title: The composition of interstellar molecular clouds.
abstract: We consider four-aspects of interstellar chemistry for comparison with comets: molecular abundances in general, relative abundances of isomers (specifically, HCN and HNC), ortho/para ratios for molecules, and isotopic fractionation, particularly for the ratio hydrogen/deuterium. Since the environment in which the solar system formed is not well constrained, we consider both isolated dark clouds where low mass stars may form and the "hot cores" that are the sites of high mass star formation. Attention is concentrated on the gas phase, since the grains are considered elsewhere in this volume.
authors.affiliation: Department of Physics and Astronomy, University of Massachusetts, Amherst 01003-4517, USA. irvine@fcrao1.phast.umass.edu
authors.forename: W M
authors.initials: WM
authors.lastname: Irvine
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D003903:Deuterium; D005118:Extraterrestrial Environment; D005740:Gases / Q000032:analysis* / Q000737:chemistry; D006859:Hydrogen; D007053:Ice / Q000032:analysis; D007554:Isotopes; D019110:Meteoroids; D015394:Molecular Structure; D009930:Organic Chemicals / Q000032:analysis*
chemical_list: D005740:Gases; D007053:Ice; D007554:Isotopes; D009930:Organic Chemicals; D006859:Hydrogen; D003903:Deuterium