INTERNAL-ROTATION AND CONFORMATIONAL PREFERENCES IN 1,2-DIARYL DERIVATIVES OF 1,1,2,2-TETRACHLOROETHANE - A H-1 DNMR AND X-RAY STRUCTURAL STUDY

The title compounds show in their H-1 NMR spectra recorded at room tem perature signals of different line-width characteristic of molecules w ith slow internal rotation around sterically crowded single bonds. Fro m the DNMR study in [H-2(6)]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 int ernal rotation processes, with different energies of activation. were extracted. The internal rotation process with higher energy barrier (D elta G = 13-14 kcal mol(-1)) was assigned to the anti reversible arro w gauche conformer interconversion, and that with lower energy barrier (Delta G = 8-9 kcal mol(-1)) to the rotation of aromatic rings aroun d the C(sp(3))-C(sp(2)) bond in the gauche conformer. The conformer po pulations measured at low temperature showed that the gauche conformer is the more abundant. The structural features of the conformers of co mpounds 1-3 were also investigated theoretically at a semi-empirical l evel with the AM1/MNDO method. The results of these calculations show that the anti conformer is the most stable one, yet the gauche form ha s higher polarity and this result could explain the increase of the ga uche population in polar solvents. For compound 3 calculations were ex tended to the whole energy hypersurface having as coordinates the angl es of rotation around the central C(sp(3))-C(sp(3)) and C(sp(3))-C(sp( 2)) bonds. The theoretical free-energies of activation, even if largel y underestimated, show that the barriers for the different internal pr ocesses follow the order: barrier for rotation around the C(sp(3))-C(s p(3)) bond > barrier for rotation around the C(sp(3))-C(sp(2)) bond in the gauche conformer > barrier for rotation around the C(sp(3))-C(sp( 2)) 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 crysta llographically dictated 1 symmetry with perfectly staggered anti confo rmation. Longer C-Cl bond lengths and smaller Cl-C-Cl bond angles than those present in structurally similar compounds indicate steric crowd ing around the C(sp(3))-C(sp(3)) bond. Solid state conformations are i n excellent agreement with the calculated ground-state rotational stru cture of these molecules.