title: A detailed investigation of proposed gas-phase syntheses of ammonia in dense interstellar clouds.
abstract: The initial reactions in two proposed gas-phase synthesis of interstellar ammonia have been studied in detail.  The rate of the slightly endothermic reaction N+(H2) --> NH+(H) under interstellar conditions has been reinvestigated under thermal and nonthermal (translational excitation) conditions based on low-temperature laboratory data.  In the thermal case, the importance or lack of importance of this reaction in synthesizing ammonia is determined by how the existing laboratory data at low temperature are interpreted.  In particular, it is unclear to what degree the greater rotational energy of laboratory H2 as compared with that of interstellar H2 affects the rate of reaction.  In the nonthermal case, whether or not this reaction is important in synthesizing ammonia depends, in addition, upon which of two experimental investigations involving this reaction at low temperature is correct.  The exothermic reaction N(H3+) --> NH2+(H) has been studied via ab initio quantum chemical methods and found to possess a large activation energy barrier.  Consequently, this latter reaction does not take place appreciably in interstellar clouds and cannot be used to synthesize ammonia.
authors.affiliation: Department of Physics, Duke University, USA.
authors.forename: E
authors.initials: E
authors.lastname: Herbst
authors.forename: D J
authors.initials: DJ
authors.lastname: DeFrees
authors.forename: A D
authors.initials: AD
authors.lastname: McLean
mesh_terms: D000641:Ammonia / Q000138:chemical synthesis*; D005118:Extraterrestrial Environment; D006859:Hydrogen / Q000737:chemistry*; D008956:Models, Chemical; D009584:Nitrogen / Q000737:chemistry; D011522:Protons
chemical_list: D011522:Protons; D000641:Ammonia; D006859:Hydrogen; D009584:Nitrogen