Symmetry Rules for Transition Structures in Degenerate Reactions

Abstract

The work of Pechukas [J. Chem. Phys. 1976, 64, 1515−1521] on predicting transtion structure symmetry from reactant and product symmetries is extended to degenerate reactions. Pechukas showed that the group of the transition structure in a one-step nondegenerate reaction must be one of the Pechukas groups common to reactant and product. It is now shown that for a one-step degenerate reaction the transition structure symmetry must either be that of one of the Pechukas groups as predicted for a nondegenerate reaction or a group constructed from one of them by adding “extra” symmetry operations to double its order. Several examples are given to clarify these rules, and a table is presented to aid in their application.