Synthesis of Monocyanopolyynes and Dicyanopolyynes with the Submerged Electric Arc - Polyynes: Synthesis, Properties, and Applications - page 324

FIGURE 15.5 Electronic absorption spectra of polyynes mixture. 5A — Polyynes
obtained by arcing graphite directly into n-octane (the top curve of 5A is an expan-
sion of the spectrum above 280 nm to show finer details). 5B — Initial spectrum of
the polyynes mixtures produced from the graphite arc in liquid nitrogen and trapped
in n-octane at the beginning of the experiment. 5C — Same as 5B but after prolonged
arcing in liquid nitrogen. 5D — Same as 5C at the end of the experiment; a final total
concentration of polyynes estimated at about 8 10 6 M was achieved.

Since the end groups of polyyne chains are affected by the nature of the
solvent where the graphite arc is struck, and since we know that usually the
polyynes are hydrogen terminated when the arc is made in hydrocarbon
solvents like n-hexane or n-octane while they are also cyano terminated
when the arc is made in acetonitrile, these spectral differences can be taken
as a first hint that the polyynes formed in liquid nitrogen are nitrogen and
hence cyano terminated.
From Section 15.3.1, we know the electronic absorption spectra of a
series of monocyanopolyynes and from literature [4] we know the spectrum
of the dicyanopolyyne C 8 N 2 whose maximum wavelength absorption band
lies just at 261 nm. In principle, it is quite obvious to expect to produce
nitrogen-terminated polyynes by arcing graphite in liquid nitrogen since the
high temperatures of the electric arc may cause the plasmalysis of mole-
cular nitrogen into atomic nitrogen which, in turn, can easily form cyano