An ESR and ENDOR study of spin relaxation of semiquinones in liquid solution

A quantitative test of the ENDOR theory for free radicals in solution has been developed experimentally using the semiquinone (SQ) radical anions of parabenzo‐ (PBSQ), duro‐ (DSQ), and 2,5‐dimethyl‐para‐benzo‐ (2,5‐DPMBSQ) dissolved in ethyl alcohol (EtOH) and dimethoxyethane (DME) solvents. It is shown that, in general, an ENDOR signal arising from a molecule containing four or less equivalent nuclei, such as PBSQ, can be analyzed rigorously whereas a molecule containing more than four equivalent nuclei, such as DSQ, which, in principle, could be analyzed rigorously, practically is best analyzed using approximate forms of the ENDOR theory. It is shown that 2,5‐DMPBSQ may be analyzed using a combination of both the rigorous and approximate forms of the ENDOR theory. The analysis involves calculation of the ENDOR relaxation parameters, T2n, Ωn, and Ωe,n, from the experimental ENDOR percent enhancement and linewidth studies. A comparison was made by performing the ESR linewidth and saturation studies to obt...