title: Hydrogen-bond symmetry in difluoromaleate monoanion.
abstract: 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 (18)O into one carboxyl group allows investigation of the symmetry of the H-bond in solution by the method of isotopic perturbation. The (19)F NMR spectra of the mono-(18)O-substituted monoanion in water, CD(2)Cl(2), and CD(3)CN 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 (18)O, not to the mere presence of the (18)O, because the mono-(18)O 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-(18)O 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.
authors.affiliation: Department of Chemistry, University of California-San Diego, La Jolla, California 92093-0358, USA. cperrin@ucsd.edu
authors.forename: Charles L
authors.initials: CL
authors.lastname: Perrin
authors.forename: Phaneendrasai
authors.initials: P
authors.lastname: Karri
authors.forename: Curtis
authors.initials: C
authors.lastname: Moore
authors.forename: Arnold L
authors.initials: AL
authors.lastname: Rheingold