Variation of BCP ellipticities in the course of the pericyclic and pseudopericyclic [2 + 2] cycloaddition reactions of cumulenes

Ellipticity changes of the electron density at the bond critical points along the [2 + 2] cycloaddition reaction paths were investigated by the AIM method applied to the MP2/aug-cc-pVDZ calculated structures. Elipticities of the σ-bonds being formed and π-bonds being broken in the ketene-vinylimine and allene-isocyanic acid cycloadditions were examined. On this basis we proposed four criteria for differentiation between pericyclic and pseudopericyclic reactions and classified the ketene-vinylimine reaction as pseudopericyclic whereas the allene-isocyanic acid one as pericyclic process. Additionally, we found the π(C=X) ellipticity profiles, X=N, O, suggesting more or less isotropicity of the C=X bond electron density over the whole reaction.