Thermodynamics of successive electron transfers Internal and solvation enthalpy and entropy variations in a series of polynitro compounds

Abstract

The thermodynamic potential separation between the successive electron transfers in a series of di- and polynitro-compounds is measured using convolution potential sweep voltammetry as a function of temperature in acetonitrile and dimethylformamide. Both conjugated molecules and aliphatic compounds of the type O 2 N−C 6 H 4 −(CH 2 ) n −C 6 H 4 −NO 2 ( n =1–4) are considered. The potential separation appears as an increasing function of temperature. This is also true for the two oxidation steps of N,N,N′,N′-tetramethyl- p -phenylenediamine in acetonitrile. The enthalpy and entropy variations between the successive electron transfers are derived from this temperature dependence. It is shown that the entropy term, mainly due to solvation, is by no means negligible. Separating the internal and solvation enthalpy and entropy terms, these last quantities appear as linearly correlated. This allows the internal enthalpy variations to be computed and discussed in terms of conjugation and coulombic repulsion.