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.