Kinetics and Mechanism of the Reaction of ^•NH₂ with O₂ in Aqueous Solutions

Abstract

The reaction of NH3 with •OH or SO4 •- radicals produces the aminyl radical, •NH2. Pulse radiolysis and laser flash photolysis techniques were utilized to study the formation of this radical, its absorption spectrum, its reaction with O2, and the mechanism of formation of subsequent intermediates and the main final product, peroxynitrite. The rates of formation of •NH2 and its absorption spectrum are in agreement with previous reports. The reaction of •NH2 with O2, however, was observed to take place much more rapidly than reported before and to involve an equilibrium of these reactants with the aminylperoxyl radical, NH2O2 •. The equilibrium is shifted toward completion of the reaction via catalyzed decomposition of this peroxyl radical, and this decomposition affects the observed rate of reaction of •NH2 with O2. The peroxyl radical deprotonates and isomerizes and finally forms NO. In the presence of O2 •-, NO is converted rapidly to peroxynitrite, ONO2 -. This product, which is stable at alkaline pH, was confirmed by γ-radiolysis of aerated ammonia solutions.

Radiolytic Reactions of Monochloramine in Aqueous Solutions

“…The NH 2 O 2 • radical has been shown before to eliminate water and produce NO. 27 The fate of the NHClO 2 • radical is unclear because it is present in a mixture with • NHCl and the decay involves various reactions. …”

“…The spectrum of the • NH 2 radical was found to have only one peak at λ max ) 530 nm ( 530 ) 80 L mol -1 cm -1 ) ( Figure 4, solid triangles), in agreement with previous determinations. 27,28 Then we determined the spectrum of • NHCl from the pulse radiolysis of N 2 O-saturated solutions containing [NH 2 -Cl] ) 3.5 × 10 -3 mol L -1 at pH 9. Two peaks were observed, at λ max ) 330 and 580 nm (Figure 4, open circles).…”

“…To model the observed kinetics in this complex system we took into account the reactions of • OH radicals with carbonate (reaction 13, k 13 ) 3.9 × 10 8 L mol -1 s -1 ) 20 and with chloramines (reaction 11, k 11 ) 5.1 × 10 8 L mol -1 s -1 ), the reactions of e aq -with N 2 O (reaction 9, k 9 ) 9.1 × 10 9 L mol -1 s -1 ) 20 and with chloramine (reaction 8, k 8 ) 2.2 × 10 10 L mol -1 s -1 ), and the reaction of H • atoms with NH 2 Cl (reaction 12, k 12 ) 1.2 × 10 9 L mol -1 s -1 ), neglecting the competing reaction of H • with OH -(k ) 2.5 × 10 7 L mol -1 s -1 ). We also took into account the self-reaction of • NH 2 (2k ) 4.4 × 10 9 L mol -1 s -1 ), 27 • NHCl (2k ) 2.4 × 10 9 L mol -1 s -1 ), and CO 3 •-(2k determined to be 1.1 × 10 7 L mol -1 s -1 , in agreement with previous values 36 ), and an estimated value for the radicalradical reaction CO 3 •-+ • NH 2 (k ) 1.5 × 10 9 L mol -1 s -1 ). The observed kinetics were best fit by assuming an equilibrium reaction 15, with k 15 ) 2.5 × 10 5 L mol -1 s -1 and k -15 ) 4 × 10 2 s -1 (K 15 ) 625 L mol -1 and ∆G 15 ) -16.5 kJ mol -1 ), and decomposition of the complex with k 16 ) 7 × 10 2 s -1 .…”

Photoproduction of nitric oxide in seawater

“…6). We used a value of 10 −11 (v/v) for atmospheric NO (Law, 2001). The atmosphere pressure was set to 101.325 kPa.…”

7 Radiation chemistry

“…210 Pulse radiolysis was utilized to study the formation of NH 2 radical, its absorption spectrum, its reaction with O 2 , and the mechanism of formation of subsequent intermediates and the main ¢nal product, peroxynitrite. 211 , and e aq À with the oxychlorides HClO, ClO À , ClO Á , ClO 2 À , and ClO 3 À have been measured. 213 The phosphonate ion HPO 3 2À of waste solutions from electroless plating processes is radiolytically regenerated only slightly into the phosphinate ion H 2 PO 2 À by reduction in the presence of alcohols.…”