Oxidation of NO˙ by small oxygen species HO2(-) and O2˙(-): the role of negative charge, electronic spin and water solvation.

The reactions of HO2(-)(H2O)n and O2˙(-)(H2O)n clusters (n = 0-4) with NO˙ were studied experimentally using mass spectrometry; the experimental work was supported by quantum chemical computations for the case n = 0, 1. It was found that HO2(-)(H2O)n clusters were efficient in oxidizing NO˙ into NO2(-), although the reaction rate decreases rapidly with hydration above n = 1. Superoxide-water clusters did not oxidize NO˙ into NO2(-) under the present experimental conditions (low pressure): instead a reaction occurred in which peroxynitrite, ONOO(-), was formed as a new cluster core ion. The latter reaction was found to need at least one water molecule present on the reactant cluster in order to enable the product to stabilize itself by evaporation of H2O.