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.

The high resolution of the electron-nuclear double resonance (ENDOR) technique has been utilized to obtain precise values for the twelve proton hyperfine couplings and to estimate the various nitrogen hyperfine couplings in solutions of the stable free radical a, a'-diphenyl-,8-picryl hydrazyl (DPPH). The results enable us to explain for the first time the rather complex hyperfme structure in the electron paramagnetic resonance (EPR) spectra of DPPH solutions. Assignment of the various couplings was aided by comparison of the observed EPR spectrum with the computer-simulated spectra and by comparison of the available theoretical and magnetic resonance data with our ENDOR data. The results provide an accurate measure of the spin densities over the entire molecule. They also provide an experimental basis for a model proposed recently to account for the "combination" lines observed in electron-electron double-resonance (ELDOR) spectroscopy.

EPR, ENDOR, and x-ray photoelectron (ESCA) spectroscopy are used here to characterize AsO4−4 and SeO3−4 radicals in KH2PO4, KD2PO4, NH4H2PO4, and ND4D2PO4 in their paraelectric and ferroelectric–antiferroelectric phases. Temperature dependence of the 75As or 77Se hyperfine structure and of the proton superhyperfine structure was used as a microscopic probe for studying low frequency (108–1011 Hz) fluctuations near the phase transitions. Above a certain temperature T*≳Tc, the EPR spectra show (the expected) axial symmetry. Below T*, the spectra exhibit the symmetry of either the ferroelectric or the antiferroelectric phase, and thus provide evidence for the formation of polarized clusters. From an analysis of the temperature dependence of the observed line shapes we deduce that the motion of the hydrogen bond protons is strongly coupled to that of the heavier ions; and the width of this mode is in the megahertz range around T*. This conclusion has been reached by using both probes in the phosphates. For arsenates, however, the SeO3−4 could not be stabilized.

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