The [4+2] Diels-Alder cycloaddition between cyclopentadiene and acrylonitrile is well-known for its asynchronous reaction pathway. In the reactant structure (Fig. ), the cyclopentadiene and acrylonitrile molecules are separated by 3.17-3.24 Å. As the reaction progressed, the two fragments moved closer to each other. At the TS structure (Fig. ), the distance between the C 4 carbon of cyclopentadiene and the C 6 carbon of acrylonitrile was reduced to be 2.09 Å. Meanwhile, the C 1 -C 7 distance was reduced only to 2.43 Å, suggesting an asynchronous cycloaddition caused by the presence of the As shown in Fig. , the reaction started with a transfer of electrons of three C-C double bonds of acrylonitrile and cyclopentadiene (shown in magenta) to the formation of the C 4 -C 6 and C 1 -C 7 bonds (shown in blue), respectively. Noticeably, a more substantial electron density change occurred between the C 4 and C 6 atoms, consistent with a short distance at the transition. At the same time, a relatively smaller electron density change was observed between C 1 -C 7 atoms, indicating a slower bond formation. In addition to the partial formation of these new σ bonds, the C 2 -C 3 bond of the cyclopentadiene also gained electron density, bringing it a partial double-bond character. The EDC from the TS to the product showed the completion of the reaction (Fig. ), where the π bond of acrylonitrile (C 6 -C 7 ) is the primary site for losing electrons while C 1 -C 2 and C 3 -C 4 bonds of cyclopentadiene continued to lose electrons (in magenta). On the other hand, the electron density on the cyano group underwent only marginal changes throughout the reaction.