Molecules in the interstellar and circumstellar medium

The first molecules detected in space were CH, CH+ and CN in the 1950s and since then the observation of over 100 molecules and molecular ions has been reported (Table 5.1), classified by number of atoms. Of all of the molecules discovered in the ISM only 12 elements are represented (H, C, N, O, S, Si, P, F, Cl, Al, Na, Mg), with H, C, N and O (sometimes referred to as CHON ) being the most abundant. The usual rules of laboratory stability seem to be irrelevant carbon for example does not need to have the requisite four chemical bonds because it is not 

A1C1 C20 c3n C4Si c2h4 CH3C2H ch3cooh ch3)2o NH2CH2COOH  

The chemistry of all of these molecules is fascinating but, concentrating on the origins of life, a detailed look at the organic species is appropriate to see what molecules are present and how they might have been formed. The only alkane detected directly in the ISM is methane but this is due to the problem of detection. All alkanes are non-polar and so do not have a pure rotation spectrum. However, there is one allowed vibration of methane that is infrared active and with the low moment of inertia of methane the vibration-rotation spectrum can be observed and a rotational progression identifies the molecule with confidence. 

There are some variations in the composition of chemical clouds as they evolve in time from dark clouds such as TMC-1 to giant molecular clouds such as Orion where the presence of light from young stars initiates photochemistry. The Orion molecular cloud chemical inventory contains several saturated species such as ethanol (CH3CH2OH) and its CN analogue (CH3CH2CN), the simplest carboxylic acid (CH3COOH, acetic acid) and methylamine (CH3 NH2). 

It is highly unlikely that all organic molecules have gas-phase formation routes and many may be formed on the surface of dust grains, probably with ice mantles, 

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