Vibrational analysis and isotope shifts of BEDT-TTF donor for organic superconductors

Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) is the best donor for 1-D and 2-D organic superconductors. Since coupling of vibrations with electronic states is critical to the nature of the superconductivity, it is essential to have a good understanding of the vibrational states. Many of the vibrational modes of BEDT-TTF were reported and assigned by Kozlov et al.(5) and more were assigned by Eldridge et al.(6) who made very complete isotope experiments. However, there remain several disagreements between these assignments. We report here the isotopic shifts for neutral BEDT-TTF based on ab initio calculations [Hartree-Fock (HF) using the 6-31G** basis set] for all fundamental vibrational frequencies. On the basis of these results, we propose new assignments for the fundamental vibrational frequencies of BEDT-TTF that satisfy the isotopic substitutions: (i) D for all H, (ii) C-13 for the central carbons, (iii) C-13 for all the double bond carbons, and (iv) S-34 for all sulfurs. The boat deformation of the neutral BEDT-TTF reduces the point group symmetry from previously accepted D-2k (or D-2) to C-2. This deformation explains the existing coincidence infrared (IR) and Raman spectrum.