ESR‐Spectroscopic Investigation of the Homogeneous Electron Transfer Reactions between Substituted p‐Phenylenediamines and Quinonediimines, and the Validity of Marcus' Theory. I. Measurements at 293 K

Abstract

Abstract The rate constants k 1 , and k 2 of the self exchange reactions between the half oxidized and the totally oxidized or the reduced forms of some p‐phenylenediamine derivatives are investigated at 293 K by ESR line‐broadening in various aprotic solvents (dielectric constants ϵ between 4.7 and 49). The dimerization constants of the radicals at 183 K and the enthalpies of dimerization are measured, too. The results are discussed in terms of Marcus' theory. In k 1 , is found to depend linearly on γ = 1/ n 2 ‐ 1/ϵ ( n = refractive index), the covered range being 0.27 < γ < 0.53. The distance parameter g is obtained from the slope and compared with values derived from different molecular models. The results are represented satisfactorily by ellipsoidal molecules with half axes obtained from crystallographic data, the distance in the activated complex being approximately the same as in the radical dimers. The inner reorganization energy is calculated by Hückel methods and compared with experimental values obtained from the intercept of the straight line ln k 1 (γ). The deviations are considerable. The calculated Marcus activation free enthalpies are smaller then the measured ones. — The slow rate constant k 2 of the second step can be computed, if the Coulomb forces between the radical cation and the dication of the quinone diimine are considered. The inner reorganization energy of the transformation between aromatic and quinoidic structures is negligible. The rate constants of different cross reactions with p‐phenylene diamines are calculated with Marcus' formula which, however, has to be extended by work terms. The symproportionation rate is nearly diffusion controlled. With some other reactions the conformity of measured and calculated rate constants is excellent.