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that similar polyyne cyclization of appropriate precursors would form larger
fullerenes such as C
70
and C
76
as well through similar pathways [22d].
Polycyclic polyynes may therefore be regarded as viable precursors for the
size-selective and even geometry-selective construction of the desired fuller-
ene structure. Besides the annealing experiments of carbon cluster ion gen-
erated by vaporization of graphite and their structural elucidation based
on the ion chromatography, the crucial role of monocyclic carbon clusters
in the formation mechanism of fullerenes has been supported experimen-
tally by their selective formation from organic precursor molecules. Thus
McElvany et al. first observed the size-selective formation of fullerene C
60
þ
by the gas phase coalescence of the cyclic polyyne C
30
þ
, which was produced
size-selectively from its well-defined organic precursor [4c].
In view of this circumstantial evidence for the polyyne route to fuller-
enes, the groups led by Rubin and Tobe developed, independently, an
approach to C
60
from well-defined, three-dimensional (3D) polyyne pre-
cursors C
60
H
6
and C
60
Cl
6
, which were generated from the corresponding
organic precursors by expulsion of stable fragments such as carbon monox-
ide [50] or indane [51] by laser irradiation. As an initial step, Rubin prepared
C
60
H
18
and three C
16
enyne bridges [52]. In the MALDI mass spectrum (negative
mode) of 28, partial dehydrogenation down to C
60
H
14
was observed, sug-
gesting the possibility of complete dehydrogenation from more unsaturated
precursors to C
60
. The desired precursor C
60
H
6
(29a) having three C
16
polyyne chains was deemed too reactive for isolation in view of the previous
studies on the synthesis of linear polyynes. Thus Rubin utilized the decarbo-
nylation of cyclophane 30 possessing cyclobutenedione units to generate 29a
[50]. As expected, in the negative mode ICR LD mass spectrum of 30, not
only C
60
H
6
but also a strong peak due to C
60
was observed, suggesting
that cyclization of the polyyne chain took place efficiently accompanied by
dehydrogenation to form the fullerene cage. On the other hand, Tobe and
Wakabayashi observed, in the positive mode LD TOF mass spectrum of
[4.3.2]propellatriene-containing precursor 31a, a peak due to C
60
þ
formed
by the loss of eight indane fragments and six hydrogen atoms from 31a [51].
The negative mode LD TOF mass spectrum of 31a showed, in contrast to
the positive mode spectrum, the peak due to C
60
H
6
formed by the loss of
all indane units. The mass spectrum also showed the small peak due to C
60
anion formed by spontaneous dehydrogenation of all hydrogen atoms from
C
60
H
6
. Moreover, hexachloro derivative 31b exhibited in the positive mode
LD TOF mass spectrum a very strong peak of C
60
þ
besides weak peaks, due
to the C
2
loss and those up to C
120
þ
which might be formed by an ion-
molecule reaction of C
60
and the subsequent fragmentation of the dimeric
cluster ion. The formation of C
60
þ
is facilitated by the electron capture by
the chlorine atoms, as supported by the observation of a strong peak of Cl
in the negative mode spectrum. The negative mode LD TOF mass spectrum
of 31b also exhibited the peak due to C
60
Cl
6
(29b
) from which stepwise
(28) shown in
Figure 6.12
which was composed of two benzene rings
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