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.
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.
Rockenbauer
Antal
A
Chemical Research Center, Institute of Structural Chemistry, H-1025 Budapest, Pusztaszeri út 59, Hungary.
Clément
Jean-Louis
JL
Culcasi
Marcel
M
Mercier
Anne
A
Tordo
Paul
P
Pietri
Sylvia
S
D003198:Computer Simulation; D003497:Cyclic N-Oxides / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D008968:Molecular Conformation; D010100:Oxygen / Q000737:chemistry*; D010103:Oxygen Isotopes; D013113:Spin Labels; D013237:Stereoisomerism; D013481:Superoxides / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics
D003497:Cyclic N-Oxides; D010103:Oxygen Isotopes; D013113:Spin Labels; D013481:Superoxides; C098505:5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide; D010100:Oxygen