On the aqueous reactions of the aminyl radical with molecular oxygen and the superoxide anion

Abstract: Two key routes of the oxidation of ammonia is the reaction between the aminyl radical with molecular oxygen and superoxide. Fundamental insights in its oxidation is of great importance to both our understanding of atmospheric chemistry, biological effects as well as the safety assessments in nuclear industry. The gas phase reactions differ largely in rate compared to the aqueous reactions. In this work, the mechanism for the aqueous reactions have been studied computationally using ab initio quantum chemical calcultions. Reaction barriers have been quantified and compared with experimental data in the litterature. Also, the absorbtion spectrum of one of the postulated intermediates is verified using TDDFT. Furthermore, ã-radiolysis experiments have been conducted to test a model for aqueous radical oxidation of ammonia by investigating the yield of one of the final products, peroxynitrite. The model consists of a large part of reported reactions relevant to the aqueous radiolysis of nitrogen containing solutions (with an emphasis on ammonia). The dependence of the yield on the chosen experimental conditions is compared to that calculated using the model. The results of the quantum chemical computations for the reaction with oxygen is in agreement with earlier experimentally reported absorption spectrum and rate of decomposition of a key intermediate, the aminyl peroxide radical. Both properties, however, only agree when a large enough number of explicit water molecules are included in the computations. The dependence of the results on the chosen number of water molecules as well as the level of theory is discussed. The evaluation of the model for radiolysis of aqueous ammonia shows that the current understanding of oxygenated systems where superoxide is in excess with respect to hydroxyl radical is good. For systems deficient in superoxide, the model fails to accurately predict the yield of the final product under investigation, peroxynitrite, and it raises the question whether some unknown process has been overlooked in earlier studies.