GEOMETRIC STRUCTURE AND TORSIONAL POTENTIAL OF BIISOTHIANAPHTHENE - ACOMPARATIVE DFT AND AB-INITIO STUDY

We present a study of the torsional potential of biisothianaphthene an d compare it to that of bithiophene. The calculations are performed at the ab initio and semiempirical Hartree-Fock (HF), ab initio post-Har tree-Fock, and density functional theory (DFT) levels. Our study has t wo major aims: (i) on the physico-chemical side, to asses the optimal conformation of biisothianaphthene and evaluate the rotational barrier s toward coplanar structures and (ii) on the methodological side, to a sses the usefulness of DFT approaches. In contrast to previous estimat es, the torsional potential of biisothianaphthene is found to differ m arkedly from that of bithiophene. For biisothianaphthene, strongly rot ated s-cis- and s-trans-gauche minima are predicted as the most stable structures. The structural analysis fully justifies the greater stabi lity of the s-cis-gauche conformer, thus explaining the ''unexpected'' s-cis-like structure observed experimentally in the crystal. The atta inment of planar conformations is prevented by the high rotational bar riers: similar to 22 kJ/mol (s-trans) and similar to 63 kJ/mol (s-cis) at the MP2 level. Aromatic polyisothianaphthene chains are therefore predicted to be highly distorted from planarity even in the solid stat e, which is of importance with regard to their electronic and optical properties. DFT calculations are shown to provide geometries very clos e to those obtained at the MP2 level. but fail in describing the energ etics of the torsional potentials because they overstabilize planar co nformers. The results allow us to propose a very efficient computation al approach for reliable determinations of conformational potentials i n conjugated compounds. The poor quality of the potentials provided by semiempirical HF methods is emphasized.