EPR and ENDOR studies of hyperfine interactions in solutions of stable organic free radicals

Abstract

ENDOR has been used to measure hyperfine couplings for all protons in solutions of the stable free radicals picryl-N-aminocarbazyl (PAC) and α, γ-bisdiphenylene-β-phenyl allyl (BDPA). The assignment of the measured couplings was aided through computer stimulation of the ENDOR spectra. The intensities of the ENDOR lines were not found to be proportional to the number of contributing protons, and the correction factor suggested by Allendoerfer and Maki was found to be necessary. The values of T2 thus obtained are ∼0.1 μsec and ∼0.05 μsec for PAC and BDPA in mineral oil at room temperature. A further check in the assignments was obtained through comparison of the experimentally observed high resolution EPR spectra with those stimulated on a computer using the proton couplings determined through ENDOR. This also yielded the nitrogen hyperfine couplings in PAC. These results complement our earlier assignment of hyperfine couplings in DPPH and confirm our earlier observations that large ENDOR enhancements can be obtained in nitrogen-centered, strongly exchanging radicals by using solvents with high viscosity and low dielectric loss and by optimizing radical concentration for large EPR signal but lower exchange rates. Well degassed mineral oil at about room temperature appears to be a good solvent. Temperature dependence of the ENDOR spectra shows dynamical changes in line splittings in DPPH and PAC. A model involving the presence of hindered rotations about the Nα–Nβ and the Nβ–Cpicryl bonds helps explain these observations. These results complement the available results on hindered rotations of picryl groups in the related nitroaromatic amines. The present work shows that for paramagnetic systems where NMR might not be successful, ENDOR could be a valuable technique for obtaining information on molecular dynamics. The ENDOR results on BDPA also resolve a controversy between earlier EPR and Knight shift studies.