KINETIC-STUDIES ON REVERSIBLE DIMERIZATION OF THIANTHRENE AND 2,3,7,8-TETRAMETHOXYTHIANTHRENE RADICAL CATIONS

Fast scan voltammetry gives kinetic evidence for the reversible dimeri zation of the radical cations of thianthrene (T) and 2.3,7,8-tetrameth oxythianthrene (TMOT) in acetonitrile. At 256 K im equilibrium constan t K(Dim)approximate to 900 for TMOT (k(Dim)approximate to 1.1 . 10(4) for T) and rate constants k(f)approximate to 1 . 10(7) and k(b)approxi mate to 1.2 . 10(4) were determined. Using the semiempirical PM3 metho d, the formation of sigma-bonded dimers is indicated. For the dimeriza tion 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 natural T and TMOT systems and their planarization du ring the radical cation formation, the heterogeneous electron transfer kinetics was found to be very fast in both cases (1.6 cm/s; 298 K). T he activation enthalpy was determined to be 14 kJ/mol in the case of T MOT. Calculations of the inner-sphere reorganization energy using the Marcus theory produced Delta H-#(infinity) = 4.6 kJ/mol for TMOT and 3 .9 kJ/mol for T respectively at infinite temperatures. This explains t he fast electron transfer kinetics and their low activation enthalpies .