How Molecules Are Formed
Many interstellar molecules are unsaturated. At first glance, this is quite surprising, as unsaturated molecules can generally add H2 in an exothermic (product-favored) process. For example, the reaction
is exothermic by about 80 kJ/mole. Because H2 is the most abundant molecule in molecular clouds, if CH2CO could react with H2, it would. Although this reaction is thermodynamically favored, it is kinetically very unfavorable at the very low temperatures (~20 K) of interstellar clouds. In fact, very few reactions between neutral molecules occur at an appreciable rate at 20 K.
The rate of the bimolecular reaction between two molecules or atoms A and B is
Rate = k [A] [B]
k = rate constant
[A] and [B] = concentrations of the reactants.
Because the concentrations of atoms and molecules in interstellar clouds is so low, in order for a reaction to occur to an appreciable extent, even over a time-scale of many years, k must be fairly large. The Arrhenius expression relates the rate constant to the temperature:
k = A e(-Ea/kBT)
A = frequency factor (fairly large for most reactions of simple molecules)
Ea = activation energy for the reaction
kB = Boltzmann constant
A reaction with an activation energy of 10 kJ/mole will occur very rapidly at room temperature, but will occur a factor of 2 x 1024 times more slowly at 20 K. Thus, only reactions with activation energies that are extremely small, zero, or negative will occur in interstellar clouds. Although few reactions between neutral molecules have sufficiently low activation energies, exothermic reactions between an ion and a molecule typically have Ea £ 0, and thus occur more rapidly at lower temperatures. This is because an ion and a molecule have a long-range attraction due to the strong ion-dipole or ion-induced dipole force. Most interstellar chemistry involves reactions between ions and molecules.
The simplest and most abundant molecule, H2, is the key to interstellar chemistry. H2 is formed by the reaction of two H atoms on a dust grain: two H atoms are adsorbed on a grain, they diffuse along the surface of the grain, eventually collide and form H2, which leaves the grain. Although there are hundreds of reactions involving the molecules that have been observed, we will just describe a set of reactions that produces H2O. This reaction is quite typical in that, although the final product is a neutral molecule, almost all of the chemistry involves ions.
First, a hydrogen molecule is ionized by cosmic radiation or ultraviolet light :
The H2+ rapidly reacts with H2, producing H3+:
The H3+ then transfers H+ to an oxygen atom:
The OH+ reacts with H2 twice, gaining an H atom each time:
Because interstellar clouds have no net electrical charge, positive ions are balanced by an equal number of electrons or negative ions; actually, the electrons far outnumber the negative ions. Positive ions react very rapidly with electrons, as the opposite charges strongly attract:
The molecules that radiotelescopes have observed reveal a great deal about the temperature, gas densities and flows in interstellar molecular clouds. In addition to motivating the study of the chemistry of exotic molecules, these observations have provided us with an understanding of how stars are formed from these dense regions of gas and dust.