Abstract In the present work we discuss the role of the Pauli antisymmetry principle (PAP) in synchronous pericyclic reactions. These reactions are allowed in the electronic ground state whenever the PAP does not act as a quantum constraint in the transition state. The possible suppression of the influence of the PAP is a peculiarity of π electron systems. The PAP is a hidden (= deactivated) variable in the π electron subspace of polyenes and (4n + 2) annulenes (n = 0, 1, 2,...). In 4n annulenes (n = 1, 2, 3,...) it leads to minority signs in the kinetic hopping matrix of the π electronic wave function and thus to an energetic destabilization. The quantum statistical difference between the above families of π systems renders possible a microscopical definition of the quantities “aromaticity” and “antiaromaticity”. The sign behaviour of the kinetic hopping elements is used in the discussion of pericyclic reactions. The present quantum statistical description of these reactions is related to the Dewar-Zimmermann and Woodward-Hoffmann rules.
