Introduction

Astrophysical environments offer chemical modelers the unusual challenge of predicting reaction rate coefficients for temperature ranges well outside those commonly encountered in the laboratory (250–2000 K). The direct extrapolation of thermal laboratory data involves implicit, often incorrect, assumptions about the dynamics of chemical reactions at extreme temperatures. Furthermore, the internal energy distributions of constituent atoms and molecules in astrophysics are frequently nonthermal, and direct application of thermal reaction rate data may not be appropriate. An understanding of the physics of chemical reactions is essential for accurate modeling (Dalgarno 1985). Elucidation of the role of various forms of energy in chemical reactions – translational, vibrational, rotational, and fine structure – now affords improved predictions of rate coefficients for astrophysical situations.

In the low-temperature environment of cold interstellar clouds, the chemical kinetics are dominated by reactions whose potential energy surfaces are without barriers. Such reactions are characterized by large exothermicities and long-range electrostatic interactions that suppress chemical barriers that may occur as collision systems approach short distances. Current models of cold interstellar cloud chemistry illustrate the importance of exothermic ion–molecule reactions (cf. Dalgarno and Black (1976), Black and Dalgarno (1977), van Dishoeck and Black (1986)). Many neutral systems are also likely to react rapidly at very low temperatures and may be important in the chemical kinetics of interstellar clouds (Graff 1989). The following section discusses dynamical characteristics of neutral reactions that are likely to be fast at low temperatures.