AN AB-INITIO MOLECULAR-ORBITAL STUDY OF THE UNIMOLECULAR DISSOCIATIONREACTIONS OF DICHLOROETHYLENE AND TRICHLOROETHYLENE

The potential energy surfaces for the unimolecular ground state elimin ation reactions of dichloroethylene (DCE) and trichloroethylene (TCE) are studied with ab initio molecular orbital calculations. By the grad ient optimization with the second-order Moller-Plesset perturbation (M P2) method and single point calculations with the fourth-order Moller- Plesset perturbation (MP4) and the quadratic single and double configu ration interaction including a triple contribution [QCISD(T)], many mo lecular elimination channels including three- and four-center HCl, H-2 , and Cl-2 elimination and H and Cl migration reactions are systematic ally examined. For cis- and trans-DCE, the three-center HCl eliminatio n with subsequent chlorovinylidene rearrangement has the lowest overal l barrier, whereas for 1,1-DCE for which the three-center path is not available, the four-center elimination has a rather low barrier. Anoth er path starting with the rearrangement of DCE isomers to 1,2-dichloro ethylidene followed by HCl elimination is not far in energy from these paths, complicating the overall mechanism of HCl elimination. The H-2 elimination from DCE isomers also can take either the three-center pa th or the 1,2-dichloroethylidene path. For TCE, though the overall bar rier to produce HCl+dichloroacetylene is the lowest for the four-cente r HCl elimination pathway, the production of HCl itself is most easily accomplished by the three-center HCl elimination, and the easiest pat h for disappearance of TCE is its rearrangement via H or Cl migration to give the 1,2,2-trichloroethylidene intermediate. Thus the overall m echanism of HCl elimination reactions from TCE can also be complicated .