Chemistry Reference
In-Depth Information
FIGURE 15.1
HPLC chromatogram of the polyynes and monocyanopolyynes
produced by the carbon arc in acetonitrile at
40
C. The chromatogram at the top
of the figure has been normalized in order to show all the relevant peaks associated
to the molecular species present in the mixture. The most intense peaks are
hydrogen-capped polyynes the others are monocyanopolyynes other than the peak
with R
t
¼
5.86min. The peak due to H-(C
C)
9
-H is not shown in the figure since it
appears at R
t
¼
9.67min and is weak. The chromatogram at the bottom of the figure
shows the relative intensity and hence the relative abundance of the various
molecular species. The most abundant species is H-(C
C)
4
-H with retention time
R
t
¼
1.65min, followed by H-(C
C)
3
-CN followed by H-(C
C)
5
-H and then by
4
6
observe a series of less intense peaks having approximately one third the
intensity of the strong peaks. The analysis of the band pattern of the elec-
tronic absorption spectra of each individual strong peak of the chromato-
gram reveals that we are dealing with a series of hydrogen-capped polyynes
R
R
0
¼
¼
H having the general formula H-(C
C)
n
-H already detected and
discussed in
Chapter 8
of this topic. The assignment is also confirmed by
comparing the retention times as discussed in previous studies [18,19].
Instead, the other series of molecules eluted between the polyynes series
show another set of homologous spectra systematically red-shifted (see
Figure 15.2
) and with a different band pattern in comparison to the normal
hydrogen-capped polyynes as clearly illustrated in Table 15.1. Based on the
regular red shift of the most intense absorption band observed in these
polyynes series, based on the fact that this series was observed exclusively
in acetonitrile solvent and based on the intermediate retention time in
the HPLC column between two ''ordinary'' hydrogen-capped polyynes,
we reached the conclusion [18] that we were dealing with a series of
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