Ab initio study of torsional potentials in 2,2′-bithiophene and 3,4′- and 3,3′-dimethyl-2,2′-bithiophene as models of the backbone flexibility in polythiophene and poly(3-methylthiophene)

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Abstract
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 β‐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.