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

Fast 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 K_Dim ≈ 900 for TMOT (K_Dim ≈ 1.1·10⁴ for T) and rate constants k_f ≈ 1·10⁷ and k_b ≈ 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.