| This thesis characterizes and quantifies a key part of the chemical evolution associated with star formation towards nearby molecular clouds by analyzing the radiation from abundant molecules and their deuterium-substituted counterparts, or deuterated molecules. As clouds evolve to form stars, molecular spectra probe the dynamics. Deuterium fractionation ratios sample the variations in temperature, density and activity of protostellar systems and offer clues into their dynamics.; We present three projects to examine the scope and scale of deuterium fractionation of ammonia, NH3, and formylium, HCO+, in low mass star forming regions. Analysis of single aperture NH2D and NH3 spectra from prestellar and protostellar cores indicates the predominance of gas-phase reactions in the production of these species. Our survey suggests that these species deplete onto grain surfaces at late times in the evolution of molecular cores into protostars. Since the collapse of protostars is rapid, deuterium fractionation of ammonia is not likely to be affected substantially by grain chemistry. This should be the case for even more massive molecular clouds such as Orion Molecular Cloud I or Sgr B2. Thus, observed NH2D/NH3 values probe the cold gas-phase evolution of molecular clouds.; The relationship between gas dynamics and star formation are explored in our survey of DCO+ and H13CO+. We extend previous analyses of the DCO+/HCO+ as a measure of the ionization fraction and magnetic field-neutral coupling of molecular clouds by examining high energy transitions. This method traces warmer, denser gas associated with near-protostellar regions and clustered star formation. Although we find that most DCO+/HCO + values are consistent with previous studies, we also discover regions where DCO+/HCO+ is larger than predicted by the paradigm of ambipolar diffusion-regulated star formation.; Single aperture surveys examine the ambient gas on ∼105 AU scales. However, only aperture synthesis studies directly probe material associated with forming protostars, on size scales of ≤103 AU. Interferometer studies of NH2D and NH3 towards NGC1333 IRAS4 and the Serpens cloud indicate that deuterated molecules are not depleted as are more common isotopes such as CO and HCO+. Thus, NH 2D probes the physical conditions of envelope gas surrounding a protostar. |
