Radioastronomical observations of our galaxy have revealed hordes of molecules in the interstellar medium. Extremely fast reactions result in the high abundance of complex organic compounds in the space between the stars. Amazingly, the key to all this is the chemistry of the helium ion!Radioastronomical observation of the galaxy has revealed a broad distribution of molecular species within the cool, low density regions between stars. Since it is only possible to observe polar molecular forms through their rotational motions, our direct knowledge of abundances of the molecular components is somewhat limited. To gain a deeper insight into the likely molecular composition of the interstellar medium, models of chemical synthesis appropriate for the cold, low density conditions are required.
Consideration of observed species shows clearly that equilibrium thermodynamic constraints are inappropriate, since in some instances high energy isomeric forms of species are quite abundant. Furthermore quite specific forms of relatively large polyatomic species are observed. In particular, the larger organic species are very unsaturated rather than saturated, as might be expected from the fact that hydrogen is by far the most abundant interstallar molecular species. The modelling of the kinetics of specific condensation from an atomic initial condition is representative of a problem of general occurance. The chemistry of the interstellar medium illustrates that complex synthesis occurs under totally abiotic conditions. The specific reactions that occur in the dark polyatomic interstellar regions are discussed in terms of cosmic ray induced primary ionisation followed by specific secondary ion molecule reactions. We show that the high abundance of complex carbon compounds is due to the chemistry of the helium ions.