Chapter 3: Gas Phase Reactions in Interstellar and Circumstellar Media

What are the types of gas phase reaction useful in constructing an effective astrochemical network? We discuss radiative association, exchange reactions, recombination reactions of ions with electrons, photoionization of atoms and molecules by starlight, collisional ionization by fast particle and by X-rays. An interesting anion chemistry is shown to be feasible. Finally, we describe briefly how chemistry in regions heated by the passage of shocks is affected by the temperature rise. We show how isotope chemistry may give rise to a variety of interstellar isotopologues. Finally, we discuss the conditions under which three body reactions may play a role in astrochemistry.