Chemistry Reference
In-Depth Information
The increasing number of nanoforms of carbon gives rise to a first taxonomic
problem. Should the aforementioned nanoforms of carbon be considered as differ-
ent allotropes? Answering this question could be accomplished by considering the
IUPAC definition of allotrope as “the different structural modifications of an
element” (IUPAC Compendium of Chemical Terminology, 2nd edition, 1997).
Although at a first glance they could be considered as different allotropes, the
scientific community considers fullerenes as the third allotropic form of carbon in
which the sp
2
carbon atoms are bonded to form spherical, tubular, or ellipsoid
structures,
thus gathering all
the above forms within the same allotrope of
“fullerenes.”
The above considerations do not, however, affect “buckyballs,” a nickname
which refers only to the former fullerenes, that is, those constituted exclusively
by closed cages of a precise number of carbon atoms, the most representative
examples being C
60
and C
70
molecules, and the most abundant and easy to obtain
fullerenes. In this chapter we will discuss the properties, chemical reactivity, and
some of the most realistic applications of fullerenes, including endohedral
fullerenes and those rare fullerenes which do not follow the “isolated pentagon
rule” (see below).
1.1 Brief History of Fullerenes
The discovery of fullerenes represents one of the most recent examples of seren-
dipity in chemistry. Fullerenes were discovered by Robert F. Curl, Sir Harold W.
Kroto, and the late Richard E. Smalley in 1985 [
1
] during carbon nucleation studies
under red giant star conditions. Only 11 years later, in 1996, these scientists were
awarded the Nobel Prize in Chemistry for “the discovery of fullerenes” [
9
-
11
]. Just
a year before, C
60
had been declared the molecule of the year by the scientific
journal
Science
.
This important finding has resulted in a new field with a broad impact in science,
thus provoking great excitement in the scientific community, ranging from chemis-
try, physics, and engineering to practical applications in materials science and bio-
medical applications [
12
]. Actually, the impact of the new fullerenes goes beyond
the scientific world and, since this molecule was first found in Texas, the State
Parliament declared Buckminsterfullerene C
60
the molecule of Texas State in May
1997.
However, a major breakthrough in fullerene science occurred in 1990 when
Wolfgang Kr¨tschmer and Donald Huffman (two astrophysicists) prepared fuller-
ene C
60
for the first time in multigram amounts [
13
] thus opening the fullerene
world to chemical functionalization and, therefore, to the unlimited imagination of
chemists for synthesizing new and sophisticated fullerene architectures. The impor-
tance of this achievement was pointed out by some of the Nobel laureate scientists.
In Smalley's own words: “Had there not been a method to make it in measurable
amounts, it would not have had an impact.” Curl also recognized this scientific
