Combined ESR and thermodynamic studies of the superoxide adduct of 5-(diethoxyphosphoryl)-5-methyl-1-pyrroIine N-oxide (DEPMPO): Hindered rotation around the O-O bond evidenced by two-dimensional simulation of temperature-dependent spectra

Antal Rockenbauer, Jean Louis Clément, Marcel Culcasi, Anne Mercier, Paul Tordo, Sylvia Pietri

Research output: Contribution to journalArticle

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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.

Original languageEnglish
Pages (from-to)4950-4957
Number of pages8
JournalJournal of Physical Chemistry A
Volume111
Issue number23
DOIs
Publication statusPublished - Jun 14 2007

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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