SYMMETRIES OF HYDROGEN-BONDS IN SOLUTION

The nuclear magnetic resonance method of isotopic perturbation can dis tinguish between single- and double-well potentials in intramolecularl y hydrogen-bonded monoanions of dicarboxylic acids. These are classic cases of a ''strong,'' symmetric hydrogen bond in the crystal. The obs erved carbon-13 isotope shifts induced by the substitution of oxygen-1 8 demonstrate that these monoanions exist as a single symmetric struct ure in a nonpolar solvent but as two equilibrating tautomers in aqueou s solution. The change is attributed to the disorder of the aqueous en vironment. These are simple counterexamples to the hope that the cryst al structure reveals the actual molecular structure in aqueous solutio n.