Radical cations of vitamin E compounds (a-, b-, gand d-tocopherols and a-, b-, gand d-tocotrienols) were for the first time successfully prepared in different solvents. Their unpaired electron distribution was solved by means of EPR and ENDOR spectroscopy and assignment of isotropic hyperfine couplings was performed. The isotropic hyperfine coupling constants calculated theoretically (UB3LYP) were comparable to those studied experimentally. Radical cations were observed to exist as an intermediate product in the formation of the neutral radical. Moreover, loosening the proton from the radical cation of a-tocopherol and from a-tocotrienol produced the neutral radical, which was detected in the ENDOR spectrum. The potency of the hydroxyl proton of the radical cation for further reactions as a measure of the biological activity of various tocopherol and tocotrienol compound is discussed.
All three radical species (radical anion, neutral radical and radical cation) from 2-methyl-1,4-naphthoquinone (vitamin were prepared in liquid solutions and measured by EPR and ENDOR/TRIPLE resonance K 3 ) spectroscopy. The methyl group protons showed the largest variation of isotropic hyperÐne coupling, from 8.3 MHz (anion) to 19.1 MHz (neutral) to 9.3 MHz (cation). Isotropic hyperÐne couplings of a proton in the quinoid ring and the methyl group protons in di †erent solutions were shown to be dependent on the relative permittivity of the solvent. A prominent alternating linewidth e †ect was detected in the EPR spectra of the radical anion of 2-methyl-1,4-naphthoquinone at di †erent concentrations and at di †erent temperatures in 1,2dimethoxyethane. This was explained as being due to the counter-cation Ñuctuation between the two quinodic oxygens. A solid dipotassium radical salt of vitamin was prepared and its EPR spectra measured. Isotropic K 3 hyperÐne coupling constants were also calculated for all three radical species using the UB3LYP density functional method.
The radical anion of duroquinone (2,3,5,6-tetramethyl-1,4-benzoquinone) was used as a model compound to
describe the low-frequency (LF) ENDOR lines observed in many radical systems. These LF lines are shown
to arise from two-photon transitions, and they occur because of the rapid cross-relaxation rates and dynamical
processes of the system. The LF lines occur when the radio frequency (RF) and microwave frequency match
the energy of an adjacent EPR transition. In practical work, LF ENDOR lines disturb the determination of
nuclei such as 2H, 13C, Na, and K, whose Larmor frequencies are in that region. Under certain conditions,
two-photon transitions were also observed in the normal proton coupling region. A method for distinguishing
normal ENDOR lines from two-photon transitions is given. The method is based on the different shape of an
ENDOR-induced EPR spectrum when it is measured from two-photon-transition ENDOR lines.
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