Theoretical study of the relative reactivity of chloroethenes with atmospheric oxidants (OH, NO3, O(3P), Cl(2P) and Br(2P))

Abstract

The reactivity of a series of chloroethenes with different electrophiles of tropospheric and stratospheric interest is analyzed by frontier molecular orbital theory and a correlation with calculated orbital energies is investigated. The reactions of CH₂CHCl; CH₂CCl₂; (Z)-CHClCHCl; (E)-CHClCHCl; CHClCCl₂ and CCl₂CCl₂ with O(³P), Cl(²P), Br(²P) atoms and with OH and NO₃ radicals were studied using semiempirical methods (AM1 and PM3) and ab initio calculations at the HF and B3LYP levels of theory with the 6-31G^** basis set, using the Gaussian 98 suite of programs. In contrast to the majority of reaction series of small radicals and molecules with alkenes and alkanes, the rate constants for the reactions with halogenated ethenes do not correlate with the ionization potential of the halogenated ethene. The energy and the carbon–carbon π-bonding form of the HOMO change on addition of chlorine atoms as substituents to the carbon–carbon σ-bonding framework of the alkenes. For the reactions studied the complete interaction HOMO–SOMO was considered, taking into account the contribution of the different atomic orbitals to the HOMO of the chloroethene through the atomic orbital coefficients, and a good correlation with the experimental values was obtained.