Hydrogen-Bond Symmetry in Difluoromaleate Monoanion

The symmetry of the hydrogen bond in hydrogen difluoromaleate monoanion is probed by X-ray crystallography and by the NMR method of isotopic perturbation in water, in two aprotic organic solvents, and in an isotropic liquid crystal. The X-ray crystal structure of potassium hydrogen difluoromaleate shows a remarkably short O–O distance of 2.41 Å and equal O–H distances of 1.206 Å, consistent with a strong and symmetric hydrogen bond. Incorporation of ¹⁸O into one carboxyl group allows investigation of the symmetry of the H-bond in solution by the method of isotopic perturbation. The ¹⁹F NMR spectra of the mono-¹⁸O-substituted monoanion in water, CD₂Cl₂, and CD₃CN show an AB spin system, corresponding to fluorines in different environments. The difference is attributed to the perturbation of the acidity of a carboxylic acid by ¹⁸O, not to the mere presence of the ¹⁸O, because the mono-¹⁸O dianion shows equivalent fluorines. Therefore, it is concluded that the monoanion exists as an equilibrating pair of interconverting tautomers and not as a single symmetric structure not only in water but also in organic solvents. However, in the isotropic liquid crystal phase of 4-cyanophenyl 4-heptylbenzoate, tetrabutylammonium hydrogen difluoromaleate-¹⁸O shows equivalent fluorines, consistent with a single symmetric structure. These results support earlier studies, which suggested that the symmetry of hydrogen bonds can be determined by the local environment