ASYMMETRY OF HYDROGEN-BONDS IN SOLUTIONS OF MONOANIONS OF DICARBOXYLIC-ACIDS

Is a hydrogen bond symmetric (single-well potential) or asymmetric (do uble-well potential)? The NMR method of isotopic perturbation of equil ibrium was used to answer this question for the monoanions of a wide v ariety of O-18-labeled dicarboxylic acids. The observed O-18-induced i sotope shifts, especially at the ipso carbons, demonstrate that these exist as a pair of equilibrating tautomers in both aqueous and organic solvents. This conclusion for organic solvents is opposite a previous one based on similar data. As a further test, primary isotope shifts, which had been diagnostic for a single-well potential, were reinvesti gated. The monoanions of 1,2-cyclopentenedicarboxylic, 3,4-furandicarb oxylic, and 3,4,5,6-tetrahydrophthalic acids have negative primary iso tope shifts and are confirmed as having asymmetric hydrogen bonds. Alt hough hydrogen phthalate has a positive primary isotope shift, it too is judged to have an asymmetric hydrogen bond, according to O-18-induc ed isotope shifts, which are considered more reliable.