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

Abstract

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 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.

Oligo‐ and Polyselenophenes: A Theoretical Study

“…[10] The HOMO-LUMO gap relates linearly to the cosine of the inter-ring twist angle, as is expected when orbital energies are proportional to the degree of orbital overlap (which is in turn proportional to the cosine of the angle between the orbitals) ( Figure 6b and 6d, inset). [51,52] As cos f= 1, at an inter-ring twist angle of zero (i.e., in a planar system) there is maximum overlap of molecular orbitals. The large change in the HOMO-LUMO gap can be explained on the basis of the effect of twisting on the frontier molecular orbitals of 6 Se.…”

Improved Reaction and Activation Energies of [4+2] Cycloadditions, [3,3] Sigmatropic Rearrangements and Electrocyclizations with the Spin‐Component‐Scaled MP2 Method

“…For example, DFT often underestimates barrier heights [6,7] and overestimates p-conjugative effects, [7,8] and MP2 uses an unbalanced treatment of electron correlation (vide infra).…”

“…This poor behaviour of B3 LYP underscores the imbalanced treatment of p conjugation. [7,8] In passing, we note that this method also occasionally has difficulties in correctly predicting the stereochemistry of Diels-Alder reactions. [21] In conclusion, SCS-MP2 gives impressively good comparisons for both the activation and reaction energies with MAE values of only 0.8 and 1.4 kcal mol…”

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes

“…In a number of cases it leads to predicting planar and more overcrowded conformations than the respective twisted X-ray geometries. There is a limited number of reports in the literature that DFT methods, including B3LYP, could overstabilize planar conformations of biphenyl and related heteroaromatic compounds [Viruela et al, 1997;Karpfen et al, 1997]. As in the X-ray structures, the acetyl groups and the anthracene systems in the mono-and diacetylanthracenes under study are essentially planar.…”