2000). This

is followed by the formation of diacetylene, triacetylene, and benzene (Cernicharo et al. 2001). In the following evolutionary stage (called the proto-planetary nebulae phase), the first spectral signatures of ICG-001 nmr aromatic compounds appear. The 3.3, 6.2, 7.7, 8.6, and 11.3 μm aromatic stretching and bending modes first make their appearance during the proto-planetary nebulae phase, and are found to become stronger in the subsequent planetary nebulae (Kwok 2000) phase (Fig. 1). These aromatic features are accompanied by aliphatic features at 3.4 and 6.9 μm selleck chemicals in the spectra of proto-planetary nebulae. The detection of out-of-plane C-H bending modes at 12.1, 12.4, and 13.3 μm suggests that the aromatic rings are not all connected to each other and there are many exposed edges of the rings (Kwok et al. 1999). Also present in the spectra of proto-planetary nebulae are broad plateau emission features at 8 and 12 μm which are due to collections of in-plane and out-of-plane bending modes of aliphatic chains (Kwok et al. 2001). Fig. 1 Infrared Space Observatory spectrum of the planetary nebula NGC 7027 superimposed on the Hubble Space Telescope image of the object. The aromatic infrared

bands (AIB) are marked in green. The identifications of these bands are given in the legend. The lines labeled in purple are atomic lines. The strengths of the AIBs show that aromatic below compounds are being produced in large quantities These observations suggest that even under the extremely selleckchem low density environment of the circumstellar envelopes, complex organics can be synthesized. One possible scenario is that

starting from acetylene, these linear molecules bend to form benzene, and all kinds of aliphatic chains get attached to the rings. The aromatic rings grow in size, possibly as the result of photochemistry. Since we know the evolutionary and dynamical timescales of the AGB (~104 yr), proto-planetary nebulae (~103 yr), and planetary nebulae (~104 yr) stages, these time scales constrains the chemical timescales that the synthesis must take place. Circumstellar molecular synthesis is therefore extremely efficient (Kwok 2004). It is interesting to note that these spectral characteristics resemble the infrared spectra seen in coal (Guillois et al. 1996), kerogen (Papoular 2001), soot (Pino et al. 2008), and petroleum (Cataldo et al. 2004). What all these compounds have in common is that they are all disorganized organic matter with mixed sp2/sp3 structures. While coal, kerogen and petroleum are remnants of life on Earth, the carbonaceous grains produced by stars condensed directly from the gas phase (similar to the formation process of soot) and are probably amorphous nanoparticles with a few aromatic islands connected by aliphatic chains (Kwok and Zhang 2011).