Introduction

Giant interstellar clouds are the most massive chemical ‘factories’ in our Galaxy containing around 80 molecules presently identified (neglecting isotopic variants) and ranging in complexity from H2 and CO to large saturated molecules such as ethanol, CH3CH2OH, and highly unsaturated cyanopolyynes including the 13-atom chain HC11N. Millimetre and sub-millimetre observations of interstellar molecules allow one to probe the densities, temperatures, and dynamics of interstellar clouds and can give information on the initial conditions for star formation. It is also of interest to understand the chemistry of these molecules since chemical kinetic modelling can be used together with observational data to constrain uncertain parameters such as elemental abundances and the cosmic ray ionisation rate. An understanding of deuterium fractionation in interstellar molecules can be used to determine the D/H ratio and thus has a bearing on cosmological models of the origin of the universe.

In recent years models of increasing chemical, physical, and computational complexity have been developed to study molecular formation and destruction in various astronomical regions. Models of interstellar cloud chemistry can be divided roughly into two classes: (1) steady-state models, in which chemical abundances are calculated through solving a coupled system of non-linear algebraic equations; and (2) time-dependent models, in which the variations of abundances as a function of time are followed through solving a coupled system of stiff, non-linear, first-order, ordinary differential equations. Traditionally, steady-state models have been the dominant tool for studying chemistry in diffuse clouds which, in the absence of shocks, reach steady-state well within their lifetimes.