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 (O ₂ ^•-) 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 O ₂ ^•- detection in chemical and biological systems. However, the nature of DMPO-O ₂H has confounded spin-trapping investigators over the years, since there has been no independently synthesized DMPO-O ₂H 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-O ₂H 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-O ₂H 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-O ₂H and DMPO-OH have also been computed.