemical reduction of chloro organic compounds the first electron should enter the LUMO- σ, even if there is a π system available.