A theoretical analysis of the acid–base equilibria of hydroxylamine in aqueous solution

Ammonia oxide (+NH3O−), a zwitterionic tautomer of hydroxylamine (NH2OH), has been proposed to explain the high reactivity of NH2OH with phosphate esters. The key parameter for kinetically significant reactions through +NH3O− is the value of the tautomeric constant KT = [+NH3O−]/[NH2OH], which it has so far proved elusive to measure experimentally. Thermodynamic cycles using CBS-QB3 and CPCM computed data have been used to estimate microscopic and macroscopic acid–base equilibria involving NH2OH, the latter in close agreement to the experimental ones, but those involving anions. Calculated KT is 2.6 × 10−2, which indicates that although molecular NH2OH is favoured, there should be enough +NH3O− in aqueous solution to be a kinetically active species.