Hydrogen bonding and proton transfer in small hydroxylammonium nitrate clusters: A theoretical study

Structures and energies of gas-phase hydroxylammonium nitrate (HAN), HONH3NO3, are determined using density functional theory and the 6-311++G(d,p) basis set. Three stable configurations are found for HAN which involve strong hydrogen bonding between hydroxylamine and nitric acid molecules. In the most stable configuration, both the oxygen and the nitrogen of hydroxylamine are hydrogen bonded to sites on the nitric acid molecule. In the less stable HAN structures only the oxygen or the nitrogen of hydroxylamine are hydrogen bonded. Two stable structures for the (HAN)(2) complex are investigated. The more stable structure is ionic, with the nitric acid proton having transferred to the nitrogen of hydroxylamine. Strong electrostatic and hydrogen-bonding interactions stabilize this structure. The other stable form of (HAN)(2) has fewer hydrogen bonds and is composed of interacting neutral nitric acid and hydroxylamine molecules. Binding energies are determined for all structures along with corrections for basis set superposition errors in the HAN molecules. Proton exchange reaction paths are studied for the HAN configurations. The saddle points for the proton exchange process are ionic forms of HAN with interacting HONH3+ and NO3- moieties. These ionic structures are 13.5 and 13.6 kcal/mol higher in energy than the neutral hydrogen-bonded complexs of HONH2 and HNO3 from which they are formed. The electrostatic attractions between the ions are sufficient to stabilize the ionic form of (HAN)(2), whereas in the HAN "monomer" the interaction energy for single HONH3+ and NO3- ions is not sufficient to compensate for the energy required for proton transfer from nitric acid to the hydroxylamine group. A correlation based on the bond-valence theory which describes the bond lengths of the hydrogen bonds is examined for the complexes. All the hydrogen bonds follow the correlation well. (C) 2003 American Institute of Physics.