Low-barrier hydrogen bonds in pyridine-dichloroacetic acid complexes

A mixture of Cl2CHCOOH- O-16(2) and Cl2CHCOOH-O-18(2) was combined with an excess of a pyridine and studied by NMR at 23degreesC, -43degreesC, and -81degreesC. The 1: 1 complexes were analyzed by isotopic perturbation of equilibrium. Plots of OH chemical shift and of C-13 isotope shift vs. pK(a) of the pyridinium ion both exhibit maxima similar to 5, when the basicities of the hydrogen-bond acceptors become matched, the hydrogen bond is strongest, and the equilibrium is maximally perturbed by the isotope. In contrast, for the complexes of pyridine and 3-picoline the isotopic perturbation seems to disappear, consistent with a low-barrier hydrogen bond in a single structure, rather than a hydrogen that can take either of two positions in a mixture of two tautomers.