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 o f tautomers with hydrogen jumping between donor atoms)? This question has received renewed interest because short, strong hydrogen bonds hav e been proposed to provide stabilization in some enzyme-catalyzed reac tions. Among the methods that have been applied are high-level calcula tions and the primary isotope shift. One of the most direct is the NMR method of isotopic perturbation, which will be illustrated with 3-hyd roxypropenal-d and with monoanions of mono-O-18-labeled dicarboxylic a cids. Both of these are found to be asymmetric. The results on hydroge n maleate and phthalate were unanticipated, since these are symmetric in crystals. The difference had been attributed to the disorder of aqu eous environment, and it was thought that these are symmetric in nonpo lar solvents, as had been indicated by the primary isotope shifts. Yet further studies of O-18-induced isotope shifts, especially at ipso ca rbons, show that a wide variety of these monoanions exist as two equil ibrating tautomers in all solvents.