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.