FORMATION OF NITRIC-ACID BY STRUCTURAL REARRANGEMENTS OF GAS-PHASE IONIC COMPLEXES OF AMMONIUM-NITRATE

A detailed study of nitric acid formation in gas-phase ionic aggregate s of ammonium nitrate is reported. Fast-atom bombardment was used to s putter cationic ammonium nitrate complexes from neat ammonium nitrate crystals. The sputtered complexes are of the form (NH4NO3)(n)NH4+ and have been detected in the range of n = 1-43. The formation of nitric a cid from gas-phase cationic complexes of ammonium nitrate is shown to occur by a rearrangement of NH4+(NO3-)NH4+, the core ion of the first solvent shell. Metastable and collisional-activation dissociation spec tra show that a completed first solvent shell exists when the core ion is solvated by the hydrogen bonding of six NH4NO3 molecules to the si x available hydrogen atoms of the two ammonium ions, yielding (NH4NO3) (7)NH4+. The metastable loss of HNO3 occurs only from complexes with l ess than six ligands; a fully solvated core ion cannot rearrange. Coll isional activation of larger complexes (n > 7), under multiple-collisi on conditions, induces sequential losses of NH4NO3 until an ion with n < 7 (incomplete first solvent shell) is formed, at which point rearra ngement may occur and one nitric acid molecule may be lost.