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

摘要

ESR spectra of cis- and trans-β-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 ~3T_4 and ~3E or ~4T_3 and E_3. For t-1, only the ~3T_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.