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 angstrom and equal O-H distances of 1.206 angstrom, consistent with a strong and symmetric hydrogen bond. Incorporation of O-18 into one carboxyl group allows investigation of the symmetry of the H-bond in solution by the method of isotopic perturbation. The F-19 NMR spectra of the mono-O-18-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 O-18, not to the mere presence of the O-18, because the mono-O-18 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-O-18 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.