NMR Quantification of Hydrogen-Bond-Accepting Ability for Organic Molecules.

The hydrogen-bond-accepting abilities for more than 100 organic molecules are quantified using 19F and 31P NMR spectroscopy with pentafluorobenzoic acid (PFBA) and phenylphosphinic acid (PPA) as commercially available, inexpensive probes. Analysis of pyridines and anilines with a variety of electronic modifications demonstrates that changes in NMR shifts can predict the secondary effects that contribute to H-bond-accepting ability, establishing the ability of PFBA and PPA binding to predict electronic trends. The H-bond-accepting abilities of various metal-chelating ligands and organocatalysts are also quantified. The measured Δδ(31P) and Δδp(19F) values correlate strongly with Hammett parameters, pKa of the protonated HBA, and proton-transfer basicity (pKBH+).