A CONFORMATIONAL STUDY OF ETHYL-SUBSTITUTED BITHIOPHENES - SEMIEMPIRICAL VERSUS AB-INITIO METHODS

Semi-empirical (AMI, PM3) and ab initio calculations (STO-3G, 3-21G) are employed to obtain the equilibrium optimized geometries and the to rsional potential surfaces of 2,2'-bithiophene as well as its 3,4'- an d 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 potenti al curves to account for the molecular relaxation, yielding a physical ly meaningful picture of the nonrigid rotation. The results given by e ach theoretical method are compared and discussed. It is found that et hyl substitution causes rather small changes in the thiophene ring str ucture. 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 fo r the 3,3'-ethyl derivative, It is also observed that the steric hindr ance created by ethyl groups is much higher than that induced by methy l substituents. (C) 1998 Elsevier Science S.A. All rights reserved.