Decomposition of the electronic activity in competing [5,6] and [6,6] cycloaddition reactions between C₆₀ and...

Fullerenes, in particular C60, are important molecular entities in many areas, ranging from material science to medicinal chemistry. However, chemical transformations have to be done in order to transform C60 in added-value compounds with increased applicability. The most common procedure corresponds to the classical Diels-Alder cycloaddition reaction. In this research, a comprehensive study of the electronic activity that takes place in the cycloaddition between C60 and cyclopentadiene toward the [5,6] and [6,6] reaction pathways is presented. These are competitive reaction mechanisms dominated by σ and π fluctuating activity. To better understand the electronic activity at each stage of the mechanism, the reaction force (RF) and the symmetry-adapted reaction electronic flux (SA-REF, JΓi(ξ)) have been used to elucidate whether π or σ bonding changes drive the reaction. Since the studied cycloaddition reaction proceeds through a Cs symmetry reaction path, two SA-REF emerge: JA'(ξ) and JA''(ξ). In particular, JA'(ξ) mainly accounts for bond transformations associated with π bonds, while JA''(ξ) is sensitive toward σ bonding changes. It was found that the [6,6] path is highly favored over the [5,6] with respect to activation energies. This difference is primarily due to the less intensive electronic reordering of the σ electrons in the [6,6] path, as a result of the pyramidalization of carbon atoms in C60 (sp2 → sp3 transition). Interestingly, no substantial differences in the π electronic activity from the reactant complex to the transition state structure were found when comparing the [5,6] and [6,6] paths. Partition of the kinetic energy into its symmetry contributions indicates that when a bond is being weakened/broken (formed/strengthened) non-spontaneous (spontaneous) changes in the electronic activity occur, thus prompting an increase (decrease) of the kinetic energy. Therefore, contraction (expansion) of the electronic density in the vicinity of the bonding change is expected to take place.