Decomposition of the Electronic Activity in Competing [5,6] and [6,6] Cycloaddition Reactions Between C60 and Cyclopentadiene

Abstract

In this work, 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 sigma and pi 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, J) have been used helping to elucidate whether sigma or pi bonding changes drive the reaction. Since the studied cycloaddition reaction proceed through a CS symmetry reaction path, two SA-REF are constructed: JA' and JA''. In particular, JA' accounts for bond transformation associated with pi bonds, while JA'' is sensitive toward sigma bonding changes. It was found that the [6, 6] path is highly favored over the [5, 6] regarding activation energies. This difference is primarily due to the less intensive electronic reordering of the electrons in the [6, 6] path, as a result of the pyramidalization of carbon atoms in C60 (sp2 to sp3 transition). Interestingly, no substantial dfferences in the pi 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