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

TL;DR

This paper investigates how hydroxyl radicals form and are neutralized in water using soft X-ray techniques, revealing new insights into their chemical mechanisms.

Contribution

The study provides a novel understanding of hydroxyl radical scavenging by TEMPO through direct electron transfer, not via a bound intermediate.

Findings

Hydroxyl radicals are generated from nitrite through photoexcitation in aqueous solutions.

TEMPO scavenges hydroxyl radicals via direct electron transfer, not through a bound intermediate state.

Soft X-ray absorption spectroscopy reveals detailed mechanisms at oxygen and nitrogen K-edges.

Abstract

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.

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