Polyynes: Synthesis with the Submerged Electric Arc - Polyynes: Synthesis, Properties, and Applications - page 164

Figure were recorded directly on the crude solution of polyynes in

dodecane and on pure dodecane (blank) using an IR cell for liquids with
CaF 2 windows and a cell thickness of 0.05 cm. It can be concluded that the
infrared stretching of the -C

8.8

C-H groups provide direct and definitive
evidence of the formation of hydrogen-terminated polyynes. Since the
molar extinction coefficient of the band at 3312 cm 1 associated with the "

C-H group is about 100mol 1 cm 1 [35], and since the absorption A of -C

the peak in Figure 8.6 is 0.09 with a cell pathlength b of 0.05 cm, using

the Lambert-Beer law (A bc), we can estimate that the concentration, c,

of the hydrogen terminated polyynes in the solution of Figure 8.6 was
9

¼ "

10 3 mol/l.

8.3.3 PAH SAS B YPRODUCTS F ORMED WITH THE P OLYYNES
D URING A RCING

When the graphite electrodes are arced into a hydrocarbon solvent like
n-hexane, as described in the experimental section, although the main reac-
tion product consists predominantly of a mixture of polyynes, it is possible
to detect other byproducts by HPLC-DAD analysis. These byproducts are
not found when graphite electrodes are arced in liquid nitrogen or are
present in extremely small trace amounts when the arc is made in certain

solvents like methanol, ethanol or water (see Table 8.1 ) .

To elucidate the nature of these byproducts, as described in the experi-
mental section, we precipitated all the hydrogen-terminated polyynes pres-
ent in an n-hexane solution as acetylides by treatment with a Cu(I) reagent.
After this treatment, it was possible to observe a profound alteration of the
electronic absorption spectrum of the hexane (see
Before the precipitation, the electronic absorption spectrum of the graphite
arced hexane solution (Figure 8.9(A)) was dominated by the absorption
bands due to the polyynes, but after their precipitation as Cu(I) acetylides it
was possible to observe the spectrum (Figure 8.9(B)) of the non-precipitable
byproducts. Of course, if the unique result of arcing between graphite elec-
trodes in hexane would have been the polyynes, after their precipitation the
hexane solution should have appeared completely clean and free from
absorption bands. This is not the case, because the HPLC-DAD analysis
of such a solution freed from polyynes reveals a plethora of products but
no trace of the polyynes. This is illustrated in Figure 8.6(A and B). In Figure
8.6(A) the polyynes are clearly distinguishable as a sharp and intense peak in
the chromatogram but they are completely absent from Figure 8.6(B) after
their complete separation as acetylides. The molecular species of Figure
8.6(B) are a mixture of PAHs. This has been established on the basis of their
retention time in the chromatogram and on the basis of their electronic
absorption spectra compared with the spectra of our standards or of Agilent
PAHs library spectra. For instance, we unequivocally identified the presence
of naphthalene, acenaphthalene, benzo[b]fluoranthene, pyrene, and crysene

solution Figure 8.9 ).