(R,R)-Tartaric Acid Dimethyl Diester from X-Ray and Ab Initio Studies: Factors Influencing Its Conformation and Packing

Urszula Rychlewska,M Hoffmann,J Rychlewski,B Warzajtis

doi:10.3390/20700106

Abstract

The conformation of dimethyl (R,R)-tartrate has been analyzed on the basis of the single crystal X-ray diffraction method as well as by ab-initio 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 ab-initio calculations has C2 symmetry and hydrogen bonds between a hydroxyl group and the nearest carbonyl oxygen. The second in energetical sequence, with an energy difference of only 1.2 kcal/mol, is the asymmetrical conformer, which differs from the lowest energy form by the rotation of one of the ester groups by 180°. Intramolecular OH...O hydrogen bonds observed in this rotamer again involve only proximal functional groups. This conformer is present in the crystal structure of the studied compound, although its conformation in the solid state is no longer stabilized by intramolecular hydrogen bonds of the type mentioned above. In the crystal, hydroxyl groups are mostly involved in intermolecular hydrogen bonds and form only a weak intramolecular hydrogen bond with each other. The planar arrangement of the α-hydroxyester moieties combined with the extended conformation of the carbon chain seems to be stabilized by the intramolecular hydrogen bonds between neighboring functional groups and by the long range dipole-dipole interactions between two pairs of CO and (β)C-H bonds.

Highlights

This work is a part of our studies on the packing and conformational behavior of (R,R)-tartaric acid derivatives, both symmetrically and unsymmetrically substituted
The only exception reported in the literature constitutes the titanium coordinated diisopropyl (R,R)-tartrate in which two ester groups are in gauche minus (G-) orientation, and the four atom carbon chain is bent [2]
While in the isolated molecule such conformation is stabilized by intramolecular hydrogen bonds in which hydroxyl groups act as proton donors to one of the oxygen atoms from the neighboring ester groups, in the crystalline state there is no indication of the presence of this type of bond

Summary

Introduction

This work is a part of our studies on the packing and conformational behavior of (R,R)-tartaric acid derivatives, both symmetrically and unsymmetrically substituted. Polvarapu et al [6] performed further VCD measurements and stated, that their results could be interpreted in favor of the T conformation of dimethyl (R,R)-tartrate only when allowing the charge flow along the central C*-C* bond. They concluded that the G+ form could not be ruled out only on the basis of VCD results and may exist in equilibrium with the T conformer. Semiempirical studies on (R,R)-tartaric acid and its dimethyl diester [11] pointed to the T conformer as the favored structure

Methods

Results

Conclusion