X-ray evidence of temperature-dependent conformational disorder of hydrogen-bonded carboxylic acid molecules in crystal structures

Abstract

FROM an analysis of X-ray data on carboxylic acids1 we have established the stability of the approximately coplanar arrangement of the non-hydrogen atoms of the acetic acid group (C–COOH) with the α-substituent in α-substituted carboxylic acids (Fig. 1). In the crystal structures of tartronic acid (X = OH) (ref. 2), aminomalonic acid (X = NH2) (ref. 3) and fluoroacetic acid (X = F) (ref. 4) the coplanar cis conformation has been found. In crystals of unsubstituted monocarboxylic acids the acetic acid group has been found to be coplanar with the β-carbon atom1.