EVALUATION AND ASSIGNMENT OF PROTON AND NITROGEN HYPERFINE COUPLING-CONSTANTS IN THE FREE-RADICAL 1-PICRYL-2,2-DIPHENYLHYDRAZYL - NMR, ELECTRON-NUCLEAR DOUBLE-RESONANCE, AND ELECTRON-NUCLEAR TRIPLE RESONANCE STUDY

Deuterium NMR and proton and nitrogen ENDOR and electron-nuclear-nuclear triple resonance (TRIPLE) measurements have been performed on the stable free-radical DPPH and its deuterium or 15N-labeled derivatives in various solvents over the temperature range 160-400 K. The combined data permit the determination of all 12 of the proton hyperfine couplings (hfcs). Nitrogen ENDOR lines were easily detected. Algebraic signs of the hfcs were assigned by TRIPLE and NMR. Four intramolecular reorientation processes were established, and for three of these the activation energies and frequency factors were determined. Unambiguous assignment of several of the hfcs to specific proton positions in the molecule was achieved by studying the temperature dependence of these reorientation processes and also by using specifically deuterated samples. A quantitative comparison of the NMR and the ENDOR/TRIPLE results is given. Within the time scales accessible by NMR and ENDOR a consistent set of hfcs has been obtained. The room temperature ENDOR data, with small modifications, give an acceptable simulation of the experimental ESR spectrum of DPPH. © 1979 American Chemical Society.