Equilibrium studies by electron spin resonance. X. Thermodynamics of ion-pair dissociation by the use of time-averaged coupling constants

Abstract

The addition of salt (KI) to solutions of para-substituted nitrobenzene anion radicals in hexamethylphosphoramide (HMPA) results in the formation of ion pairs, which in the presence of free ion can result in one of the following observations upon esr analysis: (a) simultaneous observation of the spectra due to the free ion and ion pair, (b) dramatic line width alternation of the nitrogen hyperfine lines due to a rapid interconversion of the free ion and ion pair, and (c) complete time averaging of the spectra for the free ion and ion pair. When the value for the para substituent is less than about 0.6, the rate of ion-pair formation and dissociation is slow on the esr time scale and simultaneous recording of the two spectra is observed. When the para substituent is strongly electron withdrawing, as for the case of p-dinitrobenzene, complete time averaging is observed. For intermediate cases, as p-nitrobenzaldehyde, pcyanonitrobenzene, and p-nitrobenzophenone, the esr spectra exhibit line width alternation. Thus, the larger the charge density on the NO2 group the slower the rate of ion-pair formation and dissociation. The rate of ion-pair formation and dissociation was found to be fast on the esr time scale for the 2,6-di-tertbutylbenzosemiquinone system in contrast to that for the benzosemiquinone. This is attributed to a decrease in the charge density on the oxygen atom on the 4 carbon due to the presence of the two tertbutyl groups. For the systems where the rate of interconversion between ion pair and free ion is sufficiently fast on the esr time scale the esr coupling constants were algebraically related to the ion-pair dissociation constant, and the thermodynamic parameters for this process were obtained. The results agree qualitatively with those predicted by a Hammett plot previously reported.

Rates of ion pair formation and dissociation and of electron exchange between free ion and neutral molecule in hexamethylphosphoramide

“…and those for k, are sec-1. 6 The enthalpies and preeiably when DBQ•-is substituted for another semiquinone. Thus, ki should be proportional to Keq.…”

“…The spectra of the free ion and ion pair may be superimposed upon each other (simultaneous observation), or the time between ion association-dissociation events may be small on the ESR time scale and only the time-averaged spectrum observed. 6,6 The determination of rate constants of ion pair dissociation by the use of spin relaxation techniques can be carried out only for the latter case. Further, before a kinetic study can be initiated, the thermodynamic parameters of ion pair dissociation must be known.…”