Symmetry of hydrogen bonds in solution, an overview

Is a hydrogen bond symmetric (single-well potential, unique structure with a centered hydrogen) or asymmetric (double-well potential, pair of tautomers with hydrogen jumping between donor atoms)? This question has received renewed interest because short, strong hydrogen bonds have been proposed to provide stabilization in some enzyme-catalyzed reactions. Among the methods that have been applied are high-level calculations and the primary isotope shift. One of the most direct is the NMR method of isotopic perturbation, which will be illustrated with 3-hydroxypropenal-d and with monoanions of mono-O-18-labeled dicarboxylic acids. Both of these are found to be asymmetric. The results on hydrogen maleate and phthalate were unanticipated, since these are symmetric in crystals. The difference had been attributed to the disorder of aqueous environment, and it was thought that these are symmetric in nonpolar solvents, as had been indicated by the primary isotope shifts. Yet further studies of O-18-induced isotope shifts, especially at ipso carbons, show that a wide variety of these monoanions exist as two equilibrating tautomers in all solvents.