X-ray diffraction and theoretical studies of the methyl ester of ( R,R)-tartaric acid monoamide: semiempirical and ab initio calculations of some model compounds

Abstract

In its crystal structure, determined by X-ray diffraction, the methyl ester of ( R,R)-tartaric acid monoamide crystallizes with two molecules per assymetric unit. Each molecule displays subtle conformational differences effected by 180° rotation by the methyl ester group about the C-C* bond, whereas the staggered conformation around the central C*-C* bond, with a trans arrangement of the ester and amide substituents, and the eclipsed orientation of the amide nitrogen atom with respect to the nearest hydroxyl group, remains the same in both molecules. MNDO and ab initio single-point energies for the two molecules indicate that the lower energy molecule is that with the α-hydroxyl group eclipsed by the carbonyl group rather than the methoxy oxygen atom. Conformers observed in the crystal structure differ from those obtained by full MNDO optimization in the planarity of α-hydroxyester and α-hydroxyamide residues. Ab initio calculations at the 6-31G* level on smaller systems, i.e. (2 R,3 R)-2,3-butanediol and (2 R,3 R)-2,3-dihydroxy-butanedinitrile, suggest different conformational preferences for the two model compounds (gauche vs. trans with respect to the carbon chain), and point to the importance of the intramolecular hydrogen bond in stabilizing the gauche orientation of the vicinal OH groups. A similar hydrogen bond may be present in one of two crystallographically independent molecules of the methyl ester of ( R,R)-tartaric acid monoamide, but only as a minor component in a complex system of intermolecular hydrogen bonds.