Asymmetry of the “Strongest” OHO Hydrogen Bond, in the Monoanion of (±)-α,α′-Di-tert-butylsuccinate

The large pK_a difference between first and second deprotonations of (±)-α,α′-di-tert-butylsuccinic acid has been interpreted as evidence for a short, strong intramolecular hydrogen bond in the monoanion. Incorporation of ¹⁸O 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 ¹⁸O-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 (±)-α,α′-di-tert-butylsuccinate-¹⁸O. 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 (±)-α,α′-di-tert-butylsuccinate salts show a remarkably short O−O distance of 2.41 Å, consistent with a strong hydrogen bond. However, the dicesium salt of the (±)-α,α′-di-tert-butylsuccinate dianion also shows the short O−O distance of 2.41 Å, 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 ΔpK_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.