Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) of hyperfine interactions in solutions of α, α′-diphenyl-β-picryl hydrazyl (DPPH)

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 α, α′-diphenyl-β-picryl hydrazyl (DPPH). The results enable us to explain for the first time the rather complex hyperfine 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.