Kinetic Studies on Reversible Dimerization of Thianthrene and 2,3,7,8‐Tetramethoxythianthrene Radical Cations

Abstract

AbstractFast scan voltammetry gives kinetic evidence for the reversible dimerization of the radical cations of thianthrene (T) and 2,3,7,8‐tetra‐methoxythianthrene (TMOT) in acetonitrile. At 256 K an equilibrium constant KDim ≈ 900 for TMOT (KDim ≈ 1.1·104 for T) and rate constants kf ≈ 1·107 and kb ≈ 1.2·104 were determined. Using the semi‐empirical PM3 method, the formation of σ‐bonded dimers is indicated. For the dimerization reaction in acetonitrile, enthalpy values of −79 kJ/mol for T and −56 kJ/mol for TMOT respectively were calculated. Despite the folded structure of the neutral T and TMOT systems and their planarization during the radical cation formation, the heterogeneous electron transfer kinetics was found to be very fast in both cases (1.6 cm/s; 298 K). The activation enthalpy was determined to be 14 kJ/mol in the case of TMOT. Calculations of the inner‐sphere reorganization energy using the Marcus theory produced ΔH#(∞) = 4.6 kJ/mol for TMOT and 3.9 kJ/mol for T respectively at infinite temperatures. This explains the fast electron transfer kinetics and their low activation enthalpies.