Article (Scientific journals)
Large phosphorus hyperfine coupling as a sensitive tool for studying molecular dynamics : ESR and Molecular Mechanics studies of ring interconversion in cis-2,5-diphosphoryl-2,5-dimethyl-pyrrolidinoxyl radical
Rockenbauer, Antal; Gaudel-Siri, Anouk; Siri, Didier et al.
2003In Journal of Physical Chemistry. A, 107 (19), p. 3851-3857
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Keywords :
Hyperfine Coupling; Phosphorus; ESR
Abstract :
[en] ESR spectra of cis- and trans-â-diphosphorylated pyrrolidine-N-oxyl radicals, c-1 and t-1, were studied in liquid and frozen solution. The expected 1:2:1 triplet (aP (2)) of the 1:1:1 triplet (aN) was observed for t-1; however, for c-1, the inner lines of the 1:2:1 triplet showed a dramatic broadening characteristic of chemical exchange between two equivalent conformations. Owing to the large difference in the hyperfine splitting constants (hfsc) of the exchanging phosphorus (¢aP 21 G), the coalescence temperature was unusually high (193 K, in n-pentane), and the potential barrier for ring interconversion (10.5 kJ mol-1) was easily obtained from the temperature dependence of the exchange rate. This value was in very good agreement with the value obtained for an empirical pseudorotational potential (11 kJ mol-1) that was adjusted to fit the temperature dependence of the phosphorus hfsc. For c-1, molecular mechanics calculations gave similar characteristics for the pseudorotational potential and indicated the existence of two identical minima with distorted geometries lying between 3T4 and 3E or 4T3 and E3. For t-1, only the 3T4 conformer was found to be significantly populated. Frozen solution spectra showed that the phosphorus hfsc anisotropy is higher when the C-P bond is pseudoaxial; this result can be explained by a geometry-dependent delocalization of the unpaired electron into the phosphorus 3p orbitals.
Disciplines :
Chemistry
Author, co-author :
Rockenbauer, Antal
Gaudel-Siri, Anouk
Siri, Didier
Berchadsky, Yves
Le Moigne, François
Olive, Gilles ;  Université de Liège - ULiège > Gembloux Agro-Bio Tech
Tordo, Paul
Language :
English
Title :
Large phosphorus hyperfine coupling as a sensitive tool for studying molecular dynamics : ESR and Molecular Mechanics studies of ring interconversion in cis-2,5-diphosphoryl-2,5-dimethyl-pyrrolidinoxyl radical
Publication date :
2003
Journal title :
Journal of Physical Chemistry. A
ISSN :
1089-5639
eISSN :
1520-5215
Publisher :
American Chemical Society, Washington, United States - District of Columbia
Volume :
107
Issue :
19
Pages :
3851-3857
Peer reviewed :
Peer Reviewed verified by ORBi
Available on ORBi :
since 03 March 2011

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