Chemistry Reference
In-Depth Information
FIGURE 15.3
HPLC chromatogram of the polyynes and monocyanopolyynes solu-
tion prepared by arcing graphite at
40
C, after 5 days storage at room temperature.
The monocyanopolyynes disappeared completely while the hydrogen-capped poly-
ynes are still there.
disappeared completely. In their place it is possible to record unusual spec-
tra of unknown molecules. Some of these spectra are shown in
Figure 15.4
.
These spectra are characterized by absorption bands at
350 nm which
account for the yellow color of the solution. The long-term stability of the
hydrogen-terminated polyynes in diluted solutions is well known as we have
already reported this property [20-22]. The rather high instability of mono-
cyanopolyynes in the same solution is quite unexpected. At present we do
not know what kind of rearrangement takes place on these molecules. In any
case, the knowledge that the hydrogen-terminated polyynes are much more
stable than the monocyanopolyynes may have astrochemical implications.
In fact, the latter are easily detected by radioastronomy because they possess
a dipole moment but the ordinary polyynes are not detectable because they
do not have a permanent dipole moment [11,12]. It is reasonable to propose
that the ordinary polyynes may be as abundant, or even more abundant,
than the monocyanopolyynes because of their higher stability and thanks to
the great abundance of hydrogen in the interstellar medium.
15.3.2 F
ORMATION OF
P
OLYYNES
F
ROM
E
LECTRIC
A
RC IN
L
IQUID
N
ITROGEN
B
ETWEEN
G
RAPHITE
E
LECTRODES
When the graphite electrodes are arced in liquid nitrogen the polyynes
formed in the arc are quenched into the very cold reservoir of the liquid
nitrogen surrounding the plasma ball. During arcing, part of the liquid
nitrogen vaporizes to dissipate the heat generated by the electric arc. When
liquid nitrogen vaporizes, a continuous stream of gaseous nitrogen drags the
polyynes outside the reactor (see
experimental
section) into the gas washing
bottle connected to the reactor and filled with a solvent (n-octane). When
nitrogen bubbles into the bottle, it releases the polyynes into the n-octane
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