Group theory and reaction mechanisms: Permutation theoretic prediction and computational support for pseudorotation modes in C2H and C5H rearrangements

Abstract

AbstractThe McIver–Stanton rules concerning the symmetry of transition states have a counterpart in rules concerning the permutation symmetry of single steps in degenerate rearrangements, derivable with the aid of Longuet–Higgins group theory. The generalized rules are illustrated by the widely studied PX5 polytopal rearrangements. The analysis leads to prediction of hitherto unexplored “pseudorotation” pathways for rearrangements in ethyl and homotetrahedryl cations. CNDO computations of system energies, gradients, and curvatures at critical points on the C2H and C5H surfaces indicate that symmetry‐breaking in keeping with the permutation‐theoretic predictions is a key feature of the low‐energy rearrangements of these systems. In particular, computation indicates that the C2v “classical” homotetrahedral cation corresponds to an energy maximum rather than an energy minimum, or a transition state.