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 pyrr olidine N-oxyl free radicals by computer simulation of the C-13 satell ites 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 pote ntial V = 112V1 [1 - cos(P - P1)] + 1/2V2[1 - cos2(P - P2)], where P i s the pseudorotational phase. The parameters of the potential function were adjusted in order to reproduce the measured beta-carbon coupling s and temperature coefficients. In the case of symmetrically substitut ed molecules the equilibrium conformation is the T1 twist with a poten tial barrier 11.4 +/- 2 kJ mol-1. Substituents can deform the equilibr ium conformation towards the E3 envelope. The V1 barrier characterisin g the equatorial-axial preference of substituents was found to be larg er at the ring positions 3 and 4 than at 2 and 5. The effect of multis ubstitution is described by a vector addition model of the pseudorotat ional potential, and examples are given for the assignment of cis-tran s configurations. The axial-equatorial preferences of substituents wer e determined from the linewidth variation in the nitrogen triplet patt ern. The 2-aryl and 2-ethynyl substituents revealed an axial preferenc e indicating the existence of long-range conjugation between the pi-sy stems of the NO moiety and the substituent. The substituent dependence of nitrogen hyperfine coupling is also explained in terms of the pseu dorotational model.