Mechanisms of Hydroxyl Radical Chemistry in Aqueous Solution Triggered by Photoexcitation and Probed by Soft X-rays

Hydroxyl radicals are among the most important radicals on earth, being present in the human body, the atmosphere, rivers, and oceans, contributing to mechanisms like oxidative stress in cells and the photochemistry of the troposphere, and posing a threat to aquatic life. Extensive use of fertilizers in agriculture has led to increased levels of nitrogen oxides in many rivers around the world, which are a major source of hydroxyl radicals in water. In this paper, we explore the photoinduced generation of hydroxyl radicals from nitrite and their scavenging by the radical scavenger 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) in aqueous solutions using transient soft X-ray absorption spectroscopy (XAS) at the oxygen and nitrogen K-edges. We show the photoinduced generation of hydroxyl radicals from nitrite and determine its mechanism. For the scavenging of hydroxyl radicals by TEMPO, we show that the mechanism does not proceed through a bound intermediate state between the two molecules, as has been proposed in the literature, but instead through an electron transfer.