Large Phosphorus Hyperfine Coupling as a Sensitive Tool for Studying Molecular Dynamics: ESR and Molecular Mechanics Studies of Ring Interconversion in the cis-2,5-Diphosphoryl-2,5-dimethyl-pyrrolidinoxyl Radical Antal Rockenbauer, ² Anouk Gaudel-Siri, ‡ Didier Siri, ‡ Yves Berchadsky, § Franc ¸ ois Le Moigne, § Gilles Olive, § and Paul Tordo* ,§ Chemical Research Center, Institute of Chemistry H-1075 Budapest, Pusztaszeri 59, Hungary, Laboratoire de Chimie The ´ orique et Mode ´ lisation Mole ´ culaire, UMR 6517, CNRS-UniVersite ´ s d’Aix-Marseille I et III, Case D42, 13397 Marseille, Ce ´ dex 20, France, and Laboratoire Structure et Re ´ actiVite ´ des Espe ` ces Paramagne ´ tiques, UMR 6517, CNRS-UniVersite ´ s d’Aix-Marseille I et III, Case 521, 13397 Marseille, Ce ´ dex 20, France ReceiVed: June 5, 2002; In Final Form: January 20, 2003 ESR spectra of cis- and trans--diphosphorylated pyrrolidine-N-oxyl radicals, c-1 and t-1, were studied in liquid and frozen solution. The expected 1:2:1 triplet (a P (2)) of the 1:1:1 triplet (a N ) 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 (Δa P ≈ 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 3 T 4 and 3 E or 4 T 3 and E 3 . For t-1, only the 3 T 4 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. Introduction Because of the biological importance of various five- membered rings such as those of ribose rings in nucleic acids 1 and proline residues in peptides and proteins, 2 there is still considerable interest in determining the conformational states of five-membered rings. Pyrrolidine-N-oxyl radicals (Scheme 1) form an important class of five-membered ring radicals. If their dismutation is prevented by the absence of hydrogens on the R carbons, then most of these radicals are chemically stable, and different approaches have been developed to investigate their conformational behavior. 3,4 The conformers of five-membered rings can be close to either envelope (E) or twist (T) geometries (Figure 1). Conversion among the various conformers is usually thought to occur through pseudorotation. Spectroscopic techniques often fail to give reliable information on the heights and positions of barriers to interconversion, except for few simple molecules when the microwave technique is applicable. 5 As compared to six- membered rings containing six σ bonds (sp 3 carbon or nitrogen atoms) where energetically well-separated (ΔG 0 is ca. 20 kJ mol -1 ) and slowly interconverting (the interconversion fre- quency is less than 5 × 10 8 s -1 below room temperature) conformers exist, 6 the analogous five-membered rings are rather flexible, and interconversion is fast. (ΔG 0 does not exceed 10 kJ mol -1 , and the interconversion frequency is larger than 7 × 10 9 s -1 above 200 K. 7,8a ) This difference is evident in ESR spectroscopy studies where line-width alternation due to chemi- cal exchange is often observed in the spectra of radicals centered on a six-membered ring. However, for the five-membered ring analogues, such phenomena were detected only in a few cases using the low-temperature matrix technique. 8 For five-membered ring radicals, the lack of line-width alternation is caused by the complete averaging of coupling constants due to the fast pseudorotation. Line broadening resulting from chemical exchange depends on the square of the differences in the coupling constants for * Corresponding author. E-mail: tordo@srepir1.univ-mrs.fr. Tel: 33 4 91 632 851. ² Institute of Chemistry. ‡ Laboratoire de Chimie The ´orique et Mode ´lisation Mole ´culaire, CNRS- Universite ´s d’Aix-Marseille I et III. § Laboratoire Structure et Re ´activite ´ des Espe `ces Paramagne ´tiques, CNRS-Universite ´s d’Aix-Marseille I et III. SCHEME 1 3851 J. Phys. Chem. A 2003, 107, 3851-3857 10.1021/jp0213351 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/11/2003