Combined ring inversion and side group rotation in geminal diphosphoryl substituted pyrrolidinoxyl radicals

ESR analysis of chemical exchange between four nonequivalent sites

A. Rockenbauer, Anne Mercier, François Le Moigne, Gilles Olive, Paul Tordo

Research output: Contribution to journalArticle

15 Citations (Scopus)

Abstract

The ESR spectra of three diphosphorylated pyrrolidinoxyl radicals have been studied over a large temperature range. While for the trans 2,5-disubstituted compound 1 no line width alternation was found, for the 2,2-disubstituted compounds 2 and 3, dramatic changes in the spectra were observed as a function of the temperature. These changes were explained by a four-site chemical exchange model including both ring inversion and hindered rotation around the carbon - phosphorus bonds. For radical 3, the presence of additional 5,5-dimethyl substitution can completely block rotations around carbon - phosphorus bonds for certain ring geometries, while for other ring conformations chemical exchange still occurs through combined inversion - rotation processes. An effective two-site model composed of nonequivalent sites and a superposition model composed of a pair of exchanging conformers and a pair of nonexchanging conformers were used to simulate spectrum variations. A satisfactory fit was obtained over the entire temperature range investigated. From the temperature dependence of the exchange frequencies the potential barriers for ring inversion (27 kJ/mol), for combined inversion - rotation (11 kJ/mol), and for rotation around the carbon-phosphorus bonds (14 kJ/mol) were estimated.

Original languageEnglish
Pages (from-to)7965-7970
Number of pages6
JournalJournal of Physical Chemistry A
Volume101
Issue number43
Publication statusPublished - Oct 23 1997

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Paramagnetic resonance
inversions
rings
Phosphorus
phosphorus
Carbon
carbon
Temperature
exchanging
alternations
Linewidth
temperature
Conformations
Substitution reactions
substitutes
temperature dependence
Geometry
geometry

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Combined ring inversion and side group rotation in geminal diphosphoryl substituted pyrrolidinoxyl radicals : ESR analysis of chemical exchange between four nonequivalent sites. / Rockenbauer, A.; Mercier, Anne; Le Moigne, François; Olive, Gilles; Tordo, Paul.

In: Journal of Physical Chemistry A, Vol. 101, No. 43, 23.10.1997, p. 7965-7970.

Research output: Contribution to journalArticle

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N2 - The ESR spectra of three diphosphorylated pyrrolidinoxyl radicals have been studied over a large temperature range. While for the trans 2,5-disubstituted compound 1 no line width alternation was found, for the 2,2-disubstituted compounds 2 and 3, dramatic changes in the spectra were observed as a function of the temperature. These changes were explained by a four-site chemical exchange model including both ring inversion and hindered rotation around the carbon - phosphorus bonds. For radical 3, the presence of additional 5,5-dimethyl substitution can completely block rotations around carbon - phosphorus bonds for certain ring geometries, while for other ring conformations chemical exchange still occurs through combined inversion - rotation processes. An effective two-site model composed of nonequivalent sites and a superposition model composed of a pair of exchanging conformers and a pair of nonexchanging conformers were used to simulate spectrum variations. A satisfactory fit was obtained over the entire temperature range investigated. From the temperature dependence of the exchange frequencies the potential barriers for ring inversion (27 kJ/mol), for combined inversion - rotation (11 kJ/mol), and for rotation around the carbon-phosphorus bonds (14 kJ/mol) were estimated.

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