RING PSEUDOROTATION IN PYRROLIDINE N-OXYL RADICALS - AN ANALYSIS OF C-13-HYPERFINE STRUCTURE OF EPR-SPECTRA

C-13-Hyperfine parameters have been determined for 36 substituted pyrrolidine N-oxyl free radicals by computer simulation of the C-13 satellites in the EPR spectra. The beta-carbon couplings were computed as an average value of twenty twist and envelope conformations weighted by the Boltzmann factors. The ring pseudorotation was described by a potential V = 112V1 [1 - cos(P - P1)] + 1/2V2[1 - cos2(P - P2)], where P is the pseudorotational phase. The parameters of the potential function were adjusted in order to reproduce the measured beta-carbon couplings and temperature coefficients. In the case of symmetrically substituted molecules the equilibrium conformation is the T1 twist with a potential barrier 11.4 +/- 2 kJ mol-1. Substituents can deform the equilibrium conformation towards the E3 envelope. The V1 barrier characterising the equatorial-axial preference of substituents was found to be larger at the ring positions 3 and 4 than at 2 and 5. The effect of multisubstitution is described by a vector addition model of the pseudorotational potential, and examples are given for the assignment of cis-trans configurations. The axial-equatorial preferences of substituents were determined from the linewidth variation in the nitrogen triplet pattern. The 2-aryl and 2-ethynyl substituents revealed an axial preference indicating the existence of long-range conjugation between the pi-systems of the NO moiety and the substituent. The substituent dependence of nitrogen hyperfine coupling is also explained in terms of the pseudorotational model.