Ring pseudorotation in pyrrolidine N-oxyl radicals: an analysis of 13C-hyperfine structure of EPR spectra

Abstract

¹³ C-Hyperfine parameters have been determined for 36 substituted pyrrolidine N-oxyl free radicals by computer simulation of the ¹³C satellites in the EPR spectra. The β-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= 1/2V₁[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 β-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 π-systems of the NO moiety and the substituent. The substituent dependence of nitrogen hyperfine coupling is also explained in terms of the pseudorotational model.