A theoretical scale for pericyclic and pseudopericyclic reactions

A theoretical (absolute) scale is proposed to categorize the pericyclic/pseudopericyclic nature of bonding at transition states. Such a scale is based on the electron localization function (ELF), a local measure of Pauli repulsion. Its usefulness is illustrated for settling the nature of bonding of ten cycloaddition transition states associated to the reaction between ethylene and formaldehyde and buta-1,3-dien-one and simple substituted derivatives.