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Variable-Temperature Study of Hydrogen-Bond Symmetry in Cyclohexene-1,2-dicarboxylate Monoanion in Chloroform‑d
journal contribution
posted on 2014-03-19, 00:00 authored by Charles L. Perrin, Kathryn D. BurkeThe
symmetry of the hydrogen bond in hydrogen cyclohexene-1,2-dicarboxylate
monoanion was determined in chloroform using the NMR method of isotopic
perturbation. As the temperature decreases, the 18O-induced 13C chemical-shift separations increase not only at carboxyl
carbons but also at ipso (alkene) carbons. The magnitude of the ipso
increase is consistent with an 18O isotope effect on carboxylic
acid acidity. Therefore it is concluded that this monoanion is a mixture
of tautomers in rapid equilibrium, rather than a single symmetric
structure in which a chemical-shift separation arises from coupling
between a desymmetrizing vibration and anharmonic isotope-dependent
vibrations, which is expected to show the opposite temperature dependence.