Superoxide Radical Anion Adduct of 5,5-Dimethyl-1-pyrroline N-Oxide (DMPO). 2. The Thermodynamics of Decay and EPR Spectral Properties

The formation of the superoxide radical anion (O2•-) adduct of the nitrone 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as detected by electron paramagnetic resonance (EPR) spectroscopy is one of the most common techniques for O2•- detection in chemical and biological systems. However, the nature of DMPO−O2H has confounded spin-trapping investigators over the years, since there has been no independently synthesized DMPO−O2H to date. A density functional theory (DFT) approach was used to predict the isotropic hyperfine coupling constants arising from the N, β-H, and γ-H nuclei of DMPO−O2H using explicit interactions with water molecules as well as via a bulk dielectric effect employing the polarizable continuum model (PCM). Theoretical calculation on the thermodynamics of DMPO−O2H decay shows favorable intramolecular rearrangement to form a nitrosoaldehyde and a hydroxyl radical as products, consistent with experimental observations. Some pathways for the bimolecular decomposition of DMPO−O2H and DMPO−OH have also been computed.