Cyclic Polyynes: Generation, Characterization, and Reactions - Polyynes: Synthesis, Properties, and Applications - page 112

corresponding isomerization of smaller rings (around C 36 ) activation by
collision to the buffer gas is necessary. Planar graphitic as well as bowl-
shaped isomers are not observed, suggesting that these isomers are not
involved in the carbon growth mechanism.
It was shown by collision-induced annealing experiments that relatively
large, multicyclic carbon cluster cations with more than around 40 atoms
were easily converted to fullerenes [21,22]. Annealing experiments were
carried out by von Helden et al. for medium-sized cluster cations C 30 þ to
C 40 þ , which were composed by a mixture of mono- and multicyclic isomers
(bicyclic forms as the major component) [23]. It was revealed that, in con-
trast to the larger cluster cations, most of the multicyclic forms of C 30 þ to
C 34 þ isomerized to the corresponding larger monocyclic forms of the same
carbon numbers. However, the collision-induced fragmentation also took
place by the loss of small fragments such as C 1 ,C 2 or C 3 to form smaller
cluster cations which were determined to possess fullerene structures on the
basis of their ATDs. The ratio of fragmentation increases with increasing
cluster size.
Shelimov et al. also performed detailed annealing study of medium-sized
cluster cations, C 24 þ to C 36 þ , whose major components are bicyclic ring
isomers, in order to investigate the rings-to-rings transformation [24]. These
cations are also shown to isomerize to monocyclic forms with an activation
energy of around 2.4 eV, which slightly decrease with increasing cluster size.
With larger injection energies, fragmentation to small clusters took place as
observed by von Helden et al. [23]. A model has been proposed based on the
[2 2] cyclo-additions, as shown in Figure 6.10, which explains

the formation of bicyclic isomers from two monocyclic forms (C 20 ) and its
transformation into a larger monocyclic form (C 40 ). For the formation of
bicyclic isomers, the [2

þ 2] and [4 þ

2] cycloaddition is assumed to be favorable because
the positive charge would lower the inherently high activation energy of a
symmetry-forbidden [2

þ

þ þ

2] cycloaddition. The [4 2] cycloaddition, on the

FIGURE 6.10 Possible [2 2] pathways for the transformation of mono-

cylic and bicyclic forms of carbon clusters (for C 40 ).

þ 2] and [4 þ