A conformational study of ethyl-substituted bithiophenes. Semi-empirical versus ab initio methods

Semi-empirical (AM1, PM3) and ab initio calculations (STO-3G, 3-21G∗) are employed to obtain the equilibrium optimized geometries and the torsional potential surfaces of 2,2′-bithiophene as well as its 3,4′- and 3,3′-ethyl-substituted derivatives. For the unsubstituted molecule, ab initio calculations have also been performed at the HF/6-31G∗ level. The geometries were completely optimized along the torsional potential curves to account for the molecular relaxation, yielding a physically meaningful picture of the nonrigid rotation. The results given by each theoretical method are compared and discussed. It is found that ethyl substitution causes rather small changes in the thiophene ring structure. Contrary to these results, ethylation dramatically influences the overall shape of the torsional potentials, leading to a large tilt from planarity. The barrier against planarity is found much higher for the 3,3′-ethyl derivative. It is also observed that the steric hindrance created by ethyl groups is much higher than that induced by methyl substituents.