Interplay between dipolar, stacking and hydrogen-bond interactions in the crystal structures of unsymmetrically substituted esters, amides and nitriles of (R,R)-O,O '-dibenzoyltartaric acid

The compounds analysed are the O,O' -dibenzoyl derivatives of (R,R)-tartari c acid, asymmetrically substituted by ester, amide and nitrile groups. Benz oylation does not introduce drastic changes to the molecular conformation. All investigated molecules adopt the planar T conformation of the four-carb on chain with noticeably smaller departures from the ideal conformation tha n observed in the nonbenzoylated analogs. Primary and secondary amides alwa ys orient the C=O bond antiperiplanar (a) with respect to the nearest C*-O O bond, while methylester groups adjust their conformation to that adopted by the amide substituent situated at the other end of the molecule. Tertiar y amides and carboxyl groups place their carbonyls at the same side as the nearest C*-O bond (the s form), but often deviations from coplanarity of th e two bonds are significant and higher than those observed in the nonbenzoy lated series. The results presented demonstrate the importance of dipole/di pole interactions between CO and betaC*H groups in stabilizing the molecula r conformation, and between carbonyl groups in stabilizing crystal packing of the molecules that lack classical hydrogen-bond donor groups. An illustr ation is provided as to how a small change in mutual orientation of molecul es arranged in a close-packed fashion causes a change in the character of i ntermolecular interactions from van der Waals to sandwich stacking between the benzoyloxy phenyls, and to dipolar between the benzoyloxy carbonyls. Hy drogen-bonded molecules tend to orient in a head-to-tail mode; the head-to- head arrangement being limited to cases in which terminal carbonyl groups a re situated at one side of the molecule. The orientation of the benzoyloxy substituents with respect to the carbon main chain is such that the (O=)C-O -C-H bond system often deviates significantly from planarity.