AB-INITIO STUDY OF TORSIONAL POTENTIALS IN 2,2'-BITHIOPHENE AND 3,4'-DIMETHYL-2,2'-BITHIOPHEHENE AND 3,3'-DIMETHYL-2,2'-BITHIOPHENE AS MODELS OF THE BACKBONE FLEXIBILITY IN POLYTHIOPHENE AND POLY(3-METHYLTHIOPHENE)

Ab initio molecular orbital theory is employed to calculate the gas-phase barrier to internal rotation in 2,2'-bithiophene. Ground state geometries are fully optimized at the restricted Hartree-Fock level of theory using the 3-21G* and 6-31G** basis sets. Methylation in beta-positions modulates the geometry, the inter-ring twist angle and the conformational properties of thiophene dimers. These methyl substitution effects have been assessed by calculations on the 3,4' and 3,3'-dimethyl derivatives in a number of selected conformations. A meaningful picture of the molecular relaxation on rotation is attained by allowing for full geometry optimization at both levels of calculation.