Conformational behaviour of isomeric bithienyls. An ab initio study

The conformational behaviour of isomeric bithienyls has been analysed by means of ab initio STO-3G computations optimizing the most significant geometrical parameters. The torsional potentials of bithienyls are very similar to those of the corresponding bifuranyls and are characterized by minima at planar conformations and maxima at perpendicular conformations. The energy difference between cis- and trans-conformers is very low except in the case of the 2,2′-isomer. Inclusion of the d orbitals on sulphur atoms does not alter the general trends, but significantly increases the energy barrier of the 2,2′-isomer.