(R,R)-TARTARIC ACID DIMETHYL DIESTER FROM X-RAY AND AB-INITIO STUDIES- FACTORS INFLUENCING ITS CONFORMATION AND PACKING

The conformation of dimethyl (R,R)-tartrate has been analyzed on the b asis of the single crystal X-ray diffraction method as well as by ab-i nitio quantum chemical studies. The results showed that the extended T conformation containing two planar hydroxyester moieties predominates in both ab-initio and X-ray studies. The lowest energy conformer in a b-initio calculations has C-2 symmetry and hydrogen bonds between a hy droxyl group and the nearest carbonyl oxygen. The second in energetica l sequence, with an energy difference of only 1.2 kcal/mol, is the asy mmetrical conformer, which differs from the lowest energy form by the rotation of one of the ester groups by 180 degrees. Intramolecular OH ... O hydrogen bonds observed in this rotamer again involve only proxi mal functional groups. This conformer is present in the crystal struct ure of the studied compound, although its conformation in the solid st ate is no longer stabilized by intramolecular hydrogen bonds of the ty pe mentioned above. In the crystal, hydroxyl groups are mostly involve d in intermolecular hydrogen bonds and form only a weak intramolecular hydrogen bond with each other. The planar arrangement of the alpha-hy droxyester moieties combined with the extended conformation of the car bon chain seems to be stabilized by the intramolecular hydrogen bonds between neighboring functional groups and by the long range dipole-dip ole interactions between two pairs of CO and (beta)C-H bonds.