Internal rotation and conformational preferences in 1,2-diaryl derivatives of 1,1,2,2-tetrachloroethane: a 1H DNMR and X-ray structural study

Abstract

The title compounds show in their ¹H NMR spectra recorded at room temperature signals of different line-width characteristic of molecules with slow internal rotation around sterically crowded single bonds. From the DNMR study in [²H₆]acetone solution of the 4,4′-(compound 1) and 3,3′-bis(benzoic acid) dimethyl ester (compound 2) and of the 3,3′-bispyridine (compound 3) derivative the dynamic parameters of two internal rotation processes, with different energies of activation, were extracted. The internal rotation process with higher energy barrier (ΔG*= 13–14 kcal mol^–1) was assigned to the anti⇌gauche conformer interconversion, and that with lower energy barrier (ΔG*= 8–9 kcal mol^–1) to the rotation of aromatic rings around the C(sp³)–C(sp2) bond in the gauche conformer. The conformer populations measured at low temperature showed that the gauche conformer is the more abundant. The structural features of the conformers of compounds 1–3 were also investigated theoretically at a semi-empirical level with the AM1/MNDO method. The results of these calculations show that the anti conformer is the most stable one, yet the gauche form has higher polarity and this result could explain the increase of the gauche population in polar solvents. For compound 3 calculations were extended to the whole energy hypersurface having as coordinates the angles of rotation around the central C(sp³)–C(sp3) and C(sp³)–C(sp²) bonds. The theoretical free-energies of activation, even if largely underestimated, show that the barriers for the different internal processes follow the order: barrier for rotation around the C(sp³)–C(sp³) bond > barrier for rotation around the C(sp³)–C(sp²) bond in the gauche conformer > barrier for rotation around the C(sp³)–C(sp²) bond in the anti conformer. This order fits the trend of the first two barriers, experimentally determined. For compound 3 and for the 4,4′-bisbenzonitrile (compound 4) derivative, the crystal and molecular structure was obtained from X-ray analysis: the molecules have crystallographically dictated symmetry with perfectly staggered anti conformation. Longer C–Cl bond lengths and smaller Cl–C–Cl bond angles than those present in structurally similar compounds indicate steric crowding around the C(sp³)–C(sp³) bond. Solid state conformations are in excellent agreement with the calculated ground-state rotational structure of these molecules.