3.1.1 MDA reaction on 6-6 double bond of C60:
The initial adduct, A16-6O formed between the 1st DA product (16-6) and butadiene (diene) molecule is found to be stabilized by 4.2 kcal/mol than the initial reactant pair. The reaction proceeds via the formation of a concerted six-membered transition state (TS16-6O) of activation barrier 15.7 kcal/mol in order to form R16-6O, which is considered as the second DA product. In R16-6O, the associated 6-6 bond length and the newly formed C-C bonds with the butadiene are calculated to be 1.59 and 1.56 Å, respectively. The thermodynamic feasibility of the reaction is evident from its high exothermicity value of -28.2kcal/mol with respect to A16-6O. The subsequent adduct (A26-6O) formation between R16-6Oand another butadiene molecule is energetically 32.2 kcal/mol more stable than its former adduct complex (A16-6O). A26-6O undergoes a third DA reaction via TS26-6O (Figure 2 ) of barrier height 18.0 kcal/mol to generate R26-6. This third functionalized product, R26-6 is energetically 24.3 kcal/mol more stable than that of A26-6O (Figure 3 ). The fourth DA reaction is initiated from the adduct A36-6, which is found to be situated at -64.6 kcal/mol in the energy profile diagram. The conversion of A36-6 to P46-6 requires an activation barrier of 18.2 kcal/mol (TS36-6). Moreover, analyzing the TS geometries of all three DA steps, as depicted inFigure 2 , it is observed that the bond formation between the diene and fullerene surface occurs almost to the same extent, suggesting the process to be synchronous in nature.
In short, we have explored the incorporation of a total of four 1, 3-butadiene molecules on the surface of neutral C60 via the MDA functionalization process, considering 6-6 bond connectivity to be the dienophile. Moreover, a gradual rise in the stabilization energy due to consecutive attachment of butadiene molecules to 16-6 is noticeable. The final product, P46-6 is found to be-88.5 kcal/mol downhill than the starting reactant, 16-6, suggesting the MDA functionalization procedure to be highly thermodynamically facile.