NMR conformational study of the neutralisation of meso- and rac-2,3-dimethyl- and 2,3-diethylsuccinic acids in aqueous solution

Abstract

The conformational behaviour of the meso and racemic isomers of aqueous 2,3-dimethyl-(1) and 2,3-diethylsuccinic acids (2) has been investigated at different degrees of titration. The changes of both the vicinal coupling constant between methine protons and the ¹³C chemical shifts have been determined and discussed in terms of the relative content of conformers in the conformational equilibrium.

It was found for the meso forms that different amounts of staggered conformers explain the observed changes of NMR parameters. However, for rac-1 and -2 the NMR data give evidence that besides staggered conformers, a conformer with a non-staggered conformation due to formation of an intramolecular hydrogen bond also has to be taken into consideration. At half degree of titration this conformer strongly prevails for rac-2 and seems to exist in reasonable amounts for rac-1. This is in accordance with the different K₁/K₂ ratio of both acids.