Flash photolysis of the nitrate ion in aqueous solution: excitation at 200 nm

Abstract

The flash photolysis of aqueous solutions of the nitrate ion at 200 nm has been studied. Spectroscopic observations show that no solvated electrons are produced but that three transient absorption spectra appear with maxima at <260, 300, and 450–500 nm. On the basis of kinetic measurement made at various pH Values of the initial solution coupled with product analyses, these spectra have been attributed, respectively, to undissociated peroxy-nitrous acid (tentative attribution), the peroxynitrite anion, and nitrtogen dioxide. Results obtained support the hypothesis that the following primary photolytic process occurs: NO₃^– [graphic omitted] (NO₃^–)* [graphic omitted] NO₂+ OH + OH^– The recombination of NO₂ and OH in the solvent cage gives peroxynitrous acid; this then dissociates, to give the anion. A pK value of 6·0, has been determined for peroxynitrous acid and, assuming that the rate of disappearance of peroxynitrite is controlled by the reaction, ON·O·OH → NO₃^–+ H⁺ the rate constatn of this reaction at room temperature is 1·5 ± 0·2 s^–1. The origin of molecular oxygen as a product is examined.