Density functional theory study of tetrathiafulvalene and thianthrene in acetonitrile: structure, dynamics, and redox...

The redox potentials of the organic compounds tetrathiafulvalene (TTF) and thianthrene (TH) in an explicit aprotic polar solvent, acetonitrile, have been computed using ab initio molecular dynamics simulation based on a Gaussian basis set methodology. The density functional description of the pure solvent yields a diffuse and mobile liquid, with structural and dynamical properties that are in good agreement with earlier classical models and experiment. Molecular dynamics simulation of both solute species in their neutral and radical cation states combined with free energy difference calculations result in estimates for the redox potentials of the reactions TH*+ + TTF --> TH + TTF*+ and TH2+ + TTF*+ --> TH*+ + TTF2+. The obtained values are 0.95 +/- 0.06 and 1.09 +/- 0.06 V, respectively, in excellent agreement with experimental data of 0.93 and 1.08 V. Our computational approach is based on Marcus theory, assuming quadratic free energy surfaces. We show that this approximation can still be accurate in systems, such as TH, that undergo a significant change in geometry upon oxidation. Furthermore, despite the different localization of the spin density in the radical cations, results based on self-interaction-corrected functionals and on standard generalized gradient approximations are identical to within 10 meV.