Symmetry of the Hydrogen Bond in Malonaldehyde Enol in Solution

A fundamental question about the hydrogen bond is whether the hydrogen is located in the middle of the two electronegative atoms, in a single-well potential, or else is closer to one of them and jumping between them, in a double-well potential. This question has been of interest recently because short, strong hydrogen bonds have been proposed to provide stabilization in some enzyme-catalyzed reactions. The NMR method of isotopic perturbation of equilibrium is now used to get an unambiguous answer for the intramolecular hydrogen bond of the enol of 2-phenylmalonaldehyde-α- d in CDCl 3 and pyridine- d 5 . The equilibrium isotope shift, which is large, downfield, and dependent on temperature, was measured in both 1 H and 13 C NMR spectroscopy. This result shows that the intramolecular hydrogen bond of 2-phenylmalonaldehyde enol is asymmetric, corresponding to the presence of two equilibrating tautomers.