Are there single-well hydrogen bonds in pyridine–dichloroacetic acid complexes?

The (18)O-induced isotope shifts in (13)C NMR spectra of 1:1 complexes of Cl(2)CHC(18)O(2)H with a series of substituted pyridines in CD(2)Cl(2) at low temperature reach a maximum when the acidities of the hydrogen-bond donors are matched, consistent with a mixture of tautomers, and with no drop that would indicate a single-well H-bond.