Carbon Chain Molecules in Cryogenic Matrices - Polyynes: Synthesis, Properties, and Applications - page 9

longer chains seem to develop bond alternation. The later feature lowers
the electronic energy of the chain, an effect related to the familiar Peierls

distortion of linear metals (see, e.g. [ 27 ] ). The bond alternation seems to be

accompanied by another feature in the electronic structure which turns out
to be greatly responsible for the above-mentioned simplification of the IR
spectra of long odd chains. This is a strong increase of the long range
coupling between the atoms in the chain, a coupling which extends far
beyond the nearest neighbours and which most likely originates from the
cloud of delocalized electrons.

The carbon matrix spectra can be compared with the spectrum of the

DIBs. The result is shown in Figure 1.6 . It should be realized that the DIBs

represent gas-phase spectra while the matrix data are subjected to the vari-
ous distortions mentioned above. Thus a detailed comparison is difficult.
However, some conclusions can be drawn: if the DIB carriers are neutral
carbon chains, the chain length should correspond to that of C 15 or longer.
The apparent and not yet well understood onset of DIBs at wavelengths
longwards of 440 nm may suggest that shorter chains do not survive photo-
destruction or other virulent processes in space. It should be noticed that the
grouping of DIBs fits rather well to the odd chain sequence of our matrix
spectra. All these arguments support the already otherwise established
conjecture of long carbon chains floating in interstellar clouds. We may
refer to the discovery of polyynes (e.g. of the family HC 2n þ 1 Nwith
n 5) made by radio astronomy in observing dense clouds [28]. The

polyynes have a carbon chain backbone structure. In the more rarefied
interstellar cloud medium where the DIBs are observed, the harsh condi-
tions may destroy the smaller species and strip the larger chain molecules
down to their carbon skeleton.
The matrix-isolation experiments we carried out may also have some
relevance to the still not well understood problem of fullerene formation.
The basic question is: How can such complicated molecules form with such
extreme efficiency? Fullerene yields far exceeding 10% have been reported
[29]. The initial steps of carbon condensation can be studied with matrix-
isolation techniques in a kind of slow motion, and the process can even be
stopped at will by cooling down. Basically, chains and monocyclic rings
seem to form initially and these should be regarded as building blocks for
further growth. When, during annealing we evaporated the matrix com-
pletely (see below) and checked the carbon material remaining on the
substrate for fullerenes, the result was always negative. No fullerenes were
formed. Apparently, the quenching of carbon vapor must take place in
a considerably hotter environment such that not only molecular growth
but also molecular rearrangements can take place. The cryogenic matrix
environment does not allow major rearrangements. Experiments in which
carbon vapor is condensed in a hot noble gas atmosphere indicate that for
molecular rearrangements to take place temperatures in the range 1000K
and higher are required [30,31].

¼ 0, 1 ...