The observational efforts over the last four decades, combined with the complementary stud- ies of theorists and experimentalists has led to a wealth of knowledge about the chemistry of the interstellar medium. As mentioned before, “complex” chemistry is generally confined to the interiors of dense molecular clouds. The shielded, dense, and cold conditions of these clouds allow molecules to form more easily and survive longer than is possible in more diffuse regions of the ISM. The chemistry that operates under such conditions is a unique combina-
tion of reactions in the gas phase, on the surfaces of, and in the icy mantles on dust grains. The coupling between these two domains varies depending on the local conditions in the cloud, with virtually no coupling in the coldest, most quiescent regions and strong coupling at sites of star formation where the heat and shocks from the young star drive chemical species from the grain surface/mantle into the gas phase. Gas phase chemistry, particularly in the quiescent domains of dense clouds, is dominated by exothermic ion-neutral reactions (and supplemented by a few very low barrier neutral-neutral reactions), often initiated by the key species H+3 [20]. Gas phase reactions alone, however, are limited in their potential in creating molecules larger than a few atoms in size within dense clouds, indicating that grain surface pathways or a combination of these two routes are needed to explain the large variety of complex molecules observed in regions of star formation.
Interstellar dust grains consist of a core of silicates and other oxides or carbides, sur- rounded by a mantle of ice consisting of ‘volatile’ species such as H2O and CO. In quiescent
regions of dense clouds many types of chemical reactions are inhibited on dust grains as most species are immobilized by the low temperatures (∼10 K) of the grains. In fact, at these temperatures, only atomic hydrogen will have any significant mobility because its low mass allows it to tunnel across the surface of the dust grains (though in some rare cases the migration of first row atoms such as C, N or O can play important roles in the synthesis of larger species). It was therefore thought that dust grain chemistry in quiescent regions of clouds was limited to reactions initiated by hydrogen tunneling through unsaturated bonds of molecules such as CO, followed by subsequent additions of other single atoms without a reaction barrier [21, 22]. The species formed through these processes were then thought to participate in further ion-neutral gas phase reactions as the ice evaporates near sites of star- formation. More elaborated models of complex reactions between larger radical species were
developed [23], but the hypothesized methods of initiating these pathways were considered less likely under low temperature conditions.
In recent years, however, it has been realized that energetic cosmic-rays likely also play an important role in driving complex chemistry in icy grain mantles. These particles are too energetic to initiate chemistry directly, but upon collision with dust grains/mantles can release a cascade of supra-thermal electrons that are capable of driving new chemical pathways. Both experiments [24, 25] and models [26, 27] have shown that reactions initiated by the processing of interstellar ice by electrons after collisions of cosmic-rays likely play an important role in dust grain chemistry. In these pathways, larger radical species are created through the cleavage of molecular bonds of species trapped in the ice by the electrons, which allows the pathways considered in [23] to be revisited [26]. Cosmic rays can further contribute to the chemistry in dense regions through collisions with H2 which provide the
primary source of ionization and energy input into molecular clouds, and which can generate a secondary UV field that can both process molecules in the ice of dust grains and photo- ionize molecules in the gas phase.
Beyond the addition of cosmic-ray driven reactions to the arsenal of chemical pathways, it has also been realized that as grains enter regions of star-formation, rather than under- going an instantaneous jump in temperature that promptly releases the icy mantle into the gas phase, grains are more likely to undergo gradual heating as the star matures [28]. Such gradual heating of the grains allows increased mobility of the larger radical species formed through tunneling reactions and cosmic-ray processing, further driving dust grain chemistry. It also results in the vaporization of different species at different times during the formation of a star and any attendant planetary system. For a more detailed and excellent overview of the formation of complex interstellar organic molecules, see [9].
In addition to ongoing experimental and modeling efforts to understand these different processes in the creation of complex molecules, the continued observation of known and the search for new complex molecules is of vital importance in the understanding of complex interstellar chemistry. The systematic comparison of structurally related molecules places further constraints onto chemical models by giving insights into the relative contributions of different pathways. This is the topic of Chapter 6, which explores observational searches for related cyanide molecules.