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

Abstract

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+).

Influence of 3‐Thio Substituents on Benzylidene‐Directed Mannosylation. Isolation of a Bridged Pyridinium Ion and Effects of 3‐O‐Picolyl and 3‐S‐Picolyl Esters

“…Hydrogen‐Bonding Capabilities of Pyridine Derivatives . To probe the relative influence of sulfur and oxygen on the hydrogen bond accepting capabilities of picolinyl ethers and thioethers, as in donors 24 and 27 , and the corresponding picolyl thioester and ester 26 and 28 , we synthesized the model compounds 47 – 50 (Figure 3) and adapted the NMR‐based method of Franz and coworkers [19] . Thus, equimolar solutions of 47 – 50 in CD 2 Cl 2 were mixed with 0.5 molar equivalents of a stock solution of pentafluorobenzoic acid and the 19 F NMR spectra recorded at room temperature.…”

“…To probe the relative influence of sulfur and oxygen on the hydrogen bond accepting capabilities of picolinyl ethers and thioethers, as in donors 24 and 27, and the corresponding picolyl thioester and ester 26 and 28, we synthesized the model compounds 47-50 (Figure 3) and adapted the NMR-based method of Franz and coworkers. [19] Thus, equimolar solutions of 47-50 in CD 2 Cl 2 were mixed with 0.5 molar equivalents of a stock solution of pentafluorobenzoic acid and the 19 F NMR spectra recorded at room temperature. The changes (Δδ) in the 19 F chemical shifts in the various mixtures relative to pentafluorobenzoic acid at the same final concentration serves as a measure of the relative H-bond accepting abilities of the four acceptors (Table 3).…”

“…[19] Thus, equimolar solutions of 47-50 in CD 2 Cl 2 were mixed with 0.5 molar equivalents of a stock solution of pentafluorobenzoic acid and the 19 F NMR spectra recorded at room temperature. The changes (Δδ) in the 19 F chemical shifts in the various mixtures relative to pentafluorobenzoic acid at the same final concentration serves as a measure of the relative H-bond accepting abilities of the four acceptors (Table 3).…”

“…19 F Chemical shift changes (Δδ) for the o-, m-and p-fluorines of pentafluorobenzoic acid in CD 2 Cl 2 on admixture with two equivalents of Hbond acceptor.…”

Quinine as a highly responsive chiral sensor for the 1H and 19F NMR enantiodiscrimination of N-trifluoroacetyl amino acids with free carboxyl functions

“…the aromatic quinoline ring and the quinuclidine bicycle, which are connected by potentially freely rotating bonds and, hence, are able to shape Qui conformation on the basis of the stereoelectronic requirements of interacting enantiomeric substrates. Both hydroxyl and the quinuclidine nitrogen favour hydrogen bond interactions and the basic character of the quinuclidine nitrogen, which has been very recently further enlightened and quantified, 39 makes Qui very promising as a solubility promoter for acidic chiral substrates having poor solubility in apolar solvents, frequently employed to solubilize the CSA itself.…”