A closer look to the bispericyclic transition structure leading to 4+2 and 2+4 cycloadducts in the endo dimerization of cyclopentadiene

This work delves into the study presented by Caramella and co-workers (J. Am. Chem. Soc. 2002, 124, 7) on the cyclopentadiene dimerization. We have reperformed the calculations of this work using updated methodologies, obtaining the structures of the reactants and transition states, as well as new energy profiles. Additionally, we have obtained new insights that were not included in the previous work, such as bond structures from NBO analysis, QTAIM theory, and spin density. Reactivity indices of the principal NBOs responsible for the reaction have also been obtained using a recently developed model based on conceptual DFT and Sanderson's principle of electronegativity equalization and the new results obtained have allowed us to propose reaction mechanisms that satisfactorily justify all reaction stages. This work is a study of the mechanism of the cyclopentadiene dimerization reaction, but it is also an example of applying the new model for calculating reactivity indices for NBOs. Additionally, it demonstrates the combined use of these descriptors with other types of analyses that also employ NBOs or provide a description of the molecular bonding structure. As shown in this work, the collective use of all these methodologies generally provides an adequate description of the chemical reactivity of the studied processes.