Unfavourable orbital overlap at the transition state of a symmetry allowed reaction: a theoretical analysis

Abstract

The most favourable reaction path for the approach of CH₃⁺ to acetylene to give cyclo-C₃H₃⁺+ H₂ goes through a π transition state leading from the corner-protonated cyclopropene to the products in agreement with experimental evidence; this transition structure presents a null HOMO–LUMO overlap which prevents electron density rearrangement from taking place by HOMO–LUMO interaction.