Organic molecules, interstellar

Ehrenfreund, P. and Charnley, S.B. (2000). Organic molecules in the interstellar medium, comets and meteorites. Annu. Rev. Astron. Astrophy., 38, 427-483... 

Artificial hydrothermal vents might be constructed and supplied with plausible concentrations of simple reactants such as CO, H2, NH3, and H2S. Appropriate levels of amino adds induding a small chiral excess, along with the sorts of amphiphilic molecules described above, can be rationalized by the findings from the Murchison meteorite. Organic molecules such as found in irradiated interstellar ice models, including HMT, can also be induded. The system should indude weathered feldspars, which can be modified to indude the reduced transition-metal minerals that they are known to contain. [134] Such minerals as Fe,Ni sulfides are likely to have been both present and stable in the environment of early Earth and are known [153, 155] to catalyze formation of organic molecules from simpler precursors. 

Two mechanisms have been proposed to account for the deuterium enrichment (1) for organic molecules, high D/H ratios can be explained by ion molecule reactions that occur in interstellar space and (2) for the phyllosilicates the enrichment can be produced via isotope exchange between water and hydrogen (Robert et al. 2000). 

A hundred or so different molecular species have been spotted in molecular or circumstellar clouds. Most interstellar molecules are organic molecules, that is, they are carbon-based. This indicates that chemical evolution does occur on a cosmic scale. What is more, many molecules on the interstellar list are fundamental building-blocks for the construction of biological structures. 

In cosmochemistry, we use stable-isotope fractionations to study evaporation and condensation in the solar nebula, aqueous processes on asteroids, and even ion-molecule reactions to form organic molecules in interstellar clouds. The oxygen isotopes also show large mass-independent shifts that may be related either to chemical or physical processes or to incomplete mixing of the products of nucleosynthesis. These topics will be covered in detail in later chapters. 

What observations or properties of organic molecules suggest that they formed in the interstellar medium or in the solar nebula ... 

N H2CH2C02H. Notably, about half of these interstellar molecules are carbon-based organic molecules. As discussed in Chapter 4, the atoms originated from the nuclear fusion of ancient stars. How interesting it is that these atoms then join together to form molecules even in the deep vac-cum of outer space. Chemistry is truly everywhere. 

In the past 10 years a large number of organic molecules have been found in interstellar dust clouds mostly by emission lines in the microwave region of the spectrum (for a summary see Ref. 38). The concentration of these molecules is very low (a few molecules per cm3 at the most) but the total amount in a dust cloud is large. The molecules found include formaldehyde, hydrogen cyanide, acetaldehyde, and cyanoacetylene. These are important prebiotic molecules, and this immediately raises the question of whether the interstellar molecules played a role in the origin of life on the earth. In order for this to have taken place it would have been necessary for the molecules to have been greatly concentrated in... 

Millar T (2004) Organic molecules in the interstellar medium. In Ehrenfreund P (ed) Astrobiology future perspectives. Kluwer, Dordrecht, pp 17-31 Ninomiya I, Naito T (1989) Photochemical synthesis. Academic, London, p 221 Nossal J, Saini RK, Alemany LB, Meier M, Billups WE (2001) Eur J Org Chem 2001 4167-4180... 

We have so far discussed gas phase syntheses for hydrocarbons and cyanoacetylenes. The hydrocarbons were discussed first because of the relative conceptual simplicity of their synthetic pathways and the cyanoacetylenes were discussed next because of the intense interest in these species and because their syntheses are intimately involved with those of the hydrocarbons. However, there are many different types of organic molecules in the gaseous interstellar medium and we shall attempt to discuss syntheses of a representative sample of these species. 

Despite the present uncertainty in the values of ion-molecule rate coefficients at interstellar temperature, it is appropriate at this point to ask whether or not gas phase pathways produce sufficiently large abundances of the well known organic molecules to agree with observational results. To answer this question, we must discuss the results of the detailed models which contain complex molecules and which have been published. 

It does not seem likely, based on current interstellar cloud models of in situ synthesis, that molecules of such complexity could be synthesized in the interstellar environment, unless a photochemical mechanism on grains could be invoked. However, even such a mechanism is not normally so specific as to form these species as opposed to many other types of organic molecules. 

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