3.2.1 MDA reaction on 6-6 double bond of C60:
As evident from Figure 5 , the stability of the first DA adduct for the alternative approach, i.e., A16-6S (-4.1 kcal/mol) is comparable with A16-6O. The activation barrier involved for R16-6S formation via TS16-6S is calculated to be 17.3 kcal/mol, which is only 1.6 kcal/mol higher than the corresponding direct pathway. Thus, we can say that a fruitful second DA reaction can be done by placing the butadiene molecule not only at the opposite of the first functionalization but also at the adjacent position. The addition of third butadiene molecule can be done either opposite to the first functionalization or second functionalization in R16-6S, but eventually both generate the same tri-functionalized product (R26-6) obtained in the ‘Direct’ approach. The associated adduct complex, A26-6S is placed at -36.5 kcal/mol on the energy profile diagram, which is energetically comparable with A26-6O. The transition state TS26-6S (Figure 4 ) associated with the formation of R26-6 requires a barrier height of 18.0 kcal/mol, which is exactly the same as the direct one. Similar to the second functionalization, the third functionalization is also highly exothermic as the associated product, R26-6 is -24.2kcal/mol more stable than A26-6S. For the attachment of the fourth butadiene, it will precisely follow similar pathway proposed for the ‘Direct’ approach.
Like ‘Direct’ approach, for the ’Alternative’ approach also, as shown inFigure 4 , all three DA reactions are synchronous processes as the vibrations associated with the new C-C bonds occur to an equal extent in the TS geometry.