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

Ammonia oxide ((+)NH(3)O(-)), a zwitterionic tautomer of hydroxylamine (NH(2)OH), has been proposed to explain the high reactivity of NH2OH with phosphate esters. The key parameter for kinetically significant reactions through (+)NH(3)O(-) is the value of the tautomeric constant K(T) = [(+)NH(3)O(-)]/[NH(2)OH], 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 NH(2)OH, the latter in close agreement to the experimental ones, but those involving anions. Calculated K(T) is 2.6 x 10(-2), which indicates that although molecular NH(2)OH is favoured, there should be enough (+)NH(3)O(-) in aqueous solution to be a kinetically active species. (C) 2010 Elsevier B.V. All rights reserved.