THE ASSIGNMENT OF THE NORMAL-MODES OF THE BEDT-TTF ELECTRON-DONOR MOLECULE USING THE INFRARED AND RAMAN-SPECTRA OF SEVERAL ISOTOPIC ANALOGS

Room-temperature infrared and Raman measurements on neutral bis(ethylenedithio) tetrathiafulvalene, abbreviated to BEDT-TTF, and four of its isotopic analogs are reported. The analogs are C-13(6) in which the central six carbon atoms are substituted with C-13; C-13(2) in which only the two central atoms are substituted and S-34(8) and d(8) in which all of the sulphur or all of the hydrogen atoms are substituted; respectively. The frequencies and frequency shifts of the Raman features and the infrared powder absorption features are measured. The polarized infrared reflectivity of a single crystal, and the intensities of the Raman features, help in assigning symmetries to certain resonances. By comparing these shifts to those calculated using a realistic valence force model the assigment of most of the in-plane fundamental intermolecular vibrational modes, and a few of the out-of-plane ethylene modes is achieved. The force constants are refined to produce excellent overall agreement between the measured and calculated frequencies and isotope shifts.