Chemistry Reference
In-Depth Information
OH
O
Al/HgCl
2
O
Figure 2.1. Synthesis of pentacene by reduction of pentacenequinone.
When 2D or 3D PAHs are considered then the choice becomes much larger. An
exhaustive review on the advances in the synthesis of polycyclic aromatic com-
pounds can be found in Harvey, 2004. The most used strategies are flash vacuum
pyrolysis, cross-coupling, oxidative photocyclization, Diels-Adler cycloaddition,
etc.
Linear and non-linear PAHs and their derivatives are produced not only in the
Earth and in the terrestrial environment, e.g., as air pollutants resulting from incom-
plete combustion, and artificially in the laboratory, but also in space. In fact theymay
be the most abundant organic molecules, believed to compose up to 20% of the total
cosmic carbon. Aromatic molecules have been identified in extraterrestrial matter
such as meteorites, interplanetary dust particles and comets by measurements per-
formed with the Infrared Space Observatory (ISO) satellite of the European Space
Agency (ESA) during its 1995-8 operation period (Van Kerckhoven
et al.
, 2000).
Although PAHs are not believed to be the key molecules forming the basis of life,
they may have been important intermediates in the chemical pathways that led from
space to the origin of life on Earth (Bernstein
et al.
, 1999). Irradiation of PAHs in
water ice with UV light results in the formation of a diverse mixture of organic
molecules, including ethers, quinones and alcohols. These may then further react
to form amino acids and other biochemical molecules.
The experimental conditions found in space can be partially reproduced in a
ground laboratory, i.e, at cryogenic temperatures in UHV conditions. Irradiation
of CH
4
,C
2
H
4
and C
2
H
2
ices, produced by condensation of these gases onto sub-
strates held at very low temperatures (
∼
10 K), with 7.3 MeV protons as well as
9.0 MeV He
+
2
nuclei (
-particles), aiming to simulate the interaction of galac-
tic cosmic-ray particles with extraterrestrial organic ices, generates several aro-
matic species (Kaiser & Roessler, 1997). Under such conditions, from CH
4
ice
typical FTIR absorptions of aromatic vibration modes are observed, including the
C-H stretching, in-plane as well as out-of-plane ring vibrations and deformations.
Irradiation of C
2
H
2
ice with 9 MeV
α
α
-particles produces, among other molecules,
chrysene, perylene, pentacene, coronene, etc. Thus physicists can also do chemical
