Low-Barrier Hydrogen Bonds in Pyridine-Dichloroacetic Acid Complexes

A mixture of Cl 2 CHCOOH- 1 6 O 2 and Cl 2 CHCOOH- 1 8 O 2 was combined with an excess of a pyridine and studied by NMR at 23°C, -43°C, and -81°C. The 1:1 complexes were analyzed by isotopic perturbation of equilibrium. Plotsof OH chemical shift and of 1 3 C isotope shift vs. pK a of the pyridinium ion both exhibit maxima - 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.