Geometric structure and torsional potential of biisothianaphthene. A comparative DFT and ab initio study

We present a study of the torsional potential of biisothianaphthene and compare it to that of bithiophene. The calculations are performed at the ab initio and semiempirical Hartree-Fock (HF), ab initio post-Hartree-Fock, and density functional theory (DFT) levels. Our study has two major aims: (i) on the physico-chemical side, to assess the optimal conformation of biisothianaphthene and evaluate the rotational barriers toward coplanar structures and (ii) on the methodological side, to assess the usefulness of DFT approaches. In contrast to previous estimates, the torsional potential of biisothianaphthene is found to differ markedly from that of bithiophene. For biisothianaphthene, strongly rotated s-cis- and s-trans-gauche minima are predicted as the most stable structures. The structural analysis fully justifies the greater stability of the s-cis-gauche conformer, thus explaining the 'unexpected' s-cis-like structure observed experimentally in the crystal. The attainment of planar conformations is prevented by the high rotational barriers: ~22 kJ/mol (s-trans) and ~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 state, which is of importance with regard to their electronic and optical properties. DFT calculations are shown to provide geometries very close to those obtained at the MP2 level, but fail in describing the energetics of the torsional potentials because they overstabilize planar conformers. The results allow us to propose a very efficient computational approach for reliable determinations of conformational potentials in conjugated compounds. The poor quality of the potentials provided by semiempirical HF methods is emphasized.