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

Abstract

The ¹⁸O-induced isotope shifts in ¹³C NMR spectra of 1 : 1 complexes of Cl₂CHC¹⁸O₂H with a series of substituted pyridines in CD₂Cl₂ 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.