title: Asymmetry of the "strongest" OHO hydrogen bond, in the monoanion of (+/-)-alpha,alpha'-di-tert-butylsuccinate.
abstract: The large pK(a) difference between first and second deprotonations of (+/-)-alpha,alpha'-di-tert-butylsuccinic acid has been interpreted as evidence for a short, strong intramolecular hydrogen bond in the monoanion. 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. Relative to the intrinsic (18)O-induced isotope shift at the carboxyl carbon, as measured in the diacid, an additional isotope shift of 8 ppb in methanol, 14 ppb in acetone, and 5 ppb in THF is observed for potassium hydrogen (+/-)-alpha,alpha'-di-tert-butylsuccinate-(18)O. This increase indicates that the ion exists as an equilibrating pair of interconverting tautomers and not as a single symmetric resonance hybrid. The X-ray crystal structures of the tetrapropylammonium, tetrabutylammonium, tetrabutylphosphonium, magnesium, and calcium hydrogen (+/-)-alpha,alpha'-di-tert-butylsuccinate salts show a remarkably short O-O distance of 2.41 A, consistent with a strong hydrogen bond. However, the dicesium salt of the (+/-)-alpha,alpha'-di-tert-butylsuccinate dianion also shows the short O-O distance of 2.41 A, so this cannot be taken as evidence for a strong hydrogen bond in the monoanion. Moreover, the two O-H distances in the monoanions are unequal, and the hydrogen bond is asymmetric in these crystals. It is concluded that there is no evidence for any special stabilization associated with symmetric H-bonds. The large Delta pK(a) difference is therefore not due to any feature of the H-bond itself but is attributed to the electrostatic repulsion between the carboxylates in the dianion, which is relieved in the monoanion by inserting a proton between the carboxylates.
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: Jonathan S
authors.initials: JS
authors.lastname: Lau
authors.forename: Yeong-Joon
authors.initials: YJ
authors.lastname: Kim
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