The composition of interstellar molecular clouds

We consider four aspects of interstellar chemistry for comparison with come ts: molecular abundances in general, relative abundances of isomers (specif ically, HCN and HNC), ortho/para ratios for molecules, and isotopic fractio nation, particularly for the ratio hydrogen/deuterium. Since the environmen t in which the solar system formed is not well constrained, we consider bot h isolated dark clouds where low mass stars may form and the "hot cores" th at are the sites of high mass star formation. Attention is concentrated on the gas phase, since the grains are considered elsewhere in this volume.