Theoretical study of interstellar hydroxylamine chemistry: protonation andproton transfer mediated by H-3(+)

Protonated species are known to play a key role for ion-molecule reactions in gas phase interstellar chemistry. As hydroxylamine (H2NOH) has never bee n observed as an interstellar molecule, a detailed theoretical investigatio n of the protonation of H2NOH is carried out at high level of quantum chemi cal theories (CCSD(T) and DFT-B3LYP). As protonation may occur directly by reaction with H+ or mediated by H-3(+), both processes are investigated on the nitrogen and the oxygen sites of hydroxylamine. The present results sho w that the N-protonated form is more stable than the O-protonated one and t hat the protonation initiated by H-3(+) is by far less exothermic than the other one. A particular attention is paid to the intramolecular rearrangeme nt leading from H3NOH+ to H2NOH2+ which involves a highly energetic transit ion state exhibiting proton bridged between N and O sites. As this barrier is too high to be easily overcome in the interstellar medium, an alternativ e process mediated by H-2 and involving a bridged H-3(+) as a transition st ate is considered. The calculations show that the corresponding activation energy is significantly lowered. (C) 1999 Elsevier Science B.V. All rights reserved.