Combined ESR and Thermodynamic Studies of the Superoxide Adduct of 5-(Diethoxyphosphoryl)-5-Methyl-1-Pyrroline N-Oxide (DEPMPO): Hindered Rotation around the O-O Bond Evidenced by Two-Dimensional Simulation of Temperature-Dependent Spectra
Abstract
Experiments 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 persistent
superoxide adduct of 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO). Using selectively
deuterated DEPMPO derivatives, we demonstrated that the superhyperfine pattern can be unambiguously
attributed to long-range couplings. The recording in pyridine of highly resolved spectra in a wide temperature
range, combined with two-dimensional simulation, allowed us to characterize an inverted LWA in 1 and
revealed a uniform line broadening in the spectrum of the minor DEPMPO-OOH diastereoisomer (2), with
both effects originating from a chemical exchange between conformers. When the individual spectra of 1
presenting LWA in the fast-exchange regime were simulated, four equally good fits were obtained and this
ambiguity could be resolved by using a two-dimensional simulation technique. The thermodynamic and kinetic
constants of this exchange were consistent with a rotation around the O-O bond. We propose that line
broadening effects in 1 and 2 result from this O-O rotation concerted with the pseudo-rotation of the pyrrolidine
ring.