Polyynes: Synthesis with the Submerged Electric Arc - Polyynes: Synthesis, Properties, and Applications - page 163

FIGURE 8.8 FT-IR spectra (recorded in an IR cell for liquids) of pure dodecane
(blank) and polyynes dissolved in dodecane after arcing of graphite electrodes
submerged in dodecane. The polyyne solution shows the peak at 3312 cm 1 which is
due to the stretching of the -C C-H group. This adsorption band is enhanced by the
substraction of the blank. The -C C-H stretching demonstrates that the polyynes
are hydrogen capped.

acetonitrile or other solvents (n-hexane, n-dodecane, and decalin) are treated
with Ilosvay's reagent invariably they produce a brown precipitate of copper
acetylides, which can be formed only when we are dealing with molecules
with -C C-H groups. Thus, it is demonstrated that the polyynes formed

are truly polyynes and not cyclopolyynes, alkyl-terminated polyynes or
cyanopolyynes. However, HPLC analysis has shown that, together with the
H-capped polyynes, other products are formed. These will be discussed in
the next section.
The evidence concerning hydrogen-terminated polyynes is not limited
to the electronic absorption spectra and the reactivity with Ilosvay's reagent.
In fact, in Figure 8.8 we present direct evidence of the presence of acetylenic
end groups, -C C-H, in a solution of polyynes in dodecane. The spectrum

of pure dodecane is exempt from any specific absorption band in the specific
absorption region of acetylenic end groups at 3312 cm 1 ; this can be seen in
Figure 8.8. Prolonged arcing of graphite electrodes in dodecane causes the
development of a new absorption band at 3312 cm 1 which can be enhanced
and clarified by subtracting the spectrum of pure dodecane from the spec-
trum of the polyyne solution in dodecane (see Figure 8.8). The spectra of