Polycyclic aromatic hydrocarbons (PAHs) represent one of the most abundant forms of carbon in the universe. They are seen in a wide variety of environments in space, and many varieties of these molecules are seen in meteorites as well as asteroidal and cometary dust.
Figure 1: Some examples of PAH structures.
Variants on these structures can also exist. For example, it is possible for some six-membered rings to be replaced with five- or seven-membered rings. This results in curvature of the plane of the molecule. It is also possible to replace some of the carbon atoms in the rings with other elements, for example, nitrogen. Finally, it is also possible to 'decorate' PAHs by replacing one or more of their peripheral hydrogen atoms with some other chemical group, for example, a methyl (–CH3) group or an alcohol (–OH) group.
PAHs may form in a number of different environments in space, but it is thought that most of them form in the outflows of carbon rich stars. Once formed, however, PAHs are available to participate in further chemistry that can modify them in a number of interesting ways.
Since most PAHs are relatively non-volatile compounds, they are expected to freeze out into the ices that coat cold dust grains in dense interstellar clouds. Indeed, there is some infrared spectral evidence that suggests this is case. The ices in dense interstellar clouds are dominated by the molecule H2O, but also contain many other species like methanol (CH3OH), carbon monoxide (CO), carbon dioxide (CO2), ammonia (NH3), methane (CH4), formaldehyde (H2CO), and so on.
Other work we have done shows exposure of these ices to high-energy radiation in the form of UV photons or charged particles can result in the production of a host of new molecular compounds, some of which are of astrobiological interest. What happens when PAHs are present in these ices?
Over the years we have studied the photochemistry that occurs when PAHs are placed in ices of different compositions and exposed to hard radiation. We have discovered that PAHs exposed to radiation in H2O-rich ices are easily ionized and that this can lead to a host of chemical substitutions involving their peripheral hydrogen atoms. The nature of the substitution depends on the composition of the surrounding ice. The table below shows a number of substitutions in which the site of a peripheral hydrogen atom has been altered by the addition of an extra hydrogen atoms (Hn-PAH) or replaced by another chemical groups like =O, -OH, -NH2, -OCH3, –COOH, –CH3, and –CN.
Figure 2: PAH sidegroups.
In the sections below we discuss several of these types of substitutions and their implications for astrochemistry and astrobiology.
Interstellar ices are generally dominated by the molecule H2O. When PAHs are photolyzed in H2O ices, the dominant reactions that occur are H and O-atom addition reactions. Oxygen addition reactions can lead to the formation of aromatic ethers, alcohols, and ketones (see figure below). These kinds of compounds are seen in carbon-rich meteorites and some of them are of astrobiological interest.
Figure 3: PAH + UV + H2O.
Of particular interest are oxidized PAHs that have C=O groups on their edges, i.e., aromatic ketones. The aromatic ketones contain a class of compounds known as quinones, which play a variety of key biochemical roles in living organisms on Earth.
As an example, consider the two-ring PAH naphthalene. When this molecule is frozen into an ice containing H2O and exposed to UV radiation, oxygen atoms can be added to the edges of the naphthalene, forming a number of new compounds. Some of the most abundant products are the aromatic alcohols 1-naphthol and 2-naphthol, and the quinone 1,4-naphthoquinone (see below).
Figure 4: Formation scheme of Naphthoquinone.
Hn-PAHs are PAHs that carry an excess H atom on one or more of their carbon atoms. Molecules of this type may be produced in the same carbon star outflows that are thought to produce normal PAHs. As noted above, these molecules can also be formed when regular PAHs are irradiated in ices that contain other molecular species that bear hydrogen atoms.
The carbon bonding in normal PAHs with six-membered carbon rings is all sp2 and the resulting molecules are all planar. However, the addition of an extra hydrogen atom to a carbon atom in the molecule converts that atom to sp3 tetrahedral bonding. This results in distortions of the molecule from its original planar shape. The resulting molecules produce infrared spectra that show elements of their mixed aromatic and aliphatic bonding.
These types of molecules may explain some of the more enigmatic spectral features seen in some infrared emission objects. For example, infrared emission in the C–H stretching region from the Orion Bar (an HII region) is shown below compared with our lab spectrum of hexahydropyrene (H6-pyrene) isolated in an argon matrix.
Figure 5: Comparison with the spectrum of Orion.
The 3040 cm-1 feature in the spectrum above is thought to be due to the normal aromatic C–H stretch of PAHs, but the precise identification of the weak features longward of the 3040 cm-1 emission feature have remained somewhat enigmatic. In most objects, these features, falling near 2940, 2890, 2850, and 2810 cm-1 (3.40, 3.46, 3.51, and 3.56 μm), are weak compared to the 3040 cm-1 (3.29 μm) band and show a tendency to decrease in strength with decreasing frequency. However, in the spectra of a small number of early type objects spanning the evolutionary bridge between carbon-rich giants and planetary nebulae, the 2940 and 2850 cm-1 bands (3.40 and 3.51 μm) are actually stronger than the 3040 cm-1 feature. As the spectrum of hexahydropyrene shows, PAHs containing excess H atoms (Hn-PAHs) may be able to explain this.
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Bernstein, M. P., Dworkin, J., Sandford, S. A., & Allamandola, L. J., (2001), "Ultraviolet Irradiation of Naphthalene in H2O Ice: Implications for Meteorites and Biogenesis", Meteoritics and Planetary Science, 36, 351-358
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Bernstein, M. P., Moore, M. H., Elsila, J. E., Sandford, S. A., Allamandola, L. J., & Zare, R. N., (2003), "Side group Addition to the PAH Coronene by Proton Irradiation in Cosmic Ice Analogs", Astrophys. J., 582, L25–L29
Bernstein, M. P., Sandford, S. A., Mattioda, A. L., & Allamandola, L. J., (2007), "Near- and Mid-Infrared Laboratory Spectra of PAH Cations in Solid H2O", Astrophys. J., 664, 1264-1272
Ashbourn, S. F. M., Elsila, J., E. Dworkin, J. P., Bernstein, M. P., Sandford, S. A., & Allamandola, L. J., (2007), "Ultraviolet Photolysis of Anthracene in H2O Interstellar Ice Analogs: Potential Connection to Meteoritic Organics", Meteoritics and Planetary Science, 42, 2035-2041