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(PAHs). The polyyne solution in hexane obtained after 15min arcing was
filtered, poured into 100ml of an aqueous solution of ammonia (26%),
Cu(I)Cl (2.5 g) and hydroxylamine hydrochloride (1.5 g). The organic solu-
tion was vigorously shaken with the aqueous solution for a long time and
then left to settle. All polyynes were precipitated as copper salts (acetylides)
and could be observed as copper-colored precipitates. The hexane solution
still showed absorption bands in the UV spectrum. It is clear that the poly-
ynes are accompanied by other molecules not precipitable with Ilosvay's
reagent. HPLC analysis of the solution revealed the presence of polycyclic
aromatic hydrocarbons (PAHs). About 15 components were separated by
the C8 column. One third of them show electronic spectra of ene-ynes or
cyclic polyynes. The remaining two thirds of the components eluted were
identified as PAHs on the basis of their UV spectra. Among the PAHs, naph-
thalene, acenaphthalene, benzo[b]fluoranthene, pyrene, and crysene were
unequivocally identified.
8.2.5 T
HE
E
LECTRIC
A
RC
B
ETWEEN
G
RAPHITE
E
LECTRODES AT
V
ERY
H
IGH
C
URRENT
D
ENSITY
The electric arc between graphite electrodes in hexane was repeated as
detailed in Section 8.2.1. Instead of using 10A, by connecting two power
supplies in parallel, it was possible to work with 20A at about 32 V.
Enhanced formation of carbon black was observed. The filtered hexane
solution was analyzed by HPLC with a diode array detector (DAD). The
polyynes C
6
H
2
,C
8
H
2
,C
10
H
2
, and C
12
H
2
were clearly identified. Possibly
C
14
H
2
was also present. C
8
H
2
was no more dominant, as in the usual
conditions of arcing at 10 A. In fact, at 20 A both C
6
H
2
and C
10
H
2
appear to
have a concentration comparable to that of C
8
H
2
. The concentrations of the
PAHs now appear dominant in comparison to the polyyne concentration;
exactly the opposite conditions occurred at low current density where the
polyynes are the dominant species. About 17 PAHs and other components
were eluted by the C8 column. Among them naphthalene, acenaphthalene,
and pyrene have been firmly identified.
8.2.6 E
LECTRIC
A
RC
B
ETWEEN
T
ITANIUM
E
LECTRODES IN
n-H
EXANE
The electric arc between two titanium electrodes submerged in n-hexane
was conducted in the usual manner (as described in Section 8.2.1 but by
replacing the graphite electrodes with titanium electrodes from Aldrich).
The electric arc in this specific case appeared much less intense and less
bright than in the case of a carbon arc made with graphite electrodes under
similar conditions. A gradual darkening of the solvent was noticed, pro-
gressing with the arcing time. The titanium electrodes were slightly con-
sumed and a deposit of carbon black can be observed at the bottom of the
flask; the amount increased as arcing progressed. The analysis of the filtered
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