Chemistry Reference
In-Depth Information
detected under vacuum by means of mass spectroscopy. Since the first
report on the generation of cyclic C
18
by Diederich and co-workers [2a], the
activity in this area has continually grown to report various macrocyclic
carbon molecules in a size-selective manner. The carbon clusters so far pro-
duced from the organic precursors are C
12
,C
15
,C
16
,C
18
,C
20
,C
24
,C
30
,C
36
,
C
60
, and C
78
.
In this chapter, keeping in mind the generation, characterization, and
reactions of the cyclic polyynes, the interplay of organic chemistry and
carbon cluster science during the last decade is presented. First, following
short historical remarks (Section 6.2.1), recent research activity on the pro-
duction of cyclo[n]carbons from well-defined organic precursors is surveyed
(Section 6.2.2). Second, major structural and electronic properties of mono-
cyclic carbon clusters are presented in the context of theoretical considera-
tions (Section 6.2.3), followed by observational results of photoelectron
spectroscopy (Section 6.2.4). Third, considerations on the infrared activity
of cyclic C
10
will be presented (Section 6.2.5). Finally, this chapter ends with
experimental as well as theoretical proposals for the structures of multicyclic
polyynes (Section 6.3) and their relevance to the formation of fullerenes, in
particular from polycyclic polyynes (Section 6.4).
6.2 MONOCYCLIC CARBON RINGS
6.2.1 T
HEORY AND
D
ETECTION
:A
N
H
ISTORICAL
O
VERVIEW
In 1959, during a theoretical study of the properties of thermodynamic
equilibrium of carbon vapor molecules above high-temperature graphite,
Pitzer and Clementi took cyclic molecules C
n
as well as linear C
n
into
account [14]. Using the extended Hu¨ ckel theory, Hoffmann predicted that
cyclic carbon molecules having 4m
2
p
electrons are stable in cumulenic
forms, i.e. an electron-delocalized structure showing aromatic behavior [15].
Raghavachari and Binkley compared calculated stability of linear and cyclic
clusters [16]. They found that for odd numbered clusters linear forms are the
most stable up to n
þ
9, while for even numbered C
n
of n>10 monocyclic
forms are more stable than linear forms. For even numbered small clusters
of C
4
,C
6
, and C
8
, the linear and cyclic forms were found to be almost
isoenergetic. Using ultraviolet photoelectron spectroscopy (UPS), Yang
et al. found indications of monocyclicity for cluster anions C
n
of n
¼
10
produced by laser vaporization of graphite in helium gas [17]. Ion chroma-
tography experiments indicated that small carbon cluster ions exist in linear,
monocyclic, bicyclic, and polycyclic ring structures [18-20]. The collision-
induced annealing of mass selected cluster ions revealed transformation of
the polycyclic rings into fullerene cages (see
Section 6.3.2
for details) [21-24].
Also, in the condensed solid rare-gas matrices at low temperatures, carbon
molecules are studied extensively (see also
Chapter 1
). Concerning the cyclic
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