Combined ESR and Thermodynamic Studies of the Superoxide Adduct of 5-(Diethoxyphosphoryl)-5-Methyl-1-Pyrroline N-Oxide (DEPMPO): Hindered Rotationaround the O-O Bond Evidenced by Two-Dimensional Simulation of Temperature-Dependent Spectra

International audienceExperiments were performed to elucidate the origin of the superhyperfine structure and line width alternation(LWA) seen in the ESR spectrum of the major diastereoisomer (1) of DEPMPO-OOH, the remarkably persistentsuperoxide adduct of 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO). Using selectivelydeuterated DEPMPO derivatives, we demonstrated that the superhyperfine pattern can be unambiguouslyattributed to long-range couplings. The recording in pyridine of highly resolved spectra in a wide temperaturerange, combined with two-dimensional simulation, allowed us to characterize an inverted LWA in 1 andrevealed a uniform line broadening in the spectrum of the minor DEPMPO-OOH diastereoisomer (2), withboth effects originating from a chemical exchange between conformers. When the individual spectra of 1presenting LWA in the fast-exchange regime were simulated, four equally good fits were obtained and thisambiguity could be resolved by using a two-dimensional simulation technique. The thermodynamic and kineticconstants of this exchange were consistent with a rotation around the O-O bond. We propose that linebroadening effects in 1 and 2 result from this O-O rotation concerted with the pseudo-rotation of the pyrrolidinering