AN AB-INITIO MOLECULAR-ORBITAL STUDY OF THE UNIMOLECULAR DISSOCIATION REACTIONS OF VINYLCHLORIDE

The potential energy surface for the unimolecular ground state elimination/detachment reactions of vinylchloride (VC1) is studied with ab initio molecular orbital calculations. Using second order Moller-Plesset perturbation (MP2) gradient optimizations and quadratic single and double configuration interaction including a triple contribution [QCISD(T)] single point calculations, many possible channels-three- and four-center elimination reactions of HCl and H-2, H, and Cl migrations, single atom detachments-are systematically examined. The overall barrier for the HCl three-center elimination, 69.1 kcal/mol, and subsequent vinylidene rearrangement is lower than that for the four-center elimination, 77.4 kcal/mol, the result being consistent with the most recent experiments. The H-2 elimination, with a barrier of 97.2 kcal/mol and more difficult than the HCl elimination, also proceeds via a three-center transition state, followed by an easy H migration from chlorovinylidene intermediate. The lowest energy path from VCl is the alpha-H migration with a barrier of 68.8 kcal/mol, which gives a stable intermediate, alpha-chloroethylidene, with an energy of 55 kcal/mol relative to VCl. This path cannot however proceed further because of a very high barrier for subsequent H-2 and HCl elimination, but the return to VCl provides the easiest mechanism for h scrambling in VCl. The beta-chloroethylidene species is only a marginally stable intermediate and would not contribute to the reaction.