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RESEARCH PRODUCT

Geometric Structure and Torsional Potential of Biisothianaphthene. A Comparative DFT and ab Initio Study

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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 asses the optimal conformation of biisothianaphthene and evaluate the rotational barriers toward coplanar structures and (ii) on the methodological side, to asses 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 prevent...