Influence of Hydrogen Bonds in 1:1 Complexes of Phosphinic Acids with Substituted Pyridines on 1H and 31P NMR Chemical Shifts

Two series of 1:1 complexes with strong OHN hydrogen bonds formed by dimethylphosphinic and phenylphosphinic acids with 10 substituted pyridines were studied experimentally by liquid state NMR spectroscopy at 100 K in solution in a low-freezing polar aprotic solvent mixture CDF3/CDClF2. The hydrogen bond geometries were estimated using previously established correlations linking 1H NMR chemical shifts of bridging protons with the O···H and H···N interatomic distances. A new correlation is proposed allowing one to estimate the interatomic distance within the OHN bridge from the displacement of 31P NMR signal upon complexation. We show that the values of 31P NMR chemical shifts are affected by an additional CH···O hydrogen bond formed between the PO group of the acid and ortho-CH proton of the substituted pyridines. Breaking of this bond in the case of 2,6-disubstituted bases shifts the 31P NMR signal by ca. 1.5 ppm to the high field.