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

Abstract

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 obtain values of We, the electron spin–lattice relaxation probability, and Wn, the nuclear spin-relaxation probability and then using this information to predict the observed ENDOR relaxation parameters in terms of the theory. The over-all behavior of the experimental results was found to be fully consistent with the general trends predicted by the theory and expected from the quantitative measurements obtained from the ESR studies. This work describes in detail the instrumentation and experimental methods necessary to measure relaxation parameters of organic free radicals in solution. A comprehensive section describing the effects of modulation amplitude, microwave power, coherence broadening or splitting, and pulse rate is given along with methods for analyzing ENDOR line shapes in their presence. The results and analyses of all ESR and ENDOR experiments are described in detail. Analysis of the rotational correlation time τR, obtained from the ESR studies, shows the radical anions in DME solution are most likely solvated and affected by the counterion and/or supporting electrolyte while PBSQ in EtOH is most likely dissociated. A comparison of the experimental and theoretical magnitudes of We leads to the possibility that for small molecules in liquids a Brownian model of reorientation by infinitesimal jumps need not be satisfactory and that jumps of large angle would be needed to better fit the experimental data. The effect of Heisenberg exchange on ENDOR signals is analyzed and discussed in detail, leading to the conclusion that the intermolecular electron–electron dipolar interaction as well as Heisenberg exchange make contributions to the concentration dependent portion of the linewidth. However, the latter contributes much more to the ESR linewidths as compared to the ENDOR widths for PBSQ in EtOH. The above analysis is modified to include charge effects. Finally it is shown that for methyl group ENDOR cross relaxation from modulation of the isotropic hyperfine splitting via methyl group rotation is not the dominant nuclear relaxation mechanism and all methyl group ENDOR analyses are consistent with a Wn derived from the pseudosecular terms of the anisotropic electron–nuclear dipolar interaction.