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

The hydrogen-bond-accepting abilities for more than 100 organic molecules are quantified using F and P 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(P) and Δδ(F) values correlate strongly with Hammett parameters, p of the protonated HBA, and proton-transfer basicity (p).