Organic Chemistry Across the Universe Extraterrestrial Transformation
What are the Largest Organic Molecules found in space?
There were anonymous voices singing from space in the form of spectral bands of Infrared radiation that were observed in 1973 by Gillett, Forrest and Merrill(22). The object radiating this information was unknown but over the years the same pattern of spectral bands were detected signalling from materials as different as dense clouds and planetary nebulae. The evidence was accesses and resulted in some common characteristics being attributed to the unknown substance causing these bands(23). The molecule has to be extremely stable and widespread in the galaxy as it is observed in a countless variety of conditions. The spectral bands observed are correlated or behave as a family of peaks. The bands are not always at the same frequency, but the pattern of the bands appears in the same ratio for all the spectra. It signifies a class of related molecules that have the same types of bonds but because of differing sizes of molecule they vibrate at slightly different frequencies. The fraction of IR radiation radiating from these molecules mirrors the amount of carbon available in each medium from which the light is emitted. This indicates a carbon rich molecule. The banded nature of the spectrum indicated that it is coming from gas phase molecules and not solid matter. The unidentified spectrum is of polyatomic aromatic hydrocarbons, PAH's, one of which is Coronene. They are the largest organic molecules found in space. HC11N is the largest none PAH identified and has thirteen atoms(1). It is much smaller than most PAH's and it is a confirmation of PAH's great stability that they can survive the harsh conditions of space.
The spectrum in fig.3 is the 'unidentified' spectrum.
Polyatomic Aromatic hydrocarbons are found in soot, tar and exhaust fumes on Earth. They are relatives of benzene and their stability comes from the same delocalisation of electrons above and below the molecular plane, which creates strong carbon bonds that are very unreactive. How they are formed is uncertain but one proposal is a gas phase radical reaction.
Fig.4(2)
The Figure illustrates the comparison between the spectrum observed for the interstellar object, the Orion bar shown in purple, and the spectrum obtained from the radiation of ice grains in the laboratory, in red. The similarity is evidence proving the origin of the unidentified Infrared bands as originating from the absorption frequencies of PAH's
The reaction above(22) forms a radical benzene molecule, which can react further to build up systems of fused benzene rings. This is a possible chemical reaction route to PAH's starting from the small molecule C2H2 that could take place in carbon rich circumstellar shells around stars. Below shows the most stable arrangements of the fussed rings where aromaticity and therefor the stability of the molecule is maintained.
Polyatomic aromatic hydrocarbons are huge discs, which can contain up to forty carbon atoms. Where does all the carbon come from? It gets shot out of stars and into the interstellar medium. Carbon rich stars include red giants, Wolf-Rayet stars and supernovae(22). The carbon is contained in dust particles, which swirl, spiralling out from the stars. The dust's chemical composition resembles the soot that some flames produce back on Earth. This relationship has enabled the chemical formation of these dust particles to be modelled on the data of soot formation on the Earth(22).
