EFFECT OF ORTHO SUBSTITUENTS ON THE INTERNAL-ROTATION PROCESSES AND CONFORMATIONAL PREFERENCES OF 1,2-DIARYL-1,1,2,2-TETRACHLOROETHANES - AH-1 AND C-13 NMR VARIABLE-TEMPERATURE AND X-RAY STRUCTURAL STUDY

The conformational behaviour of 1,2-diaryl-1,1,2,2-tetrachloroethanes, ortho,ortho'-disubstituted with CN (1), F (2) and CO2CH3 (3), and of the mesityl derivative (4), has been studied in solution by variable t emperature H-1 and C-13 NMR spectroscopy. The internal rotation proces ses around the exocyclic C-C and central C-C bonds appear to have simi lar energy barriers and thus behave differently from the compounds wit hout ortho substituents, studied previously, which have a higher barri er for the interconversion of anti/gauche conformers. At low temperatu re, compounds 1-3 exist as mixtures of gauche and anti conformers with the former prevailing; compound 4 is almost exclusively in the gauche form. The solid state crystal and molecular structure has been determ ined for compounds 2 and 3. The molecule of compound 2 has point symme try ($) over bar 1, which defines perfect staggering around the centra l C-C bond with the aromatic rings anti. The structure is affected by statistical disorder due to two alternate orientations of the phenyl r ings. The crystal structure of compound 3 indicates one asymmetrical m olecule in a gauche conformation. Statistical disorder involves both t he methoxycarbonyl substituents at the phenyl rings; these latter disp lay almost the same orientation with respect to the C(sp(3)) carbons, in both compounds.