Cheri A. McFerrin

TL;DR: In this paper, a chemical spin trapping agent, 5,5-dimethyl-1-pyrroline-Noxide (DMPO), was used in conjunction with electron paramagnetic resonance (EPR) spectroscopy.

Abstract: Additional experimental evidence is presented for in vitro generation of hydroxyl radicals because of redox cycling of environmentally persistent free radicals (EPFRs) produced after adsorption of 2-monochlorophenol at 230 °C (2-MCP-230) on copper oxide supported by silica, 5% Cu(II)O/silica (3.9% Cu). A chemical spin trapping agent, 5,5-dimethyl-1-pyrroline-Noxide (DMPO), in conjunction with electron paramagnetic resonance (EPR) spectroscopy was employed. Experiments in spiked O water have shown that ∼15% of hydroxyl radicals formed as a result of redox cycling. This amount of hydroxyl radicals arises from an exogenous Fenton reaction and may stay either partially trapped on the surface of particulate matter (physisorbed or chemisorbed) or transferred into solution as free OH. Computational work confirms the highly stable nature of the DMPO−OH adduct, as an intermediate produced by interaction of DMPO with physisorbed/chemisorbed OH (at the interface of solid catalyst/solution). All reaction pathways have been supported by ab initio calculations. ■ INTRODUCTION Resonance-stabilized, environmentally persistent free radicals (EPFRs) (semiquinone, phenoxyl, cyclopentadienyl, etc.) can form on the surfaces of fine particles and persist almost indefinitely in the environment. Redox cycling of adsorbed EPFRs may be a source of reactive oxygen species (ROS), such as hydroxyl radicals (•OH), superoxide anion radicals (O2 • −), hydrogen peroxide (H2O2), etc. 1 These results were partially supported by later works. Recently, a chemical spin trapping agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) in conjunction with electron paramagnetic resonance (EPR) spectroscopy was employed to measure the production of ROS in an aqueous suspension of particle-associated EPFRs derived from adsorption of 2-monochlorophenol (2-MCP) on 5% Cu(II)O/silica (3.9% Cu) particles. It was established that hydroxyl radicals are generated by a surface-mediated redox cycle, with the resulting hydroxyl radicals remaining completely or largely on the surface such that they cannot be readily scavenged to form secondary organic radicals in quantities detectable using currently available methods. The surface-bound hydroxyl radical as well the reduced metal in the immediate vicinity are responsible for the enhanced activity of the particles. The concentration of hydroxyl radicals was measured at ∼1 μM for a 140 min incubation of EPFR-containing solution. Failure to form secondary radicals using standard scavengers, such as ethanol, dimethyl sulfoxide, sodium formate, and sodium azide, suggests that caution must be used to interpret free hydroxyl radical generation in solution. There is the dilemma: first, hydroxyl radicals may form on the surface via a non-homogeneous reaction of H2O2 because of “site-specific OH production” known as “site-specific Fenton reaction”. A fraction may react with the target (in our case, with DMPO), and the remainder may be released into solution as free OH without any significant effect on the scavengers (because of the low concentration of hydroxyl radicals). On the other hand, the significance of the concerted reaction between a metal site, H2O2, and a target (here DMPO) without participation of OH in the general process cannot be excluded. In other words, it is always challenging and in most cases unclear to ascertain the origin of OH radicals. The large problem is that the DMPO−OH adduct (as an indicator for free OH) may also be formed by nucleophilic addition of water to DMPO catalyzed by a transition-metal impurity (or through intermediate DMPO radical cation). The non-radical nucleophilic reaction of water has been proposed to be a significant pathway for the formation of DMPO−OH radical adducts, even during a Fenton reaction; i.e., 80−90% of the total DMPO−OH in O-enriched water was due to irondependent nucleophilic addition of water. However, the same authors also discuss a water-independent mechanism of DMPO−OH formation and how an Fe or Cu ion-induced nucleophilic addition of water to DMPO may be significantly suppressed in experiments performed in most common buffers. These arguments are the main reasons for performing the spin-trapping experiments using O-labeled water in the presence of EPFRs associated with CuO/SiO2 nanoparticles. We provide here additional evidence of in vitro generation of hydroxyl radicals by EPFRs produced from the adsorption of 2Received: March 13, 2014 Revised: July 8, 2014 Accepted: July 18, 2014 Published: July 18, 2014 Article

TL;DR: In this article, the reactions of OH with molecular chlorine, methane, and propane were studied experimentally using a pulsed laser photolysis/pulsed-laser-induced fluorescence.

TL;DR: In this paper, the authors used B3LYP/6-31G(d,p) and BHandHLYP/631G (d, p) to calculate the energy of combustion-generated semiquinone and phenoxyl radicals.