Theoretical study of interstellar hydroxylamine chemistry:: protonation and proton transfer mediated by H3+

Protonated species are known to play a key role for ion-molecule reactions in gas phase interstellar chemistry. As hydroxylamine (H(2)NOH) has never been observed as an interstellar molecule, a detailed theoretical investigation of the protonation of H(2)NOH is carried out at high level of quantum chemical 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 show that the N-protonated form is more stable than the O-protonated one and that the protonation initiated by H(3)(+) is by far less exothermic than the other one. A particular attention is paid to the intramolecular rearrangement leading from H(3)NOH(+) to H(2)NOH(2)(+) which involves a highly energetic transition state exhibiting proton bridged between N and O sites. As this barrier is too high to be easily overcome in the interstellar medium, an alternative process mediated by H(2) and involving a bridged H(3)(+) as a transition state is considered. The calculations show that the corresponding activation energy is significantly lowered. (C) 1999 Elsevier Science B.V. All rights reserved.