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 Δδ
p
(19F) values correlate strongly with Hammett parameters, pK
a of the protonated HBA, and proton-transfer basicity
(pK
BH+).
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