Conformational Study and Chiroptica Properties of Chiral Dimethyl- Ethylenedithio-Tetrathiafulvalene (DM-EDT-TTF)

The enantiopure dimethyl-ethylenedithio-tetrathiafulvalene (DM-EDT-TTF) donor exists as biaxial (ax, ax) and biequatorial (eq, eq) conformers in equilibrium. DFT calculations combined with free energy surface (FES) analysis establish that the (ax, ax) form is more stable by similar to 0.7 kcal.mol(-1) than the (eq, eq) one and that the interconversion process involves a sequential conformational change through a boat type (ax, eq) conformer with an activation energy of similar to 3 kcal.mol(-1). TD-DFT calculations show similar, but opposite in sign, CD bands for the two conformers. A Boltzmann type average of the two CD curves, corresponding to a ratio of three (ax, ax) for one (eq, eq) conformers in equilibrium in solution, provides an excellent agreement with the experimental curve.