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Nanomaterials Fabrication 83

Saunders reported the existence of the endohedral He@C 60 and
that form when C is exposed to a pressure of around 3 bars
Ne@C 60 60
of the appropriate noble gases [158]. Under these conditions it was
possible to dope one out of every 650,000 C 60 cages with a helium
atom. Endohedral complexes with He, Ne, Ar, Kr, and Xe as well as
numerous adducts of the He@C 60 compound have also been proven
[159] with operating pressures of 3000 bars and incorporation of up
to 0.1 percent of the noble gases. While the isolation of single atoms
of the noble gases is not unexpected, the isolation of N@C , N@C ,
60
70
and P@C 60 is very unusual. Unlike the metal derivatives, no charge
transfer of the pnictide atom in the center to the carbon atoms of the
cage takes place.

Chemically functionalized fullerenes. Although fullerene has a conju-
gated aromatic system, its reactivity is very different from planar
aromatics, as all fullerene carbons are quaternary, containing no hydro-
gen, which renders characteristic substitution reactions of planar
aromatics impossible. Therefore, only two types of primary chemical
transformations exist: redox reactions and addition reactions. Among
those two, addition reactions have the largest synthetic value in
fullerene chemistry as they can also function as a screening probe for
the chemical properties of fullerene surfaces. Another remarkable fea-
2
ture of fullerene addition chemistry is its thermodynamics. The sp
carbon atoms in a fullerene are pyramidalized. This dramatic variation
from planarity draws great strain energy, especially in C 60 fullerene. The
1
strain energy is ca 8 kcal mol , which is about 80 percent of its heat
of formation. So the relief of strain energy to fullerene cage resulting
from a moderate number of addends bound to the fullerene surface is
the major driving force for addition reactions as reactions leading to
3
sp hybridized C atoms strongly relieve the local strain of pyramidal-
ization, as shown in Figure 3.38. As a consequence, most additions to C 60
and C 70 are exothermic reactions; however, this energy decreases as
number of addends increase.
Another important feature for fullerene carbons is its rehybridization
2
of the sp and the p orbitals corresponding to the derivation from
planarity. Calculations have shown that average hybridization at carbon

A
101.6° [A] 110.9°
Figure 3.38 Strain release after
addition of addend A to a pyra-
midalized carbon of C 60 .
2
3
C ("sp ") C -adduct (sp )
60
60