Ab initio determination of geometries and vibrational characteristics of building blocks of organic super-conductors: TTF and its derivatives

Molecular behavior of the building block {[2-(1,3-dithiole-2-ylidene)-1,3-dithiole] tetrathiafulvalene (TTF)} of organic superconductors have been investigated along with its three derivatives, namely, {[2-(1,3-dioxole-2-ylidene)-1,3-dioxole]tetraoxafulvalene (TOF)}; [2,2]-bi -[[1,3] oxathiolylidene]Der I and 2-(3H-Furan-2-ylidene)-[1,3] oxathioleDer II. The properties of the molecules such as molecular geometries, frontier MOs and vibrational spectra have been investigated by using DFT method at the B3LYP level employing 6-311++G(d,p) basis set. The geometrical parameters and atomic charges on various atomic sites of the TTF, TOF, Ders I and II suggest extended conjugation in these systems. The present calculations lead to the reassignments for of some of the fundamentals and new interpretations for some of the observed IR and Raman frequencies. One of the two modes involved in the Fermi resonance giving rise to the doublet 1555 and 1564 cm−1 needed to be revised and another doublet 3083 and 3108 cm−1 could be interpreted as a Fermi resonance doublet. Out of the two ν(CC) modes under the a1 species, the lower frequency mode is assigned to the ν(CC) of the ring and the higher one to the ν(CC) of the central CC bond contrary to the assignment reported in literature. The conducting properties of these molecules depend mainly on this mode.

The IR and Raman spectra of the tetrathiafulvalene are computed with the help of Gaussian 03 by using DFT method. The computed spectra are reproduce here.Figure optionsDownload as PowerPoint slideHighlights
► TTF has non-planar and other three molecules have planar structures.
► All these molecules exhibit extended conjugation across the molecules.
► Molecular conduction depends on their geometrical and vibrational characteristics.
► In going from TTF to other molecules, central ν(CC) is strongly increased.