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

Chimia 2018, 72, 389-393

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 0.7 kcal·mol-1 than the (eq, eq) one and that the inter- conversion process involves a sequential conformational change through a boat type (ax, eq) conformer with an activation energy of 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.